Fast Drill Borehole Management

SPE 134580

Borehole Quality Design and Practices to Maximize Drill Rate Performance Dupriest F.E., Elks Jr. W.C., Ottesen S., Pastusek P.E., Zook J.R., ExxonMobil Development Company, Aphale C.R., ExxonMobil Upstream Research Company

Copyright 2010, Society of Petroleum Engineers This paper was prepared for presentation at the SPE Annual Technical Conference and Exhibition held in Florence, Italy, 1922 September 2010. This paper was selected for presentation by an SPE program committee following review of information contained in an abstract submitted by the author(s). Contents of the paper have not been reviewed by the Society of Petroleum Engineers and are subject to correction by the author(s). The material does not necessarily reflect any position of the Society of Petroleum Engineers, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper without the written consent of the Society of Petroleum Engineers is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of SPE copyright.

Abstract In 2005 the operator implemented a workflow that ensured drilling performance limiters were identified, redesigned, and extended continuously. The use of mechanical specific energy surveillance to address bit limiters and dysfunction has previously been published. The purpose of this paper is to discuss additional practices that have been developed to extend the non-bit performance limiters, particularly those related to borehole quality. There have been over 40 non-bit performance limiters identified and redesigned globally. While these are diverse, those with the greatest global impact were found to be tied directly to borehole quality. Consequently, in 2008 the performance management workflow was modified to increase awareness of borehole quality as a performance limiter. The result was that acceptable borehole quality became defined as that which would not limit footage per day. Quality is now redesigned to the "economic limit of performance" in the given interval. The economic limit of performance is a significantly higher standard than the common industry objective for borehole quality, which is to achieve low trouble time and run casing successfully. The average drilling footage per day drilled by the 23 operations that have been active since the performance management process was implemented has improved by about 63%. Instantaneous drill rates have typically increased 100-300%. Advances in bit and non-bit limiters appear to have contributed equally, and the majority of the gain in non-bit limiters has come from improved borehole quality. Other gains have come from related limiters, such as an increased understanding of the manner in which cuttings transport and tripping operations are controlled by borehole quality. The paper discusses the technical models that are used to understand the major borehole limiters, the engineering design and the real-time practices that have been developed, as well as the field results. Introduction Performance management tools tend to share a core process, which is the basic plan-do-analyze cycle. This is seen in important early works like the process used by the industry to optimize hydraulics (Lummus, 1970), to the "technical limit" workflow in the late 1990's (Bond et.al., 1996 ) and the operator's "limiter redesign" workflow implemented in 2005 (Dupriest et.al., 2005; Dupriest, 2006). Each reflects the intuitive process through which progress is made in any endeavor, which is to identify an issue, make changes to address it, and then repeat the process based on the results. While the fundamental cycle is the same, the detailed workflows differ, and probably should. To be effective, a performance management process must be consistent with a variety of factors, such as the company's risk management culture, its technical resource base, the availability of internal training resources, and the complexity and diversity of its operations. The gains shown in Fig. 1 since the rollout of the Fast Drill Process (FDP) suggest that the work process has been effective. These were largely mature programs in which the expected early learning curve gains had already been achieved (Brett, 1986). While there are elements of the workflow that would be effective in any organization, it should be noted that this effectiveness also reflects the degree to which the workflow is consistent with the organization's capabilities and culture. The key elements that one might expect to work universally have been previously discussed (Dupriest, 2006). Other details may be uniquely tailored to the operator's own strengths, well mix, or operating environment. The workflow is built on the insight that factors that limit performance do so by essentially limiting the ability to apply greater weight on bit (WOB). These are systematically identified and redesigned so that desired increase in WOB becomes possible.

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The traditional drill-off curve shown in Fig. 2 is used notionally to illustrate the concept. Rate of penetration (ROP) should increase linearly with WOB if the rock cutting process is efficient. If weight is applied and a non-linear response is seen, some form of dysfunction is occurring, which is to say the indention exposure of the cutting structure is not increasing linearly with WOB. Progress is then referred to as bit-limited.

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Fig. 1 - Increase in footage per day for each operation that has been active since the global implementation of the performance management process. The operations shown occurred over a period of approximately 4 years, from 2005 to mid-2009. Previous papers have described the manner in which mechanical specific energy (MSE) surveillance is used to detect and quantify the degree of this dysfunction (Dupriest, et.al., 2005). The fact that the MSE rises the further the performance diverges from the straight line is used diagnostically by the driller in real time to manage the drilling parameters and in post analysis for redesign. To date, the operator has trained approximately 3000 contractor and vendor personnel in this method. While MSE is effective in identifying bit limiters, approximately 60% of the operator's worldwide footage appears to be limited by factors other than the bit. In this footage the MSE is low and uniform, indicating that we are operating with linear behavior and a simple increase in WOB will yield the desired increase in ROP. However, in this footage we're unable to apply the desired WOB because other non-bit limiters prevent it. The state of operations this results in is commonly referred to as control drilling. A few examples of factors that result in control drilling are shown on the straight line portion of the curve in Fig. 2. More than 40 non-bit limiters have been identified across global operations. A key observation is that all limiters, regardless of their nature, notionally lie on the straight line. This means that at any moment in time there can be only one limiter that prevents further increase in WOB. The workflow is then simply to identify the current limiter and redesign or change operational practices to extend it. The objective of redesign is to extend the limiter to a level that causes another factor to become the limiter, and then the team works to extend this new limiter. The fact that there can only be one limiter is important because it allows the allocation of scarce technical resources to be prioritized to achieve the greatest business impact. The high level of focus on a single limiter has also tended to drive the organizations technical understanding to deeper levels, which has resulted in a variety of unique design and operational practices.

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Founder PointFounder PointFounder Point The linear response model helps prioritize the work effort. Notionally, there can only be one limiter at a point in time

Fig. 2 - Notional depiction of relationship between performance limiters and weight on bit. Performance is increased by practices or redesign that extend the WOB that can be utilized, whether the limitation is due to bit dysfunction or a non-bit limiter.

