Copyright 2000, IADC/SPE Drilling Conference

This paper was prepared for presentation at the 2000 IADC/SPE Drilling Conference held inNew Orleans, Louisiana, 23–25 February 2000.

This paper was selected for presentation by an IADC/SPE Program Committee followingreview of information contained in an abstract submitted by the author(s). Contents of thepaper, as presented, have not been reviewed by the International Association of DrillingContractors or the Society of Petroleum Engineers and are subject to correction by theauthor(s). The material, as presented, does not necessarily reflect any position of the IADC orSPE, their officers, or members. Papers presented at the IADC/SPE meetings are subject topublication review by Editorial Committees of the IADC and SPE. Electronic reproduction,distribution, or storage of any part of this paper for commercial purposes without the writtenconsent of the Society of Petroleum Engineers is prohibited. Permission to reproduce in print isrestricted to an abstract of not more than 300 words; illustrations may not be copied. Theabstract must contain conspicuous acknowledgment of where and by whom the paper waspresented. Write Librarian, SPE, P.O. Box 833836, Richardson, TX 75083-3836, U.S.A., fax01-972-952-9435.

AbstractA new concept and process for real-time monitoring andcontrol of wellbore stability establishes the drilling parametersrequired to optimize the drilling process and thereby reducethe potential for wellbore instability and subsequentunscheduled events or lost rig time. Surface and downholemeasurements, recorded while drilling, are used to makeregular updates to a model of the wellbore and to revise thedrilling plan accordingly.

The first step in the process is the generation of amechanical earth model (MEM) using information obtained inoffset wells and field and regional data. The proposed welltrajectory for a new well is projected into the MEM and a setof stability parameters is generated for a given initial drillingplan. The product identifies potential danger zones within awell plan.

During drilling, real-time data, including logging-while-drilling (LWD), measurements-while-drilling (MWD), surfacemechanical measurements, and fluids and solids monitoringinformation, are used to diagnose the state of the wellbore.Any significant hole instability is detected and a warning isgiven to the driller. The state of the wellbore is compared tothe model, and any revision required to align the predictedwith the actual state is made. This real-time update of themechanical model is then used to predict the future state of thewellbore, in front of and behind the bit, for the given drillingplan. If the drilling plan can be improved, a revision will berecommended; for instance, reduction in the rate ofpenetration, increase in mud weight and circulation, and

change in hole direction. The drillers can independentlyevaluate their own recommendations for changes to thedrilling plan and then decide on the best course of action. Theprocess also provides a record of wellbore stabilityinformation that can be input to the field description for use infuture wells and continuous improvement of the drillingprocess.

Use of this concept was validated on the Valhall field inthe Norwegian sector of the North Sea. Extended-reachdrilling (ERD) to downflank targets has been problematic inrecent years; there is a high risk that wells will be suspendedor abandoned because of problems associated with wellboreinstability in this very weak overburden.

The Real-Time Wellbore Stability Control (RTWBSC)project team produced an MEM for the Valhall field, workingclosely with the drilling engineers to develop a well plan for aproposed ERD well. Implementation involved providingwellsite support to coordinate monitoring and detection ofwellbore instability from real-time data, and on-line support inthe drilling office to interpret data, update the MEM and revisethe well plan. Through this process the team proposed andimplemented a strategy of drilling the well in controlled statesof failure—not a conventional drilling approach. The wellsuccessfully reached its target ahead of schedule and a plannedstring of intermediate casing was not required, mud losses (aprevious problem contributing to instability and cost) wereminimal and the well was cased to below the unstableoverburden intervals.

