IADC/SPE 122276

MPD: Beyond Narrow Pressure Windows Sagar Nauduri, SPE, TAMU, George H. Medley, SPE, Signa Engineering Corp., Jerome J. Schubert, SPE, TAMU

Copyright 2009, IADC/SPE Managed Pressure Drilling and Underbalanced Operations Conference and Exhibition This paper was prepared for presentation at the IADC/SPE Managed Pressure Drilling and Underbalanced Operations Conference and Exhibition held in San Antonio, Texas, 12–13 February 2009. This paper was selected for presentation by an IADC/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 International Association of Drilling Contractors or the Society of Petroleum Engineers and are subject to correction by the author(s). The material does not necessarily reflect any position of the International Association of Drilling Contractors, or 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 International Association of Drilling Contractors or 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 IADC/SPE copyright.

Abstract Managed Pressure Drilling (MPD) is an enabling technology that is highly sought after in the drilling industry in recent years. MPD is generally associated with narrow pressure windows, i.e., staying between either pore pressure (Pp) or formation stability and fracture pressure (Fp). However, operators are starting to realize MPD has a broader range of application, beyond navigating narrow pressure windows and other traditional applications such as Dual Gradient Drilling and Pressurized MudCap Drilling. Over the past couple of years, MPD’s range of application has increased, as can be seen from the examples presented in this paper. New offshore and onshore MPD projects in the Gulf of Mexico (GoM), North Sea, North America, South America, Africa and Europe are being designed for objectives like improving Rate of Penetration (ROP), reducing formation invasion, validation of pore pressure data, reducing incidents of stuck pipe and mitigating the effects of differential sticking, reducing ballooning or hole breathing and minimizing Non-Productive Time (NPT). MPD is not a universal solution or panacea and improper application of the technique may ultimately cost more than conventional drilling methods. However, with comprehensive planning and execution by experienced engineers, MPD can help actualize project objectives that otherwise might be impossible with conventional techniques. Introduction MPD is a name for a collection of old, modified, and new technologies, referred to as VARIATIONS or METHODS of MPD. MPD can help in achieving specific purposes (such as eliminating a casing string or avoiding formation damage in a particular section of hole), or in solving particular drilling problems (such as drilling through narrow pressure windows or lost circulation zones), or in meeting definite project constraints, such as quality, time or safety (Nauduri, Medley, 2008). Although the MPD acronym was coined recently (Hannegan, Wanzer, 2003; Hannegan, 2005) in 2003, and the IADC UBD committee gave an official definition in 2004, many techniques associated with MPD have been in existence well before the turn of the Twentieth Century. There are accounts of such techniques being used even in the late 18th century and early 19th century. MPD in part evolved from its precursor, Underbalanced Drilling (UBD), in that both drilling methods typically incorporate much of the same drilling equipment and pressure management equipment. Similar equipment is used to drill both UBD and MPD wells and the benefits of the techniques are, in many cases, identical. However, since there is no continuous flow of fluids to the surface, MPD wells are often perceived to be safer than UBD. Until recently based on the applications, four major classifications or “Variations” of MPD have been recognized – Constant Bottomhole Pressure (CBHP), Dual Gradient Drilling (DGD), Pressurized Mudcap Drilling (PMCD) and Zero Discharge or Health Safety and Environmental (HSE). Some of these variations are achieved by using more than one different technique or method of MPD together (Medley, Reynolds, 2006). There are other applications of MPD, however, that do not fit into these broad categories of MPD.

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Typical MPD applications – Traditional and Advanced MPD is almost invariably associated with drilling wells with narrow windows between pore-pressure or formation stability and fracture pressure gradient. Classically, a CBHP variation with surface backpressure (BP) helps to drill through these zones. Drilling infill wells in normally or severely depleted reservoirs typifies the CBHP application.

DGD typically incorporates a subsea mud lift pump to facilitate MPD in deepwater (>1,000 ft) and ultra deepwater (>6,000 ft) drilling in the GoM. DGD can help reduce the number of casing points and assist in reaching target depth with a reasonable pipe diameter. This is an example of narrow pressure window, or rather a flattened pressure window, and in many such cases the only possible way to drill a well is to use DGD.