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SPE 134580 3

The process also differs from many in that specific performance objectives are not established, as they would be in a management by objectives system. The industry has traditionally used objective-based mechanisms such as target wells, well budgets, elimination of NPT (Kadaster, 1992), or calculated technical performance limits (Bond, 1996) to move the organization toward a desired end point. In this continuous improvement workflow limiters are identified and extended without a targeted end point, or regard of the actual performance achieved (Deming, Walton 1986). Each well program identifies the current limiter for each interval, the plan to extend it, the risks associated with the plan, and the mitigations for those risks. While footage per day is used as the metric to measure progress, it is not used as an objective. Early progress was dominated by the new insights MSE surveillance provided on rock cutting limiters, which include bit balling, bottom hole balling and vibrations. Many of the significant new bit-related practices that arose from this have been previously published (Dupriest et.al. February 2005, Dupriest November 2005, Remmert et.al. 2007, Bailey, et.al., 2008, Dupriest et.al. 2009, Sowers, et.al. 2009, Bailey et.al. 2009). By late 2006 it was recognized that the same level of gains were not being achieved in non-bit limiters, particularly those related to borehole quality. While MSE provides a very distinct metric for identifying bit dysfunction, there was no simple tool to determine the degree to which borehole quality was limiting performance. There is also an industry history of making this judgment based on whether non-productive time (NPT) was acceptable and casing run to bottom and cemented, rather than on whether borehole quality was limiting footage per day performance. The result was that the focus on continued improvement in hole quality was difficult to maintain. It was necessary to develop additional metrics and to rationalize the redesign process. A detailed study of wells with high NPT showed that virtually all major events were preceded by near misses, such as tight hole, packoffs, or cavings. It was also observed that the manner in which drill crews respond to near misses results in preventative operations with large hidden costs becoming routine, even when they were not needed. Examples include reaming on connections, additional circulation time, or reaming out of hole. On a global basis, the hidden cost of these operations was believed to be greater than the actual NPT. The importance of near miss identification has also been reported as a key tool in other borehole management efforts (Aldred, 1999). The decision was made that near misses would be used as the primary metric in driving redesign of borehole limiters, much as MSE was used for bit limiters. The concept is similar to the safety pyramid (Heinrich, 1931) in that there are multiple levels at which near misses can be identified and addressed. An example of the work process as it is applied to borehole stability is shown in Fig. 3. When near misses are addressed at lower levels in the well planning and execution process, hidden costs decrease, footage per day increases, and NPT is eliminated. Not only does the response to near misses reduce the minor hidden time that controls footage per day, but because it lies lower in the pyramid it also eliminates the NPT events that have traditionally been the focus of the industry's borehole design practices

Logical Potential NPTRedesign logical risks in the well plan as if they were a near miss

Historical Near MissRedesign to eliminate near misses

observed in offset wells

Real-Time Near Miss

React to every event observed in real-time

NPT

i.e., tight hole reported on trips, high LWD failure rate, fishing operations, enlarged hole on calipers, seepage losses, reaming to reduce drag, packoff with high ROP .............

i.e., high overbalance, marginal MW for stability, limited hydraulics for hole cleaning, small drilling window, extreme throw ................

i.e., drag in sands on trips, packoffs while reaming, torque fluctuations, unusual friction factors, cavings on shaker, sweep results, seepage losses, ballooning, cuttings load trends, bridges after trips in vertical wells, bit damage patterns, MSE and vibrations data ...........................................................

i.e., stuck pipe, reaming time, LWD failures, sidetracks, lost returns, inadequate formation evaluation, unscheduled casing, poor well productivity, fishing operations.................. .................

Logical Potential NPTRedesign logical risks in the well plan as if they were a near miss

Historical Near MissRedesign to eliminate near misses

observed in offset wells

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NPT

i.e., tight hole reported on trips, high LWD failure rate, fishing operations, enlarged hole on calipers, seepage losses, reaming to reduce drag, packoff with high ROP .............

i.e., high overbalance, marginal MW for stability, limited hydraulics for hole cleaning, small drilling window, extreme throw ................

i.e., drag in sands on trips, packoffs while reaming, torque fluctuations, unusual friction factors, cavings on shaker, sweep results, seepage losses, ballooning, cuttings load trends, bridges after trips in vertical wells, bit damage patterns, MSE and vibrations data ...........................................................

i.e., stuck pipe, reaming time, LWD failures, sidetracks, lost returns, inadequate formation evaluation, unscheduled casing, poor well productivity, fishing operations.................. .................

Fig. 3 - Near misses are used as the primary diagnostic to identify borehole limiters. Finally, it was necessary to establish guidance to ensure performance was not actually over designed. The underlying philosophy was already in place, which was that limiters would be redesigned to the "economic limit of performance". Logically, equipment would continue to be redesigned and new operational practices developed until the cost to extend the current limiter exceeded the value of the gain in performance. The difficult challenge is in projecting the value. For example,


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if mud weight (MW) is increased to reduce hole enlargement, what will the gain be in cuttings transport efficiency, and how much increase in footage per day will this enable? Other economic benefits are more easily predicted. For example, if a team is reaming each stand due to pulls in tight hole, elimination of the borehole pattern or other root cause would result in elimination of measurable reaming time. The potential value in a redesign is also judged from the broader experience of the organization through a well-connected global network of concept champions and a uniform process for conducting field trials. Also, research programs have been initiated to further understand the physics of key limiters to support the development of new science-based practices and improve the ability to predict potential performance gains. The enhancements to the Fast Drill Process workflow were implemented globally in mid-2008. This paper includes discussion of the three major borehole limiters and some of the key practices developed to extend them. These include instability limits, hole cleaning limits, and limits created by vibrationally induced patterns. The incremental impact of this effort through mid-2009 is shown in Fig. 4.

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Fig. 4 - Plot shows trend in average of the performances of each drilling operation since the global implementation of the workflow. Early gains were largely due to extension of bit performance limiters, while more recent gains have tended to occur through increased focus on borehole performance limiters. Borehole Stability Performance Limiters The dominant borehole performance limiter is instability. The resulting hole enlargement creates high NPT, sidetracks, reduces cuttings transport efficiency and results in very significant hidden costs. Consequently, improvement in stability was given a high priority. This was accomplished through changes in both well design practices and real time drilling practices. Initially there was a concerted effort to conduct detailed stability analysis on a greater number of wells. The criteria established for selecting the candidates for well-specific borehole stability assessments were: All business-critical wells Wells with offset history of instability Wells with inclination >40 Wells within complex geologic or tectonic setting Last year (2009) detailed analyses were conducted on approximately 100 wells out a total of 626 wells drilled by the operator. Another significant change was to incorporate the near miss philosophy into the stability design process. Examples of actions that might be taken at each level are shown in Fig. 5. Experience with the detailed analyses and field implementations have lead to the general conclusion that all wells should be drilled with the highest practical MW. If analysis shows this mud weight to still be inadequate, changes are made to the well design and/or operational practices and their cost and effectiveness are evaluated. This is a logical process for any operator, but what may differ is the end point for the redesign. When instability is viewed as a limiter to footage per day performance, rather than simply as a possible cause of NPT, the desired quality of the borehole becomes much higher. The economic limit of redesign is not reached until the cost impact of historical or logical near-misses are reduced to the expenditure required to address them. The economic limit of redesign considers the cost of such events as intermittent pack-offs, tight hole and excessive back-reaming on trips that typically is not captured by the NPT statistics, but may in some cases represent significant time and cost. A rapid response to real-time events call for increasing the MW immediately if cavings morphology is indicative of instability. And finally, thorough root cause analysis are conducted following any significant NPT event such as stuck pipe or lost returns to ensure new practices and / or well design changes eliminate the true root cause and not simply allow the team to survive with the current condition