IntroductionWellbore instability is a major problem during the drilling ofmany oil and gas wells. Often quoted as costing the industrybetween 0.6 and 1 billion dollars per year,1 it currently leads tomajor difficulties in such diverse areas as the North Sea,Argentina, Nigeria and the Tarim basin.2,3 A recent, well-documented spectacular example of the cost savings availablefrom improved handling of wellbore instability is available forthe Cusiana field operated by BP Amoco and partners inColombia. Wellbore instability was very severe there, leadingto costs per well of tens of millions of dollars. An integratedapproach to the problem led to large reductions in these

IADC/SPE 59121

When Rock Mechanics Met Drilling: Effective Implementation of Real-Time WellboreStability ControlI.D.R. Bradford, SPE, Schlumberger Cambridge Research, W.A. Aldred, Schlumberger, J.M. Cook, SPE, SchlumbergerCambridge Research, E.F.M. Elewaut, Netherlands Institute of Applied Geoscience TNO, J.A. Fuller, SchlumbergerHolditch Reservoir Technologies, T.G. Kristiansen, SPE, BP Amoco Norge and T.R. Walsgrove, Consultant

2 BRADFORD, ALDRED, COOK, ELEWAUT, FULLER, KRISTIANSEN, WALSGROVE IADC/SPE 59121

costs.4,5 A fundamental aspect of this approach was to acceptthat wellbore instability was inevitable and to manage it ratherthan to eliminate it.

The drilling industry historically addresses wellboreinstability issues in two ways. The first approach treats theproblem on an ad hoc basis; for specific problem formations,data and cores are collected and the drilling history isanalyzed, allowing the formulation of a set of empirical rules.In the Valhall field of the North Sea, for example, wells drilledthrough the Middle Eocene formation at inclinationsexceeding 65º are at high risk. These rules do reducenonproductive time. However, they do not identify theunderlying instability mechanism and do not appropriatelyrelate it to drilling operations so that the full benefit of thisknowledge is realized. Furthermore, many of these empiricalrules apply to a well, and all need to be taken into account todetermine the drilling parameters (e.g., mud flow rate, rate ofpenetration, pump pressure, trajectory). Techniques exist tosolve this type of problem, but these are often not applied andcan lead to an inadequate set of drilling parameters that cantrigger wellbore instability. The second approach is based onlog interpretation methods that estimate the safe mud weightwindow using rock strength and in-situ stress state predictionsbased primarily on sonic logging. The calculations are made,however, within the framework of classical rock mechanicswhere it is assumed that the maximum and minimum mudweights are governed by the onset of breakouts and fractures,respectively. Several common modes of wellbore instability(e.g., fractured shales, fault reactivation) are not amenable tothis classical approach. The description of wellbore stability is,therefore, generally incomplete.

Both approaches can be applied before or after, but notduring, drilling. Any lessons learned from data or experiencegathered on a well can therefore only be applied on subsequentwells in the same field. As a result, several wells can be drilledbefore the minimum cost construction technique is found. Thissignificantly increases both the capital required for fielddevelopment and the cycle time. Managing boreholeinstability in real time would potentially allow learning to beimplemented on the current well so that the optimalconstruction technique is achieved over the minimum numberof wells. Such an approach has not, however, been possibleuntil recently because of technical constraints. The followingdevelopments now make it feasible:1. There is increasing availability of MWD data.6

2. Wellbore deformation and failure mechanisms, and theirrelation to stress state, are better understood.7

3. There is improved understanding of how drilling practices(e.g., frequency of wiper trips, swab and surge pressures)influence instability and of how, in turn, instabilities ofdifferent kinds influence drilling.

The RTWBSC concept uses real-time measurements andinterpretation to manage wellbore instability (real-time heremeans essentially during drilling of the well; some real-timedata arrive immediately as a formation is being drilled, butother data can be delayed by up to a few hours). Although

wellbore instability can be classified as either mechanical(e.g., failure of the rock around the hole because of highstresses, low rock strength, or inappropriate drilling practice)or chemical (damaging interactions between the rock,generally shale, and the drilling fluid), the integration ofunderstanding of chemical and mechanical damage remainsproblematic despite intensive efforts throughout the oilindustry. Accordingly, the RTWBSC process (a) determineswhether a particular drilling problem is mechanical orchemical in origin, (b) deals with the mechanical aspects andmakes recommendations, based on known rules of thumb, ifthe problem is chemical in origin.