Another traditional application of MPD is PMCD, which has been used to drill highly fractured or cavernous formations where total or near-total mud losses have been experienced. PMCD is used to drill wells in countries like Kazakhstan, Malaysia, Indonesia, Angola and the United States, where no other drilling method could be used to safely reach the target (Colbert, Medley, 2002). PMCD has also been applied in offshore GoM and Bay of Bengal (India). Point of Constant Pressure (PoCP) is a variant/modification of CBHP where the choke point of the narrow window is located in a spot in the wellbore other than the bottom of the hole. Other CBHP applications are using CBHP to drill through multiple zones of constriction or using CBHP to drill through a severely depleted formation.

The emphasis for most of the typical applications discussed above is still on one or two tiny constriction(s) that make the hole un-drillable without MPD. The current applications and objectives of MPD have gone beyond these narrow pressure window constraints. The key reason for this wider gamut of applications is the typical byproduct of using MPD on a well –lowering the overbalance pressure. Perhaps the greatest advantage of MPD (when compared to conventional drilling methods) is the ability to lower the dynamic overbalance pressure. This effect is usually observed through most of the open hole section, with the CBHP and DGD MPD variations, while drilling and circulating mud. BHP in conventional drilling In conventional drilling, Bottomhole Pressure (BHP) is the sum of the mud’s hydrostatic pressure, the pressure required to overcome the annular friction, and the weight of the cuttings. This is represented as:

CONVCONVCONVCONV cuttingsFrictionMWBHP ++= Under static conditions, the FrictionCONV and cuttingsCONV components are both typically zero. Therefore, the above equation becomes

CONVCONV MWBHP = in the static condition, while the above equation is unchanged for the dynamic condition. Hence, an additional “”FrictionCONV + cuttingsCONV component exerts additional pressure in the conventional dynamic condition, resulting in additional overbalance. Note that the MWCONV component remains unchanged in both static and dynamic conditions. BHP while using CBHP variation In the CBHP variation, BHP is lowered or controlled by carefully managing the mud rheology, mud weight, solids content of the mud, applied BP and cuttings concentration in the annulus. The BHP while using CBHP MPD variation can be represented with the following equation:

CBHPCBHPCBHPCBHPCBHP cuttingsFrictionBPMWBHP +++= As discussed earlier, the “FrictionCBHP + cuttingsCBHP “ component is zero under static conditions, lowering the BHP by that value compared to dynamic BHP. In order to control the static BHP or bring it very close to the dynamic BHP, backpressure (BPCBHP) is applied in the static condition such that:

CBHPCBHPCBHP cuttingsFrictionBP +≈ Typically, the “BPCBHP” component is zero or a very small number under dynamic conditions. Thus

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DYNAMICCBHPDYNAMICCBHPDYNAMICCBHPCBHP

STATICCBHPCBHPSTATICCBHP

BHPcuttingsFrictionMWBPMWBHP

−−−

−−

=++≈+=

Maintaining the static and dynamic BHPs very close to each other enables the driller to either move the BHP close to the Pp or away from the Pp to suit the requirement. Typically, staying close to Pp provides greater benefits rather than staying away from it, unless the wellbore collapse limit is higher that the Pp. In such a case, the BHP is maintained slightly above the wellbore collapse gradient. To maintain BHP closer to Pp and reduce the overbalance, the “MWCBHP” is reduced to a value lower that the “MWCONV”. The mud circulation rate, rheology and other parameters are designed in such a way that the dynamic BHP is slightly higher than the Pp. The required BP is calculated and is applied when the pumps are switched off, in order to avoid any influx. BHP in DGD variation Considering DGD MPD variation using a subsea mudlift pump, where the subsea pump adds energy at a point close to the sea floor, thus providing two mud gradients in the wellbore pressure profile. The BHP in this case is reduced numerically by the amount of pressure the sub sea pump is pumping mud into the return lines. When the wellbore attains static condition, BP equivalent to the dynamic frictional pressure drop is applied. Thus, the overburden is considerably reduced using DGD. Expanded MPD applications Lowering the dynamic overbalance can offer positive effects such as increasing RoP, decreasing surge and swab effects, reducing formation invasion, and enhanced control of kicks and lost circulation zones. These benefits of MPD have now become so critical in many new projects (onshore and offshore) that operators often view them as required goals rather than desirable side effects, even when the typical narrow pressure window is absent. ROP improvement Reduction in the dynamic overbalance reduces the differential pressure at the rock bit interface. This reduces the force with which a broken chip or piece of rock is held in its place. Hence, lesser force and time are required to displace the broken chip from its former position. Therefore, the rate at which the cuttings are removed from the rock or hole increases, which in turn increases the ROP of the drill bit or the rate at which new hole is created. Improved ROP is a direct benefit of reduction in the overbalance pressure. In one North Sea project, MPD was used to obtain better penetration rate and to stay close to the formation pressure. Many UBD projects are designed wholly to obtain better ROP. However, achievement of this benefit with MPD is preferred since it is accompanied by fewer issues or concerns with safety compared to UBD.