SPE 134580 5

Logical Potential NPTMitigate hazards to the economic limit of re-design

Historical Near MissTreat near misses observed in historical

data as actual NPT

Real-Time Near Miss


NPT

Conduct root cause analysis and address in response plan

Raise mud weight immediately if cavings morphology indicates instability

Model borehole stability in all wells with offset history of instability

Model borehole stability in high angle (>40) wells, weak rock, high tectonic or complex geology. Use maximum possible mud weight



data as actual NPT

Real-Time Near Miss


NPT



data as actual NPT

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NPT

Conduct root cause analysis and address in response plan

Raise mud weight immediately if cavings morphology indicates instability

Model borehole stability in all wells with offset history of instability

Model borehole stability in high angle (>40) wells, weak rock, high tectonic or complex geology. Use maximum possible mud weight

Fig. 5 - Borehole stability risk triangle, showing examples of the actions that might be take to eliminate near misses and NPT related to instability at each phase of well design and execution. When maximizing the mud weight other factors may become limiters. The four concerns most commonly sited by the industry are lost returns, differential pressure sticking, destabilization of naturally fracture formations, and reduced drill rate. When planning mud weights borehole stability has become prioritized over lost returns, because historically the consequences of lost returns are less severe (costly) than the consequence of wellbore instability. In the majority of cases, lost returns do not result in a significant reduction in integrity. When a loss fracture is created it maintains a closing stress equal to the far field stress. The extended reach wells in which instability is most severe tend to have losses due to circulating pressure which can be manipulated in many ways. If circulating pressures or MWs are reduced to below the fracture closure stress, losses stop. Additionally, fracture closure stress (FCS), engineered particle non-aqueous fluids (EP-NAF) and managed pressure drilling (MPD) methods are available to mitigate the losses, if needed. In contrast, borehole enlargement is irreversible and its cost impact is very large. Stuck pipe events due to differential pressure sticking have been practically eliminated through practices implemented by the operator over the last 8 years and this is no longer considered an unmanageable risk when raising MW for stability. These practices have previously been published (Dupriest et. al., 2010). The concern for pressure penetration into fractured rock has led the industry to limit, or even reduce, the MW. A combination of engineered blocking solids and higher mud weights have been shown to be more effective in promoting stability in fractured rock (Ottesen, 2010). A similar concept has also been proven to be successful in stabilizing cleated coals with higher MW (Zeilinger, et. al., 2010). The theoretical impact of increased MW on ROP is usually minimal. The increased borehole pressure causes the in-situ effective stress to increase, which results in elevated confined rock strength. The change is usually small and the resulting loss of depth of cut (DOC) can be regained by increasing the weight on bit. There are situations where an increase in MW can dramatically reduce the ROP by causing the onset of bit dysfunction (bit or bottom hole balling), but these limiters can be extended through other redesign practices. The most severe impact of instability on footage per day occurs when pack-offs and stuck pipe result from inadequate hole-cleaning in and around enlarged sections of the borehole. Borehole instability (hole-enlargement or breakout) obviously impacts both hydraulics, hole-cleaning, and lost returns. The additional volume of rock generated as a result of instability needs to be effectively removed from the borehole, affecting the ECD and therefore also the lost returns potential. Hole-enlargements also have a negative impact on hole-cleaning, because the annular velocity is reduced in the enlarged area, resulting in potential cuttings and cavings accumulations. Borehole stability, hydraulics and hole-cleaning are therefore clearly coupled and can, for this reason, not be individually studied or assessed in isolation. These challenges led to the development of a new approach that exploits detailed understanding of the physics of borehole dynamics during drilling and the power of Quantitative Risk Analysis (QRA) technology (Ottesen et. al., 1999, Kline et. Al., 2005). This borehole quality QRA technology is used to optimize the well design architecture and drilling parameters for each individual hole-section. Choice of rig specific equipment such as drill sting dimensions and pump capacity are also incorporated into the analysis. The borehole quality QRA technology integrates underlying physical models on a risk analysis platform, allowing the probability of drilling success to be determined based on multiple drilling concerns. Large-scale flow loop experiments were conducted to validate the physical relationships (limit states) between hole-enlargement and hole-cleaning, tripping, back-reaming and casing running operations. The QRA approach allows optimum drilling parameters to be determined, such as MW and flow rate, and the overall probability of drilling success for a given hole-section. Probability of drilling success is defined as the probability of not encountering a well bore condition that may lead to a major NPT event or create a performance limiter.
















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However, the chance of success is also indicative of the likelihood of high hidden costs associated with managing poor hole quality. A typical result from a QRA assessment for a hole section is presented in Fig. 6.

75 ftROP= 50 m/hrQ = 750 gpm

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98% probability of drilling success, considering wellbore stability, hole

cleaning & lost returns.

XXX SS depleted to 6.40 ppg.Assumes 5-1/2-inch DP.

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Fig. 6 - Quantitative risk analysis (QRA) assessment for selection of mud weight and flow rate to maximize borehole quality and footage per day. Designs with higher probability of success have lower NPT and hidden cost time. In this example, borehole stability, hydraulics, hole cleaning and lost returns are assessed for a proposed high inclination 9-7/8-inch hole-section. The graph shows probability of drilling success as a function of MW and flow rate, considering borehole stability, hydraulics, hole cleaning and lost returns. In this example, using a stability MW of 12.0 ppg and a flow rate of 750 gpm will result in minimal hole breakout and a quality borehole with good cuttings transport at an ROP of 50 m/hr (165 ft/hr). The borehole is expected to look much like the 3D borehole image in the top left hand corner of Fig. 6. If the well is drilled with a MW of 11.5 ppg there is still a relatively high probability that this hole section can be drilled "successfully", even though there now will be areas of significant breakouts, particularly in the side of the borehole. The borehole will now look more like the middle 3D image on the left hand side of Fig. 6. While the geologic objectives may be achieved, near misses will likely drive the crew to conduct more reaming, back-reaming and tripping. Instantaneous ROP may also be constrained due to reduced cuttings transport efficiency in the enlarged sections of the borehole. If the mud weight is further reduced to 11.3 ppg, we risk initiating uncontrolled failure around the entire borehole circumference as indicated by the 3D borehole image in the bottom left hand corner in Fig. 6. With this borehole quality significant NPT or failure to meet objective has a high probability of occurring and, at best, the hole-section can be completed with significant hidden cost and reduction in overall drilling performance. The traditional use of NPT and running casing to bottom as metrics for success has lead the industry to drill many wells resembling the intermediate and last examples shown above. Continuous redesign to the economic limit of performance tends to drive quality to the higher levels shown in the first image. An initial hole-quality QRA assessment is performed for all major new drilling projects at the concept selection stage, to establish drilling feasibility and to determine the highest performing well design based on available drilling, geotechnical and geological information. Additional data requirements are also identified at this stage. Individual well-specific QRA assessments are then performed ahead of each new well as the drilling campaign progresses, incorporating additional data and knowledge being acquired. Real-time monitoring of wellbore quality is achieved by description of cavings morphology, torque & drag and ECD monitoring, log evaluations and analysis, etc. This "relentless re-design" (to the economic limit) work-flow is illustrated in Fig. 7.