The four main components of this process are described inthe next section. The first component is a wellbore modelconsisting of the trajectory, in-situ stress state, rockconstitutive parameters and all types of instabilitymechanisms, together with a description of the drillingpractices. It is constructed through the two approaches bywhich wellbore instability is currently addressed, and it usesoffset well data, drilling experience and in some cases aseismic survey to define the geological structures. Theaccuracy of the model depends on the information available,but it always provides a framework against which real-timeobservations and interpretations are judged. The secondcomponent is the data acquisition program, which defines thetypes of data and sampling rate necessary to provide a reliablediagnosis of the instability mechanisms, their severity and theconditions under which they occur. The third component is asoftware tool that accepts data from a wide range of sourcesand manages the data flow, diagnoses the instabilitymechanisms, and quantifies both their severity and correctivedrilling practices. A key part of this third component is therefinement of the subsurface model. The fourth component is acommunication tool, such as an intranet Web site, that acts asa data repository and enables rapid dissemination ofinformation and recommendations.

The RTWBSC process was validated on an ERD well inthe Valhall field of the North Sea (see “Valhall field test”section). This field, operated by BP Amoco Norge, is locatedin offshore blocks 2/8 and 2/11 in the Central Graben area ofthe southern part of the Norwegian North Sea. It wasdiscovered in 1975, when the exploration well 2/8-6encountered over 100 m of hydrocarbon-bearing section inLate Cretaceous chalk formations. Production began in 1982from the highly porous Tor and Hod chalk formations.8

Valhall was originally developed to recover reserves of250 million barrels. There are ongoing projects to increaserecoverable reserves to 1000 million barrels.9 One projectinvolves accessing downflank reserves in the far northern andsouthern parts of the field through ERD wells. Although this iseconomically attractive, because of the potential for significantgains in recoverable reserves over a relatively small cycletime, wellbore instability is a major problem: There is a highrisk that wells will be abandoned or suspended before reachingtheir target. This factor, together with the availability of acomprehensive data set, meant that Valhall was suited to

IADC/SPE 59121WHEN ROCK MECHANICS MET DRILLING: EFFECTIVE IMPLEMENTATION OF REAL-TIME WELLBORE STABILITY CONTROL 3

demonstrate the value and viability of real-time detection andcontrol of wellbore instability.

Real-time wellbore stability control processThe process uses four main components: the MEM, a dataacquisition program, data management software and acommunication system. The implementation of the processand its components, for drilling optimization, is shownschematically in Fig. 1 and has three phases:1. In the design phase, relevant data are gathered and the

MEM is constructed. The wellbore stability and drillingplans are then formulated: these are taken into accountduring the design of the data acquisition program.

2. In the execution phase, the drilling process is monitoredand data are aquired to detect instability.

3. In the evaluation phase, which also occurs during drilling,real-time data are interpreted, the MEM is updated asnecessary and recommendations relating to drillingpractices are made to the rig crew. Interpretation of real-time data should be made within the context provided bythe MEM and the wellbore stability predictions:assessments of the validities of the interpretation and/orthe MEM will be more reliable.

The four components are discussed further in the followingparagraphs. The implementation of the design-execute-evaluate cycle is discussed in the “Valhall field test” sectionand is illustrated using events that occurred during the drillingof the well.

Planning. Before drilling, the optimal, or least damaging, wellconstruction techniques are identified through prognoses of thegeology and instability mechanisms likely to be encounteredand estimates of the conditions, including the stress state, that“trigger” the mechanisms.

In areas where drilling has occurred, the geology can becharacterized using offset well data such as logs andgeological reports, perhaps combined with a seismic survey. Inareas where no exploration has occurred (the case in Cusiana),it is necessary to rely on a geological prognosis only, albeitone now aided by geological modeling software tools.10,11

The process of analyzing the likely instability mechanismsand estimating their trigger conditions is described in thefollowing paragraphs.