Validation of pressure Validation of pressure is a classic application of “Walk the Line” MPD. Reducing the Equivalent Circulation Density (ECD) and lowering the dynamic BHP as close as possible to the Pp has evolved into an accepted technique for validating or determining the pressure regime. At least one major operator has utilized MPD to “find” the pore pressure in an exploration well where pressure profile was not well defined. The various potential pressure profiles developed from offset wells and other available geological data were inconsistent. The operator decided to use CBHP MPD variation to stay close to an agreeable pressure profile using surface-backpressure and was able to successfully validate or establish a definite Pp regime for the field. This technique is closely related to the enhanced kick and loss detection category of MPD, discussed below. Formation invasion mitigation Mitigating formation invasion is another advantage of lower overbalance and has been another important objective for UBD projects. Higher overbalance increases the pressure differential across the openhole between the formation fluids and the wellbore fluids, forcing drilling mud or filtrate into the formation. Since MPD can help maintain a lower overall BHP and reduce the quantity of fluid invading into the formation, a reduction in the formation invasion is typically witnessed in CBHP and DGD projects.

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Enhanced kick and loss detection Enhanced kick and loss detection is a useful byproduct of using MPD. MPD requires additional equipment to obtain better control of the wellbore pressure profile by monitoring and detecting variations in the fluid flow and volume. This also enables early detection of an influx from the formation or loss of fluids into the formation. Early detection of kicks and losses can reduce NPT and prevent undetected kicks and blowouts. With the increasing depth and complexity of offshore and onshore wells, kick-loss cycles have become a very difficult drilling menace. MPD has proven invaluable in such critical wells, where equipment like Coriolis Flow meters and Micro-Flux control meters are utilized to obtain enhanced kick-loss detection. Additional MPD Benefits

MPD can offer additional advantages as a result from reducing the gross overbalance in the wellbore by using MPD.

Minimizing the effects of wellbore ballooning (or “breathing”) is a positive effect of reducing the overbalance in the wellbore. Reducing ballooning is similar to kick-loss detection, albeit on a smaller scale. MPD can reduce the pressure exerted on the formation during drilling, which can lead to wellbore ballooning. This reduces the volume of fluid the wellbore releases when it is breathing out, thus also helping in better kick detection. Wellbore “fingerprinting” can be accomplished by using advanced hydraulic software to distinguish between wellbore ballooning and kick-loss conditions. Another advantage of reducing overbalance in the wellbore is reducing occurrences of stuck pipe due to differential sticking. Using MPD means having better control of the fluid properties like rheology, solids content and mud weight. This actually gives better control of wellbore pressure profile. Now the drilling fluid design properties can be modified in such a way that properties including mud viscosity, mud density, and annular friction can be controlled within much more flexible limits because the MPD system allows for multiple methods of changing the BHP quickly. Incorrect Application The expansion of MPD technology has been accompanied by an increase in improper application of the technology. Incorrect application can include improper candidates or applying MPD in formations where conventional drilling techniques might make a better fit. Though probably trite, the old saying “When the only tool you have available is a hammer, every problem looks like a nail” is an apt description of this “one-size-fits-all” approach to MPD. Proper Candidate Selection is discussed adequately elsewhere in industry literature, and consequently will not be discussed in detail here. However, problems related to candidate selection continue to appear, usually related to poor understanding of the goals and objectives of MPD, but also related to a poor understanding of the basic concept of MPD.