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Earth Model- In-Situ Stresses - Pore Pressure- Rock Strength

Model Predictions- Mud Weight - Flow Rate- ECD- SPP- ROP

Well Application(Validation)

Improved Understanding (calibration / Re-design)

Rig Equipment- Pump - SPP Limit- Drill Pipe / BHA

Well Design- Trajectory- Casing Points- Hole Size- Casing Size- Drilling Fluid

Well Data- Hole Caliper / MW - Cuttings / Cavings- LOP- Logs (Sonic etc.)- SPP- PWD

Earth Model- In-Situ Stresses - Pore Pressure- Rock Strength

Model Predictions- Mud Weight - Flow Rate- ECD- SPP- ROP

Well Application(Validation)

Improved Understanding (calibration / Re-design)

Rig Equipment- Pump - SPP Limit- Drill Pipe / BHA

Well Design- Trajectory- Casing Points- Hole Size- Casing Size- Drilling Fluid

Well Data- Hole Caliper / MW - Cuttings / Cavings- LOP- Logs (Sonic etc.)- SPP- PWD

Fig. 7 - Elements of the iterative process used to redesign hole quality to the economic limit of performance. The hole-quality methodology and work-flow described above was adapted for the Bass Strait, Australia, drilling campaign that started in 2007. The improvement in hole-quality as a result of consistently using higher than historic MWs is evident in the caliper logs presented in Fig. 8.

Well A26 - Mud Weight = 9.6 ppg Well A21a - Mud Weight = 11.4 ppg

Fig. 8 - Comparison of caliper logs after utilizing QRA-based designs to balance mud weight, flow rate and desired hole quality to maximize the achievable drill rate and footage per day The caliper log from the 8-1/2-inch hole section in well A26, a well drilled during an earlier drilling campaign with a MW of 9.6 ppg, show hole enlargements in excess of 12.5 inches. Well A21a was drilled during the current drilling campaign (2008) using a MW of 11.4 ppg. The caliper log from the 8-1/2-inch hole section in well A21a show some indications of instability but the improvement in hole quality compared with well A26 is evident. The formations penetrated in these two hole-sections contain shales with inter-bedded coal stringers. Further improvement in the hole-quality of wells subsequent to well A21a was achieved by additions of engineered blocking solids to prevent fluid and pressure penetration into the coal fracture network, making the increased MW more effective in stabilizing the coals (Zeilinger, et. al., 2010). Other details of the campaign and

A21aA26

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Wellbore Stability Polar Plot

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redesigns arising from the performance management workflow have previously been published (Kilroy, 2009). The use of the QRA modeling resulted in a balance of mud weight, flow rate, fluid properties, drill string, hole and casing architecture that yielded a significant increase in the sustainable drill rate and also a significant reduction in hole quality related NPT. The results are shown in Fig. 9.

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FDPBMIHole Quality NPT

Fig. 9 - Bass Strait, Australia drilling performance summary showing reduced trouble time and increased footage per day NPT related to borehole quality was reduced from 7.8% in 2007 to 3.2% in 2008, and finally to 0.8% in 2009. Over the same time period the ROP, as measured by the performance management metric increased from 1,475 to 2,283 feet per day, an improvement of 55%. In previous programs, considerable time was spent conditioning the hole after reaching total depth. When this time is included in the pre and post comparisons, the metric improved from 1,518 ft/day to 6,463 ft/day, an improvement of 326%, reflecting a significant reduction in hidden costs. This trend is seen in most areas with soft formations where the drill time may be small, so any time spent after reaching total depth conducting hole conditioning, tripping, or back-reaming in preparation for running casing may become a significant percentage of the total well. Hole Cleaning Performance Limiter Many factors influence the drill rate at which the cuttings load may become unacceptable. The measure of the adequacy of hole cleaning has generally been the ability to drill to total depth with reasonable NPT and no stuck pipe. However, in the ROP management workflow redesign of the hole cleaning rate continues until the next step is proven to be uneconomic or the another limiter becomes dominant, such as the logging while drilling (LWD) data acquisition rate. As with borehole instability, the economic limit of redesign in hole cleaning has proven to be a much higher and cost effective standard than NPT or stuck pipe. Ideally, there would be no circulating or reaming time on connections and no hole conditioning during trips or prior to running casing. This would be maintained while extending the ROP to some level at which another limiter becomes dominant (i.e., surface solids processing, cuttings injection). The factors that control the cuttings transport rate vary with hole angle. In the early 1990's the operator developed a model to predict the hole cleaning efficiency for various factors, including hole angle (Rasi, 1994). In low angle wells, the transport efficiency is determined by the rate at which the cuttings slip downward in the vertical flow stream. If cleaning is inadequate, the equivalent circulating pressure (ECD) from the cuttings load may approach integrity or packoffs may occur, usually after making connections when there is no fluid flow. In the operator's well mix it is rare for the drill rate to be limited in low angle wells, even at 500-800 ft/hr. The cuttings slip rate can be controlled with rheology. Low velocity in enlarged hole remains the primary concern. In this situation, hole enlargement is the true performance limiter and the redesign effort shift to stability. In high angle wells, the slippage of cuttings downward as they travel laterally results in the continuous deposition of a layer of material on the bottom. These cuttings beds begin to develop at around 40 of inclination. As the bed grows in height, the flow area above it declines and the fluid shear acting on top of the bed increases. At some point the deposition and removal rates equal each other and an "equilibrium bed height" is established (Rasi, 1994). One factor that influences the bed height is the cuttings generation rate, or ROP. As ROP is increased the bed height grows which restricts the flow area and ECD may approach integrity. The bed may also become more prone to pack off and stick pipe. It may also create tripping or casing running hazards. When the performance management workflow was implemented in 2005 the operator had already implemented many of the practices that allowed challenging wells to be drilled with low NPT and minimal stuck pipe (Elks, Jr, W.C. 2002). These included : 1) using maximum rotation and maximum circulation (MRMC) both while drilling and when circulating bottoms up