Review of offset well construction. This review shouldinclude the drilling phase, with trips and casing runs. Attentionis typically focused on (a) mud losses, cavings rates andmorphology, geological reports and any (partial or full) stuckpipe incidents and (b) relating instability issues to theoperation (tripping, backreaming) and comparing the muddensity and/or equivalent circulating density (ECD) to thepredicted stable mud weight window.

The product of this review includes the instabilitymechanisms and their severity, indexed to true vertical depth(TVD) or, more generally, incorporated within an earth model.Any key factors influencing the instability, such as well orbedding inclination, should also be noted.12 The instability

mechanism at a given depth is categorized as either breakouts,sloughing, natural fractures, weak planes, drilling-inducedfractures, faulting, undergauge hole, interbedded sequence,overpressured formation, unconsolidated formation, mobileformation, permeable formation or chemical activity. This listis not exhaustive; further categories can be envisaged. Theseverity of the instability is categorized as low, medium orhigh.1. A low severity problem is one for which symptoms exist,

but no remedial action is required.2. An instability of medium severity has noticeable

symptoms; minor action is required either to inhibit theproblem or to deal with its consequences. An example isminor breakouts manifested by an increased cavings rate,or perhaps even a partially stuck pipe. The hole cleaningcould be emphasized (to deal with breakout debris withoutstopping breakouts) or the mud weight could be increasedby a small amount, thus inhibiting the problem.

3. A problem of high severity is a potential well-stopper.Without major remedial action (running casing), a totalloss of borehole integrity is highly likely and will result ina sidetrack or abandonment.

Density, sonic and gamma ray logs. Data can beconstructed using logs from several offset wells. The sonic logshould ideally consist of compressional and shear slownesses.In many cases, however, only compressional slowness isavailable: an empirical correlation is then needed to derive theshear wave speed. These data form the primary input for theMEM, which consists of the in-situ stress state, the formationconstitutive parameters and the failure mechanisms. Theaccuracy of the MEM can be enhanced by correlating (a) thelog-derived results to point data, such as information fromcores or leakoff tests, and (b) quantities such as sonicvelocities to constitutive parameters such as formationstrength.13 The MEM and proposed well trajectory may thenbe used to predict the safe mud weight window.14

The instability evaluation must be combined with otherfactors considered during well planning, such as mudhydraulics, hole cleaning, torque and drag calculations, andcasing programs. A discussion of how the relevant factors areintegrated exceeds the remit of this paper. It is evident,however, that many iterations are required before the finaltrajectory and drilling practices are decided.

Planning in Valhall. The geological structure of Valhall isdominated by a central uplift, elongated about a North-Northwest axis.8 Otherwise, the stratigraphy is relativelyuniform, with formations varying a little in thickness anddipping away from the center of the field at an angle ofapproximately 5o to the horizontal. Figure 2 shows a genericstratigraphic column.

Owing to the relatively uniform geology, 1D mechanicalearth models (where the properties are only a function ofTVD) are adequate for wellbore stability purposes in this field.The structure of Valhall is not, however, entirelyaxisymmetric, so it was necessary to construct MEMs that are

4 BRADFORD, ALDRED, COOK, ELEWAUT, FULLER, KRISTIANSEN, WALSGROVE IADC/SPE 59121

locally valid for the northern and southern parts of the field.Since the RTWBSC concept was validated in an ER welldrilled in a northwesterly direction (Figs. 3 and 4), attention isrestricted in the remainder of this paper to the MEMconstructed for northern Valhall. The MEM derived prior todrilling is shown in Figs. 5 and 6.15 The associated mudwindow, derived using an undrained linear elastic-brittlemodel, is shown in Fig. 7.16 It is important to note, however,that in cases where the geological structure and/or rockbehavior is more complicated (e.g., a salt diapir), fullynumerical techniques, such as finite element analyses, arenecessary to model the in-situ stress state and derive the mudweight window.