Understanding Constant Bottomhole Pressure MPD An incorrect application may result from an incorrect understanding or assumption about what MPD can accomplish, for example, a misunderstanding of how CBHP actually affects the entire annulus. The graphs below (Fig 1) show how BHP at a casing shoe at 15,000 ft is affected when an attempt is made to maintain BHP constant at the bit as the well is drilled deeper and deeper, for cases where true vertical depth increases (i.e., a vertical well) and then with vertical depth remaining the same but measured depth increasing (i.e., a horizontal well). At any given measured depth, merely holding the pressure constant for both drilling and making connections results in significant changes in the pressure applied to the shoe. The graph shows that the equivalent mud weight at the shoe can increase more than 0.3 ppg in a vertical well and nearly 0.5 ppg in a horizontal wellbore while holding pressure at the bit constant (connection pressure at bit depth held the same as circulating pressure at bit depth).

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Fig 1 - BHP Held Constant at the Bit

Not only does the overall pressure increase, but the difference in pressure at the shoe between circulation and static at any given depth can also change by as much as 0.3 ppg in the cases shown. As pressure at the shoe changes the formation is cyclically loaded, potentially weakening the rock. If wellbore stability is an issue, weakening the formation can in turn lead to unexpected premature collapse of the wellbore.

The graphs below (Fig 2) show the effect on BHP (i.e., pressure at the bit) of maintaining a Point of Constant Pressure (PoCP) at the intermediate casing shoe (15,000 ft).

Fig 2 - BHP Held Constant at the Casing Shoe

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When bottom hole pressure is held constant at the casing shoe rather than at the bit, the overall effect is less cyclic load on the exposed formations. In all cases shown above, holding the pressure at the shoe constant results in a more constant pressure applied at the bit during connections, regardless of whether or not the wellbore is vertical or horizontal. However, the difference between circulating and static BHP at the bit can still vary by as much as 0.25 ppg, an effect not shown in the figure. The term “Constant Bottom Hole Pressure” can obviously be somewhat misleading, and a full evaluation of exactly what changes are taking place downhole during the MPD process should be carried out prior to attempting to conduct “CBHP MPD” operations. The Importance of Hole Cleaning Another example of poor understanding of the application of MPD is a failure to recognize and account for the significance of cuttings in the wellbore. Poor hole cleaning has long been acknowledged as one the largest obstacles to success in underbalanced drilling (Tian, Medley, 2000). With MPD, hole cleaning is perhaps even more significant (Tian et al, 2007). The upper limit is typically a frac-gradient or other leak-off limit. The general tendency is to decrease circulation rate in order to minimize friction and reduce the potential to exceed the upper limit. However, reduced circulation normally results in reduced ability to clean the hole. Since the mud-weight may be closer to Pp while utilizing MPD, the rate of penetration may actually be higher than in conventional drilling. The combination of increased cuttings loading and decreased circulation rate has a multiplying effect, increasing the chance of pack-off in the annulus, high torque and drag, and eventually worse problems such as stuck pipe, twist off, etc. The early indicators of a hole-cleaning problem may be misdiagnosed as a wellbore stability problem, especially when the lower limit of the pressure window is the wellbore stability or collapse gradient, as is common in MPD applications. Importance of Appropriate Equipment Arrangement Use of an inappropriate equipment arrangement can also lead to problems. Other papers have discussed the use of too much equipment and the use of inappropriate equipment with regard to the more traditional application of MPD (Goodwin et al, 2008; Medley, et al, 2008). The expanded application of MPD to other drilling problems can be, again, accompanied by this same problem. However, the only real danger in using too much equipment or too high a level of complexity for the chosen application may be primarily economic in nature. The project may be a rousing technical success but fail in an economic sense. On the other hand, the use of too little equipment or too much simplification in the equipment arrangement runs a very real risk of causing a technical failure, which almost invariably will result in an economic failure as well. For example, if the expanded application of MPD in question is enhanced kick and loss detection or validation of pressure regime, leaving out a precise means of accurately measuring flow from the annulus makes no technical sense at all. Likewise, if the MPD application will be undertaken on a location with limited availability of personnel and/or inexperienced personnel, failure to include automation of BP imposition may virtually guarantee a technical failure of the process. MPD systems can be simplified too much, to the point of near uselessness.