SPE 134580 9

prior to tripping out of the hole, 2) circulating multiple bottoms up prior to tripping to ensure the cuttings bed was reduced to a safe tripping level, 3) monitoring torque and drag (T&D) friction factor trends in real time at the rig site while drilling, tripping and running casing to provide the rig supervisor an indicator of the worsening hole condition (i.e. excess cuttings bed), 4) real time cuttings return volume monitoring to provide the rig supervisor a measure of hole cleaning efficiency and 5) backreaming multiple cycles at high RPMs (120 200) prior to the connection to condition the borehole and reduce the bed height and ECD. While these practices have been successful, they are focused on NPT avoidance and not necessarily the economic limit of redesign for footage per day. In some sense, the objective is not to drill with a clean hole but to succeed with "the dirtiest hole possible", which is to say the highest ROP and equilibrium bed height that does not result in packoffs or ECD that exceeds integrity. The question becomes, what determines the dirtiest hole possible? This is a more complex problem than what determines how clean the hole is? As redesign occurred in hole cleaning operations, several key observations helped to redefine the assumed limits. First, very high drill rates were achieved in horizontals that exceeded industry recommendations by a large margin. In one drill team, the IROP (instantaneous ROP) in the horizontal pay was routinely maintained at 1000-1500 ft/hr with no history of packoffs. Second, research in a full scale flow loop showed hole enlargement to dominate the cuttings transport process (Fig. 10). Cuttings transported easily in gauge hole. Packoff events were seen to occur in, or around, the enlarged sections. The behavior was consistent with observations from the field. For example, the high drill rates achieved in the previously mentioned horizontals were in gauge sandstone wellbores. And third, as a result of redesign to extend vibrational bit limiters, the organization became more aware that poor management of bit whirl could also create enlargement. The impact was that when hole cleaning was observed to be the limiter the response became to redesign the hole quality as much as the fluid rheology, flow rates, or other traditional approaches.

Fig. 10 - Full scale testing showing the packoff process developing as the top of the drill collars enters the enlarged area and the stored mass is mobilized by the increased velocity around the collars. In similar testing in plexiglas boreholes of uniform diameter the wellbore was easily cleaned with no packoffs. The hole cleaning simulations resulted in several key learnings and observations. Gauge hole was seen to clean quickly and easily (i.e. cuttings bed is removed) around the BHA using typical flow rates and drill string rotation speeds Enlarged hole is almost impossible to clean-up even when the BHA is backreamed through the enlarged area. Another observation, depicted in Fig. 11, was that the area above the cuttings bed appeared to be relatively constant in both the gauge and enlarged areas.

A1

A2

A1 A2

EquilibriumBed Height

A1

A2

A1 A2

EquilibriumBed HeightEquilibriumBed Height

Fig. 11 - The cuttings bed reaches an equilibrium bed height in the gauge hole and the enlarged hole that leaves approximately the same open flow area in each hole section.

Dune and Packoff Rapid mobilization of stored mass











10 SPE 134580

This is consistent with the equilibrium bed height concept, which is that the bed will grow until the cuttings deposition rate equals the erosion rate. Regardless of hole size, this occurs when the fluid velocity above the bed reaches the equilibrium rate, which occurs at a given bypass area. The implication is that wiper trips or other hole conditioning operations do not improve hole cleaning if the cleaning problem is dominated by enlargement, which it usually is. The enlarged area will refill immediately when drilling resumes. Another conclusion was that only one interval of enlarged hole may dominate the hole cleaning performance for the entire hole section, including the rathole below the previous casing shoe. When modeling hole cleaning the design flow rates and rheologies are now based on worst case hole enlargements The most unique perspective to come out of the research is that it may not be possible to pack off due to high drill rate during steady state conditions. As the bed height increases the velocity and erosion rate increase. The system may become self-limiting. While drill rates may become limited by the ECD caused by declining flow area, it may be possible to drill rate very high rates without packoffs if the integrity allows it. Altogether, hole enlargement is believed to be the dominant factor in hole cleaning efficiency. Three potential root causes of enlargement are 1) wellbore instability due to stresses or the fluid type utilized, 2) side cutting from bit whirl while drilling or while reaming, and 3) inadequate filtration control and MW when drilling unconsolidated sands Although there has been redesign and changes in practices to address all three, the most common issue is stress-induced instability, which has been previously discussed. Mud weights have been increased globally to the point that ECD is now close to integrity in most high angle operations, even when there is little history of NPT. While increased MW and other design changes have been beneficial, there have also been other changes in operating practices related to hole cleaning. Because whirl was seen to cause enlargement, MRMC was revised to MRMCwow (maximum rotation and maximum circulation without whirl). Although higher rotary speeds may aid hole cleaning this may be offset if whirl is allowed that results in enlargement. A testing protocol was developed to determine the off-bottom speeds that minimized whirl. The low whirl speeds have generally been found below 120 RPM and the impact on hole cleaning in those teams that have reduced RPM appears minimal. More significantly, after MW is increased to reduce hole enlargement, reaming and circulating time was almost eliminated in many operations without adverse effect on hole cleaning. A typical connection practice is now to reduce rotary speed (3/4 to 2/3 of the on-bottom RPM) prior to coming off bottom to mitigate bit whirl and ream upward only a minimal distance (i.e. 10 20 ft) during the time it takes to circulate cuttings above the BHA, which is typically 2-4 minutes. Fig. 12 shows data from a connection during a field trial. The chart shows instantaneous ROPs of 150 - 200 ft/hr followed by a connection with minimal reaming and shortened circulation times. This field trial was in 12-1/4 hole at 66 inclination in a field known to have wellbore instability.

Fig. 12 - Time based drilling data showing connection practices with minimal circulation and reaming








SPE 134580 11

The following plots show another field trial in which the original hole was drilled using the standard high angle reaming practices that were revised to mitigate whirl during the geologic sidetrack. WOB was increased to mitigate on-bottom drilling whirl and the connection practices were modified to mitigate off-bottom bit whirl. Fig. 13 shows the comparison of instantaneous ROPs which were doubled from 3250m to TD as a result of increased WOB and MSE management.