The “classical” rock mechanics approach just describeddetermines the risk of breakouts and mud losses. It is,however, increasingly recognized that many wellbores,especially those drilled at higher inclinations, fail because ofinstability mechanisms that are not amenable to this approach.Examples of such mechanisms include fractured zones, mobileformations and faulting. Practical quantitative orsemiquantitative modeling of these instabilities requiresdevelopment. Currently, issues pertaining to them are handledin a “soft” manner: drilling histories are analyzed to identifythe location and severity of “nonclassical” failure. Thedominant instability mechanisms for the discussed well areshown in Fig. 8. Medium and high severity instabilities aredenoted by the thick vertical dotted and solid lines on the rightside of the figure, respectively. Experience indicates that thenaturally fractured zone lying between 2000 and 2200 m TVD[4160 and 4570 m measured depth (MD)] poses the mostsevere risk, particularly if the well inclination through thiszone exceeds 65o. The region from 1510 to 1850 m TVD(2370 to 3680 m MD) contains rock with weak beddingplanes; it becomes more unstable with time.

Drilling strategy. The combination of the mud window(Fig. 7) and analyses of other hazards (Fig. 8) indicated it wasimpossible to drill the well without continuous rock failurebecause simultaneous remedies to all the instabilities did notexist:1. The mud weight needed to be high to avoid both

breakouts and underbalanced drilling.2. The mud weight needed to be less than the minimum in-

situ horizontal stress to prevent fluid loss, particularly intothe fractured zone between 2000 and 2200 m TVD.

To formulate a strategy for drilling the well, it was necessaryto assess the risk posed by each instability:1. Breakouts are a controllable failure. This type of failure is

either self-stabilizing (breakouts tend to stop growingafter reaching a certain size) or can be controlled byremedial actions (increasing mud weight preventsbreakout development), or both.

2. Destabilized fractured zones are an uncontrollable failure.This type of failure, once initiated, cannot be stoppedeasily and is expected to become ever more severe.

Thus, the strategy for the well was to prevent destabilization ofthe fractured zone between 2000 and 2200 m TVD. This

approach is contrary to conventional drilling practices, whichemphasize breakout control. This strategy involved thefollowing:1. A relatively low mud weight. It was accepted that this

would induce breakouts. The resulting cavings were dealtwith using hole-cleaning procedures and rate ofpenetration (ROP) control. The mud weight was increasedin steps of 0.1 lbm/gal only if the rate of cavings influxinto the annulus overwhelmed hole-cleaning capabilities.

2. Specific attention, within the monitoring program(discussed below), to cavings and mud losses to provide awarning of a destabilized fracture zone.

Recommendations for drilling parameters, such as ROP, couldonly be quantified as drilling progressed and trends forparameters such as ECD became established.

Data acquisition. A reliable diagnosis of the instabilitymechanisms, their severity and their trigger conditionsrequires a combination of MWD and LWD measurements,mud analysis, geological/micropalaeontological analysis andother surface information such as hookload and mud flow rate.The variety of data is notable and necessary because (a)wellbore instability and the influence of operations, togetherwith the relationship between them, are very complex, and (b)the process cannot rely on any single source of information.Thus, sensible interpretations require integration of allavailable information. It is also important that the samplingrates are such that interpretations can be provided on anappropriate timescale.

Clearly, data acquisition programs are designed on anindividual well basis, taking into account the nature and riskposed by the anticipated hazards, together with other factorssuch as budget constraints, formation evaluation requirementsand contingency plans. The benefits provided by acquiringspecific types of data and desirable sampling rates aresummarized below: use and flow of the data are discussed inthe following paragraphs.

LWD measurements can include annular pressure, caliper,gamma ray, resistivity (phase and attenuation; i.e., shallow anddeep, respectively) and compressional slowness:1. Annular pressure is an important measurement. It can be

used to (a) determine the risk of mud losses or shearfailure, (b) assess hole-cleaning effectiveness, and (c)evaluate annular cuttings/gas loading.