Importance of Adequate Preparation Every good project manager knows that inadequate planning is a top reason for project failure. In spite of this, the industry continues to undertake projects seemingly at the last minute, and even when minimal planning reveals that success is unlikely, the project continues on once started. This is never good, and in the case of MPD, it can be catastrophic. Preparation though includes more than just good planning. It must include the resources required to carry out the plan. Appropriate training, delivered at the correct time, is an integral part of any successful MPD project. Good training delivered to the wrong personnel at the wrong time is a wasted effort. This is most obvious in the case where training is delivered too late to be of any benefit. Often, though, the problem is training delivered too early. Case histories abound where training was delivered far in advance of MPD application. Once the application presented itself on site, almost none of the trained personnel were available, and those who were available had forgotten the differences in MPD and conventional drilling because they had most recently been practicing conventional techniques. The best training for MPD, whether traditional or expanded application MPD, is conducted just days prior to the implementation of the operation. This is especially true of the hands-on type rig crew training. Appropriate procedures, prepared with appropriate expertise, are a necessity. The basic procedures of MPD application are similar, no matter the application. However, the details related to each specific application, though they may be small, are critical. This is especially true with the newer, expanded applications of MPD. Even procedures for “twin” wells in the same reservoir typically will require some modification between wells. The greatest misunderstanding with regard to procedures is the assumption that proper execution of MPD is related only to those components of MPD that have been added to the drilling system. Nothing could be further from the truth.

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Any MPD application must be approached from a holistic viewpoint and must address the entire drilling system, from the nature of the bit to the crown of the rig. An experienced drilling engineer should prepare all MPD procedures. Likewise, an experienced drilling supervisor familiar with all aspects of the drilling process and capable of handling any drilling situation should be in responsible charge of execution of the procedures. Conclusions

1. MPD’s niche has grown out of its initial tight margins application to a wider range of applications, including validating pressure profiles, mitigating formation invasion, advanced kick and loss detection.

2. Good understanding of the MPD techniques and the prospect, careful planning of the MPD procedures and proper sequence of execution of these procedures is very important for successfully completing of an MPD project.

3. Need for training all the crew for potential hazards, slip-ups and methods of execution of the MPD procedures cannot be overemphasized and continues to increase in importance with the introduction of expanded applications for MPD.

4. MPD can offer a viable technique to drill wells that cannot be otherwise drilled using the conventional drilling techniques.

References

1. Colbert, John W. and Medley, George, 2002: “Light Annular MudCap Drilling – A Well Control Technique for Naturally Fractured Formations,” SPE 77352, presented at the 2002 SPE Annual Technical Conference and Exhibition in San Antonio, Texas, 29 September – 2 October.

2. Goodwin, Robert, Medley, George H., and Reynolds, Patrick B.B., 2008: “Understanding MPD Complexity Levels,” Hart’s E&P, October, pp.37-39.

3. Hannegan, Don M.: "Managed Pressure Drilling in Marine Environments – Case Studies.” SPE 92600, Presented at the 2005 SPE/IADC Drilling Conference, Amsterdam, February 23-25, 2005.

4. Hannegan, Don M. and Wanzer, Glen; “Well Control Considerations—Offshore Applications of Underbalanced Drilling Technology,” SPE/IADC 79854, pp. 1-14, (Presented at the 2003 SPE/IADC Drilling Conference).

5. Medley, George H., Moore, Dennis, and Nauduri, Sagar, 2008: “Simplifying MPD – Lessons Learned,” SPE/IADC 113689, presented at the 2008 IADC/SPE Managed Pressure Drilling and Underbalanced Operations Conference and Exhibition, Abu Dhabi, UAE, January 28-29.

6. Medley, George H. and Reynolds, Patrick B.B., 2006: “Distinct Variations of Managed Pressure Drilling Exhibit Application Potential,” World Oil, Vol. 227, No. 3, March, pp.41-45.

7. Nauduri, Sagar, and Medley, George, 2008: “Managed Pressure Drilling; Chapter Ten – MPD Candidate Selection,” published by Gulf Publishing Company, pp. 261-284.

8. Tian, Shifeng, Medley, George H., and Stone, Charles R., 2007: “Parametric Analysis of MPD Hydraulics,” IADC/SPE 108354, presented at the 2007 IADC/SPE Managed Pressure Drilling and Underbalanced Operations Conference and Exhibition, Galveston, Texas, March 28-29.

9. Tian, Shifeng and Medley, George, 2000: “Re-evaluating Hole Cleaning in Underbalanced Drilling Applications,” paper presented at the International Association of Drilling Contractors (IADC) Underbalanced Drilling Conference and Exhibition, held in Houston, Texas, August 28-29.