2250

2450

2650

2850

3050

3250

3450

3650

3850

4050

42500 5 10 15 20 25 30 35 40

Instantaneous ROP (m/hr)

Mea

sure

d D

epth

(m)

Sidetrack HoleOriginal Hole

Fig. 13 - Instantaneous ROP in the original hole versus the sidetrack hole. Both holes had similar trajectories with inclinations up to about 85. Surprisingly, Fig. 14 shows the ECD in the sidetracked hole, which was drilled at twice the ROP using the same MW and drilling assembly, was less than that in the original hole by as much as 0.2 ppg. Tripping the drill string on elevators had been problematic in this field due to tight spots, routinely requiring extensive backreaming to get out of the hole. In addition, pack-offs were common while backreaming, occasionally causing lost returns or stuck pipe. On the sidetracked hole, the drill string was tripped out of the hole on elevators with minimal problems even after MRMCwowing the hole clean at reduced RPMs. Backreaming out was not required. Since this sidetrack hole was drilled, all subsequent drill wells have been drilled with the same practices. Tripping on elevators has become commonplace and backreaming has not been required to trip out of the hole. Casing was successfully run to bottom on all wells. The specific reasons for the reduced ECD at the higher ROP are not known, but the result points to the fact that drill rate should not be constrained by assumed hole cleaning limits.

10.0

10.2

10.4

10.6

10.8

11.0

24002500

26002700

28002900

30003100

32003300

34003500

36003700

38003900

40004100

4200Measured Depth (m)

ECD

(ppg

e)

FG @ Shoe, EMW Surface MW Original Hole ECD Sidetrack Hole ECD

Sidetrack Hole

Original Hole

Fig. 14 - ECD in the original hole versus the sidetrack hole.


12 SPE 134580

A clear concern is that the hole cleaning limit is found by having a stuck pipe event. A controlled field trial process was developed to ensure the limit is recognized when it is reached and risks are mitigated The Max IROP (maximum instantaneous rate of penetration) test determines the drill rate that can be maintained with no significant circulation or reaming on connections. Connection practices are held constant, typically around 5-7 minutes, while the instantaneous drill rate is increased steadily as each stand is drilled. The maximum instantaneous drill rate is that at which the ECD equals integrity when the stand is down. Because ECD is a product of the equilibrium bed height, if it is acceptable prior to the connection it should be acceptable afterward, with no significant circulating time required to further reduce the bed height. The process focuses on minimizing connection time rather than maximizing instantaneous ROP. If instantaneous ROP is excessive, the result will be unacceptable ECD and longer circulation times to reduce the bed height. The Max IROP test differs from some practices in that it does not limit ROP based on observed torque or drag. T&D are expected to increase as the equilibrium bed height grows and the "dirtiest hole possible" is achieved. Torque is only a concern if it actually approaches the mechanical limit of the string or system, and not because it is showing an increasing trend. The process also does not utilize predictive software or cuttings transport modeling. The limits are established empirically with the in-situ conditions through the testing described above for monitoring ECD. In soft rock the PDC bit must be designed with the proper depth of cut control to allow adequate WOB to mitigate whirl and its undesirable effects, including hole enlargement, reduced bit and downhole tool life, and poor steering response. Fig. 15 shows an example field trial where the WOB is staged up from about 20k lbs to about 55k lbs in a 78 high angle 12-1/4 hole over about 400 m. The ROP increases from 25 m/hr to about 45 m/hr while MSE becomes more consistent (i.e. less variation in cutting efficiency).

Fig. 15 - WOB ramped up during Max IROP testing in 12-1/4 hole. Fig. 16 and 17 show data from one operation routinely using the above noted practices, including drilling with long gauge (4 plus) bits equipped with depth of cut control. Fig. 16 shows the digital drilling data for the 12-1/4 hole section which was drilled at sail angle of about 87. WOB increased from about 4k lbs to about 8k lbs while IROP increased from about 200 ft/hr up to almost 300 ft/hr with a sustained rate of about 250 ft/hr. MSE stays fairly constant but illustrates the relationship of WOB vs. IROP vs. MSE. At about 8,960 ft, the WOB almost doubled whereas the MSE dropped significantly and the ROP increased significantly indicating improved efficiency and reduced vibrations. Fig. 17 shows the ECD and IROP over the hole section. ECD remained fairly steady as the IROP was increased even up to the 300 ft/hr level indicating hole cleaning was not compromised even at the higher drill rates. Drill teams worldwide continue to relentless re-design their bits, BHAs, connection practices, whirl mitigation practices, etc. to eliminate hole cleaning limiters, improve borehole quality and overall performance. One drill team recently drilled an 80 plus










SPE 134580 13

high angle 17-1/2 hole section with IROPs up to 150 m/hr (500 ft/hr). ROPs are reaching levels that few personnel believed possible in high angle holes.

Fig. 16: Ramp up of WOB and ROP in 12-1/4 hole section.

Fig. 17 - ECD and ROP plot for 12-1/4 hole section.

Vibrationally Induced Borehole Pattern Limiters The manner in which vibrationally induced borehole patterns limit footage per day is typically indirect. In most cases the pattern results in hidden costs and loss of rig time, rather than major NPT. Like stability, the design objective for treatment of borehole patterns has shifted from elimination of NPT and getting casing on bottom to redesigning to reduce their amplitude to the economic limit of redesign. When the total hidden costs are considered, significant changes in operational practices, bits

14 SPE 134580

and BHA designs are justified. Several papers have been written that show that excess side cutting of the bit often leads to borehole patterns. The causes, performance limitations created, and methods to mitigate repeating borehole patterns are discussed in this next section. Fig. 18 is an example 3D image of a caliper log of a 6 inch hole showing a significant one inch amplitude borehole spiral.

Fig. 18 One inch amplitude spiral pattern with a 4 foot period

These repeating borehole patterns, or oscillations, are a result of the feedback between the contact points in the BHA and hole already drilled. (Pastusek, et.al, 2003) When the near bit stabilizer (or other contact point) encounters a small ledge or perturbation in the borehole it is pushed to the side as it drills past this perturbation. This lateral force causes the bit to cut sideways at the same time. When the stabilizer reenters smooth hole and become quiet, the bit also becomes stable and drills gauge hole. The pattern repeats itself. Figure 19 shows the BHA used in the given well superimposed on the pattern. The feedback relationship between the first stabilizer and bit can be seen in the degree to which the period of the pattern matches the distance between the two .