2. Resistivity measurements can be used to evaluate mudinvasion into fractured or permeable zones and faults.

3. Compressional slowness can be used to determineformation strength or flag overpressured domains.

The evolution of time-dependent instabilities can be assessedusing the appropriate time-lapse data.

MWD and surface measurements must include deviation,inclination, ROP, pump pressure, rotation rate in revolutionsper minute, downhole torque, downhole weight on bit, surfacetorque and hookload, possibly combined with turbinerevolutions per minute. The data are principally used todetermine the risk of stuck pipe and hole-cleaning

IADC/SPE 59121WHEN ROCK MECHANICS MET DRILLING: EFFECTIVE IMPLEMENTATION OF REAL-TIME WELLBORE STABILITY CONTROL 5

effectiveness. The combination of ROP and ECD data enablesannular cuttings and/or gas loading to be managed.

Mud logging data should, for safety reasons and losscontrol, consist of mud flow rate in and out, total active tankvolume, change in the total active tank volume, averagebackground gas and maximum background gas. Periodic mudmeasurements—such as rheological parameters and fluid loss,and the percentages of oil, water and solids—are alsodesirable, not least to aid interpretation of annular pressuredata.

A cavings analysis greatly reduces the ambiguity ininstability diagnoses; rate (i.e., volume), size range, averagesize, morphology, lithology, and source depth are desirablemeasurements. It should also be noted if the cavings are old(in a cuttings bed for several days) or are new (just becomedetached from the wellbore wall). This is discussed inAppendix A.

LWD, MWD and surface information should be monitoredcontinuously during drilling and also while tripping, providedthe driller is pumping out of hole at a sufficiently high flowrate. It is advisable to conduct cavings analyses at 30-minintervals, with periodic mud logging data gathered every fewhours. All data should be indexed to date, time, hole depth andbit depth to identify the effect of specific operations. Last et al.correlated greatly increased cavings volumes with trips andback-reaming.4

An appropriate selection of these measurements forms thebasis of any data acquisition program that is part of a real-timewellbore stability control process. It is not an exhaustive list;other key data may be required depending on the nature of theinstability. For example, if swelling shales are a severeproblem, further mud analysis may be required. It is also not a“must have” list; the approach to real-time detection andcontrol must be flexible so that no measurement is critical.

The data acquisition program for the Valhall field testconsisted of surface measurements, mud and cavings analyses,and extensive MWD and LWD measurements. The benefitsprovided by this program are discussed in the “Valhall fieldtest” section.

Decision support software. The process summarized in Fig. 1is embodied, to a significant extent, in the decision supportsoftware shown in Fig. 9 and is designed for use on a Pentiumlaptop computer. This package contains data manipulation,evaluation and visualization algorithms that help the usermake efficient, effective real-time decisions. It is not intendedto be an automated drilling optimization tool.

The package supports the user in five main areas:predicting instability mechanisms and their trigger conditions,diagnosing the wellbore state using real-time data, updatingthe earth model to ensure consistency between the predictedand the diagnosed states, providing recommendations to thedriller, and visualization.

Predicting the instability mechanisms and their triggerconditions has been discussed. Algorithms enable users tobuild trajectories and MEMs; safe mud weight windows are

calculated with an undrained elastic-brittle theory.Diagnosing the wellbore state using real-time data involves

the integration of a number of disciplines; namely, geologicalanalysis, drilling mechanics, formation evaluation, wellborestability and mud logging (mud analysis and palaeontology).This is a complex process requiring human judgment,particularly to distinguish wellbore instability and poor holecleaning. Diagnoses made within the context provided by theMEM and the planning analysis are more reliable than thosemade using only the real-time data.

After the diagnosis is completed, the current wellbore stateis compared to the model; human judgment determines if thetwo are consistent. If inconsistencies exist, it is necessary toupdate the MEM.