Fig, 19 BHA illustrated on top of pattern All steerable bits are capable of sidecutting, but if the bit design or whirl cause this to be excessive the bit will be pushed even further to the side and the system pivots about the next contact point in the BHA (i.e., second stabilizer). This leads to an exponential growth in the pattern until the body of the BHA makes contact with the borehole and limits further increases. In contrast, if the side cutting action of the bit is reduced, the BHA will flex and the repeating pattern will decay toward zero. Thus for a given formation and BHA, the magnitude and existence of a pattern is determined by the rate of side cutting of the bit . This is dependent on whirl, formation strength, ROP, RPM and WOB and the lateral aggressiveness of the bit. All of these are controllable through changes in design or practices, except the formation strength. The most apparent effect of patterns is tight hole. If the root cause of the tight hole is not understood, the crew may adopt practices to deal with the symptoms rather than redesign to eliminate the vibrationally induced patterns themselves. The increased torque and drag from patterns can lead to back reaming on connections and wiper trips or even underreaming to create additional clearance to run casing. This does not show up as NPT, but adds unproductive time and reduces footage per day. The extra torque created as the stabilizers engage the patterns can also lead to stick-slip, which is a particularly problem in long reach wells where the torque may become the primary performance limiter. Stick-slip can often be reduced by decreasing WOB, but this may increase whirl which in term may cause the amplitude of the pattern to become even greater. Patterns also impact performance by reducing weight transfer. When part of the weight on bit is taken on the stabilizers the DOC and ROP decline. Note that downhole weight on bit measured by MWD tools does not reflect this because the MWD load and torque sensors are typically above the first few stabilizers which are taking the load. The effect of this load can often be seen the in wear created at the end of the stabilizers as they attempt to drill the humps in pattern. If tight hole or loss of weight transfer are seen to reduce footage per day they become the object of redesign the workflow. However, there are numerous causes of tight hole and diagnostics are needed to ensure the redesign is appropriate to solve the root cause. The 3D images shown below provide a clear diagnostic, but these are not available while drilling and other indirect methods are required to anticipate or detect the presence of patterns. Whirl is assumed to increase the side cutting action to a degree that dominates all other corrective actions so the detection of whirl is taken to correlate with the likelihood of patterns. Mechanical specific energy (MSE) surveillance is the primary tool used by the operator to detect bit whirl. While LWD sensors provide valuable data to help protect the BHA from high shock levels, they do not provide an indication of the sidecutting occurring at the bit. The MSE provides a direct measurement of the




SPE 134580 15

rock cutting dysfunction caused by the lateral movement of the bit which creates sidecutting. The presence of sidecutting can also be inferred from the fracture and wear patterns left on the bit. In firmer rocks, ( 15-20ksi) the shoulder cutters and gauge trimmers will often show significant fracture events if whirl is occurring instead of smooth wear. Bit whirl in softer formations may not fracture the cutters, however gauge pad rounding and wear on the blades behind and below the cutter tips can often be seen with close inspection. Forward BHA whirl can be detected by wear on one side of the BHA and/or stabilizers, while reverse BHA whirl will often show up as rounded gauge pads and gouge marks. When a bit whirls, it must drill a slightly oversized hole, which may result in the inability of rotary steerable tools to apply adequate side force to steer. Consequently, the inability to steer may be a primary indicator of whirl and the degree of sidecutting occurring. If the evidence suggests vibrational patterns are the root limiter, there are changes that are considered in both design and practices to ensure the patterns exhibit decay rather than gain. The BHA is redesigned to reduce the amplitude of its vibrational shape at the given operating parameters. BHA whirl creates sidecutting by tilting the bit axis, which increased the lateral load on the trim cutters and their DOC. Proprietary software is used along with an iterative field trial process to develop the quietest BHA The bit gauge design is also modified to constrain sidecutting to that required to achieve the desire build rates (Pastusek, et. al., 2005). The length of the gauge pads and their relief relative to the gauge trimmers are the primary bit design factors influencing the gain that may develop in the pattern. For a fixed relief, increasing the gauge length reduces the side cutting response of the bit. The expectation was established that all bits would be designed with gauge lengths of 4 inches or more (Dupriest et.al,. 2009). Six and eight inch gauges are now being deployed in most directional and straight BHAs where the required dog leg has been 4/100 and less. The key operational parameter to suppress bit whirl while drilling is to maximize the WOB and DOC (ROP) within the other drilling limits. MSE is actively monitored for signs of whirl and used continuously to determine the WOB and RPM parameters that minimize it. It is important to reduce the reliance on LWD vibration data to manage bit whirl. Significant sidecutting may be occurring at the bit, despite very low levels (

16 SPE 134580

BHA3 PDC to 3632

Fig. 20 Effect of WOB on pattern amplitude Fig. 21 Pattern eliminated to section TD Conclusions In 2008 a global performance management workflow built on "limiter redesign" thinking was modified to emphasize the redesign of borehole related performance limiters. Significant additional gains were seen beyond those that were achieved earlier from MSE surveillance and the redesign of bit dysfunction. Near-miss recognition and response practices where developed to increase the focus on borehole limiters, and the expectation for redesign was shifted from NPT reduction to elimination of near misses. The near miss concept was applied broadly and became the primary metric for identifying borehole limiters and the need for redesign or changes in practices. In addition to achieving gains in footage per day, the practices that have eliminated low-level near miss events have intrinsically eliminated the higher level NPT costs and increase the certainty of success, particularly in narrow margin wells.

BHA2 Roller cone to 1414

BHA3 PDC to 3632

3D Caliper Images

SPE 134580 17

The redesign effort has resulted in some practices that are not routine in the industry. These are largely related to one of three areas, which are cuttings transport, management of instability, and mitigation of vibrationally induced borehole patterns. Key related practices for increasing footage per day are: - Creation of the expectation that borehole quality will be redesigned to the economic limit of performance, rather than to a

level that only eliminates NPT and allows casing to be run to bottom; - Use of near miss events surveillance to drive redesign, rather than NPT; - Use of MW which results in ECD close to integrity; - Utilization of engineered particle fluids in conjunction with higher MW to stabilize fractured shales and coals; - Redesign to drill the "dirtiest hole possible"; - Develop and utilize operational practices to mitigate differential pressure sticking potential to allow use of higher MW for

stability; - Use of "Max IROP" test protocol to determine the drill rate corresponding to the dirtiest hole possible; - Discontinued use of torque and drag trends to establish hole cleaning limits; - Develop and utilize Quantitative Risk Analysis (QRA) assessments to balance priorities when determining stability MW; - Utilization of cuttings characterization and robust near miss recognition and response practices to adjust MW in real time; - The primary mitigation for vibrational patterns is to reduce whirl through redesign and the real time use of MSE to adjust