When the predicted and diagnosed wellbore states agreeadequately, recommendations either to suppress theinstabilities or minimize their consequences can be made tothe driller. For example, increasing mud weight will reduce theamount of breakouts, whereas decreasing the ROP will reducethe rate at which breakouts are exposed, resulting in less debrisin the annulus given constant flow and rotation rates. Therecommendations should apply over the entire open-holeinterval or a specified subsection of it. The aim is to optimizethe condition of the complete open-hole section and not tofocus on remedial actions required just at the bit.

Visualization is a key component of the support tool; thequality of the real-time decisions depends strongly on theready and unambiguous assimilation of the output of theRTWBSC process. For example, Fig. 10 shows the predicteddamage zone around a borehole resulting from shear failure. Itis immediately evident that the failure is extensive enough towarrant increasing the mud weight to suppress the failure;hole-cleaning procedures would not be able to cope with thedebris that would fall into the annulus.

Communications. Decisions on well construction are made atthe wellsite and in the office. The influence exerted by eachlocation varies according to the operator, the level ofactual/anticipated risk and the maturity of the fielddevelopment program.

The distribution of wellsite data and the procedures forimplementing decisions resulting from the RTWBSC analysismust be compatible with working practices; there should beparticular attention on communication.

During the Valhall field test, the RTWBSC process wasmanaged in the office by wellbore stability specialists workingwith an existing team of drilling engineers. A Schlumbergerengineer trained in drilling risk management was at thewellsite to ensure (a) the necessary measurements were takencorrectly and (b) the data flowed efficiently to the relevantpeople at the wellsite and in the office. This engineer was alsoresponsible for communicating recommendations for wellborestability at the wellsite and for conducting the cavingsanalysis. Although these recommendations are usually madeby office-based personnel, a suitably trained engineer canmake recommendations independently in some situations.

6 BRADFORD, ALDRED, COOK, ELEWAUT, FULLER, KRISTIANSEN, WALSGROVE IADC/SPE 59121

The acquired data are generally analyzed by personnel ofdiffering disciplines (geologists, drilling engineers, mudloggers, formation evaluation and wellbore stabilityspecialists), both at the site and in the office. This jointevaluation requires a reliable link with sufficient bandwidthbetween rig and office and a readily accessible repository forthe information. The experience gained from Valhall and fromthe BP Amoco ETAP field has shown that a Web site canfulfill this requirement.

Training classes on wellbore stability in general andcavings monitoring in particular were given to all drilling andmud logging crews going offshore on Valhall. The crewsresponded positively to these classes, which focused onavoiding, rather than reacting to, instability problems.

Valhall field testThe field test began with the drilling out of the 13 3/8-in.casing shoe at 1610 m (Fig. 8) and continued until thereservoir was penetrated at 5602 m (Point C). The sectionbetween the casing shoe and Point A was drilled using a rotarysteerable assembly with a 12.25-in. bit and a 14-in. three-armstabilized reamer. Sections AB and BC were drilled withconventional steerable assemblies having 12.25-in. bits.

Drilling from casing shoe to Point A (1610-3832 m MD).The 13 3/8-in. casing shoe was drilled out using a mud weightof 14.2 lbm/gal; a leakoff indicated that fluid loss occurred atpressures exceeding 15 lbm/gal. During drilling, ECD dataindicated that a safe lower bound to the minimum horizontalstress was 15 lbm/gal over the interval 1610 to 2040 m MD.

The mud weight had to be raised to 14.6 lbm/gal by 2200m MD to reduce background gas levels from 20% (gas peaksof 35% were observed). These high gas levels were consistentwith the drilling hazards prognosis (Fig. 8) and resulted frommatrix gas being released into the annulus as rock was crushedbeneath the bit. The necessity for further mud weightincreases, which would have led to the destabilization of thecritical fractured zone between 4160 and 4570 m MD, waseliminated by slowing the ROP to below 30 m/h (Fig. 11).This action reduced the rate at which gas was released into theannulus and, combined with the mud weight increase of 0.4lbm/gal, eventually led to background gas levels decreasing toless than 5%.