parameters; - The higher the ROP the lower the amplitude of vibrational patterns, and the converse is also true; - Extend gauge lengths to a minimum of 6" to reduce the amplitude of vibrationally induced patterns. Acknowledgements The authors wish to thank the numerous operator, vendor, and contractor personnel who have worked to development new practices to extend borehole-related performance limiters. The advances described have been implemented and refined by 23 drill teams over the last four years and each has contributed in significant ways. We would particularly like to thank the many industry experts who have shared their expertise and enthusiasm for their chosen fields of study. Each change in practices has arisen from a deeper understanding of "how things really work" and we are thankful to our business partners for their willingness to share their unique knowledge. Nomenclature Mud Weight MW Non-Aqueous Fluid NAF Engineered Particle NAF EP NAF Managed Pressure Drilling MPD Fast Drill Process FDP Mechanical Specific Energy MSE Non-Productive Time NPT Bottom Hole Assembly BHA Weight on Bit WOB Rounds Per Minute RPM Rate of Penetrations ROP Instantaneous ROP IROP Quantitative Risk Assessment QRA Logging While Drilling LWD Equivalent Circulating Pressure ECD Maximum Rotation and Maximum Circulation MRMC MRMC without Whirl MRMCwow Torque and Drag T&D Depth of Cut DOC Gallons Per Minute GPM Pounds Per Barrel PPG Feet Per Hour FPH Measurement While Drilling MWD References Aldred W.D., "Improving Drilling Efficiency Through the Real Time Application of PERFORM, Performance by Risk

Management", SPE 57574 presented at Middle East Drilling Technology Conference, Abu Dhabi, UAE, 8-10 November 1999

Bond D.F., Scot P.W., Page P.E., Windham T.M., "Step Change Improvement and High Rate Learning are Delivered by Targeting Technical Limits on Subsea Wells", SPE 35077 presented at IADC/SPE Drilling Conference, New Orleans,

18 SPE 134580

USA, 12-15 March 1996 Brett J.F., Millheim K.K., "The Drilling Performance Curve: A Yardstick for Judging Drilling Performance", SPE 15362

presented at 61st SPE Annual Technical Conference and Exhibition, New Orleans, USA, 5-8 October 1986 Dupriest F.E., "Comprehensive Drill Rate Management Process to Maximize Rate of Penetration", SPE 102210 presented at

SPE Annual Technical Conference and Exhibition, San Antonio, USA, 24-27 September 2006 Dupriest F.E., Witt J.W., Remmert S.M., "Maximizing ROP with Real Time Analysis of Digital Data and MSE", IPTC 10706

presented at International Petroleum Technology Conference, Doha, Qatar, 21-23 November 2005 Dupriest F.E., Koederitz W.L., "Maxmizing Drill Rates with Real-Time Surveillance of Mechanical Specific Energy", SPE

92194 presented at SPE/IADC Drilling Conference, Amsterdam, The Netherlands, 23-25 February 2005 Dupriest, F.E., Sowers, S.F., Maintaining Steerability While Extending Gauge Length to Manage Whirl, paper SPE/IADC

119625 presented at the SPE/IADC Drilling Conference and Exhibition held in Amsterdam, The Netherlands, March 1719 2009.

Dupriest, F.E., Elks Jr., W.C., Ottesen, S., "Design Methodology and Operational Practices Eliminate Differential Sticking", paper IADC/SPE 128129 presented at the IADC/SPE drilling conference, New Orleans, Louisiana, USA, 2-4 February, 2010.

Elks, Jr. W.C., Masonheimer, R.A., Extended-reach Drilling Develops Sacate field, offshore California, Oil & Gas Journal, March 11, 2002, pg 45-55.

Ernst S., Pastusek P.E., Lutes P., Effects of RPM and ROP on PDC Bit Steerability, paper SPE/IADC 105594, presented at the 2007 SPE/IADC Drilling Conference in Amsterdam, The Netherlands, 2022 February 2007.

Kadaster A.G., Townsend C.W., Albaugh E.K., "A Total Quality Management Tool For Drilling in the 1990s", SPE 24559 presented at 67th SPE Annual Technical Conference and Exhibition,Washington D.C., USA, 4-7 October 1992

Kilroy, D.D., Dupriest, F.E.: Practices implemented to achieve record performance in narrow margin drilling in the Bass Strait extended reach, paper SPE 125246 presented at the SPE ATCE, New Orleans, Louisiana,USA, 4-7 October, 2010.

Kline, W. E., Chandler, T. K., Keller, S. R., Ottesen, S., Gupta, V., Tenny, M., "Physics-Based Well Design - Beyond the Learning Curve", presented at the IPTC conference, Doha, Qatar, 21-23 November, 2005.

Lummus J.L., "Drilling Optimization", published in the Journal of Petroleum Technology, November 1970, pg 1379-1388 Pastusek P.E., Brackin V., A Model for Borehole Oscillations SPE 84448, presented at the SPE Annual Technical

Conference and Exhibition held in Denver, Colorado, U.S.A., 5 8 October 2003 Ottesen, S., Zheng, R.H., McCann, R.C., "Borehole Stability Assessment Using Quantitative Risk Analysis", SPE/IADC

52864 presented at SPE/IADC Drilling Conference, Amsterdam, The Netherlands, 9-11 March, 1999. Ottesen, S., "Wellbore Stability in Fractured Rock", IADC/SPE 128728 presented at SPE/IADC Drilling Conference, New

Orleans, Louisiana, USA, 2-4 February 2010. Pastusek, P.E., Brackin V., Lutes P., A Fundamental Model for Prediction of Hole Curvature and Build Rates with Steerable

Bottomhole Assemblies, SPE 95546, presented at the 2005 SPE Annual Technical Conference and Exhibition held in Dallas, Texas, U.S.A., 9 12 October 2005.

Pastusek, P.E., Cooley, C.H., Sinor, L.A., Anderson, M., Directional and Stability Characteristics of Anti-Whirl Bits With Non-Axisymmetric Loading, paper SPE 24614, presented at the SPE ATCE, Washington DC, October 4-7, 1992.Zeilinger, S., Dupriest, F.E., Turton, R.: Utilizing engineered particle drilling fluid to overcome coal drilling challenges, paper IADC/SPE 128712 presented at the IADC/SPE drilling conference, New Orleans, Louisiana, USA, 2-4 February, 2010.

Rasi, M., "Hole Cleaning in Large, High Angle Wellbores", SPE 27464 presented at IADC/SPE Drilling Conference, Dallas, USA, 15-18 February 1994

Remmert S.M., Witt J.W., Dupriest F.E., "Implementation of ROP Management Process in Qatar North Field", SPE 105521 presented at SPE/IADC Drilling Conference, Amsterdam, The Netherlands, 20-22 February, 2007

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Fast Drill Borehole Management