Wellbore stability in this section was controlled followingthe strategy outlined previously. A mud weight of 14.2 lbm/galprevented significant breakouts after the shoe was drilled out(Fig. 7). Subsequent mud weight increases resulted solelyfrom the overpressure problems described, as hole cleaningcoped with the levels of debris in the annulus caused bybreakouts.

Cavings analysis indicated no failure had occurred as aresult of weak bedding planes while drilling this section (Fig.8), although the instability mode became active during one trip(discussed below). The cavings rate is shown in Fig. 12.During the drilling of this section (0 to 100 hr approximately)the cavings rate remained reasonably steady, although there

was a reduction at around 3650 m MD caused by a packoff.The steady cavings rate resulted from the use of a rotarysteerable tool and the absence of severe wellbore instabilities.

The ECD was constrained by ensuring the ROP did notexceed 30 m/h: this rate controlled the cuttings loading andgas levels in the annulus. The ROP limit was deduced bycorrelating annular pressure while drilling and ROP data.Figure 13 shows a typical case. During the period 33 to 36 hr,the ROP exceeded 30 m/h and the ECD increased gradually asthe cuttings loading in the annulus increased. Partial packoffsthen occurred, causing the ECD to become highly erratic.Subsequently, the ROP was reduced to below 30 m/h and thehole was cleaned more effectively by increasing both therevolutions per minute and flow rate. The ECD became morestable and decreased gradually to 15.1 lbm/gal, indicating theECD effects were a result of inadequate hole cleaning ratherthan continued wellbore instability.

As drilling proceeded, mud weight rose to 14.6 lbm/galand the ECD increased above the estimated minimumhorizontal stress (Figs. 6 and 7) to between 15 and 15.2lbm/gal, without mud losses. The minimum horizontal stresswas therefore assumed to be 15.2 lbm/gal in the section 1610to 3832 m. Although this value is a lower bound of hσ , it is

more accurate than the previous hσ estimate. Figure 14 showsthe refined model of the in-situ stress state.

A severe problem occurred at 3649 m, where a fault wasencountered. This fault was diagnosed using resistivity,gamma ray and mud loss data, as shown on Fig. 15. It can alsobe inferred from this data that a packoff occurred below theLWD resistivity tool where the ECD sensor is housed. Thesurface pump pressure increased significantly while the ECDremained constant. The reason for the packoff is uncertain, butit is due to either fault movement or rubbilized rock, whichcan occur around faults, blocking the annulus. This incidentcaused seal failure on the rotary steerable system, leading tolubricant loss. The assembly had to be pulled out of hole afterdrilling to 3832 m MD (Point A on Fig. 8). Specificprocedures for wellbore stability control were developed forthese trips and are discussed separately.

The other key problem encountered in this zone was thepresence of limestone stringers at 2943, 3258, 3290, 3305,3330, 3350, 3546, 3508, 3550, 3596, 3645, 3650, 3668 and3795 m MD. When the bottomhole assembly (BHA) waspulled back through these stringers, there was a tendency topack off. It is thought that while the limestone stringersremained in gauge, hole enlargements either side of themresulted in lower mud velocities, which led to the formation ofcuttings beds. Accordingly, during circulation periods theBHA was positioned away from these stringers. At the sametime, to limit damage in the weakest formations, the MEMwas used to select the strongest zones for rotation of the BHA(Fig. 5).

Drilling from Point A to the reservoir (3832-5602 m MD).This section was drilled in two stages (AB and BC on Fig. 8)with conventional steerable assemblies having 12.25-in. bits.

IADC/SPE 59121WHEN ROCK MECHANICS MET DRILLING: EFFECTIVE IMPLEMENTATION OF REAL-TIME WELLBORE STABILITY CONTROL 7

Wellbore stability control in this section consisted of ensuringthe ECD did not exceed 15.2 lbm/gal (this initial constraintwas later relaxed to 15.35 lbm/gal) to avoid destabilizing thenaturally fractured zone. This was difficult given the largeamount of sliding that occurred, and there was a strongemphasis on hole cleaning and ROP control procedures. Thestability of caving beds was also a source of concern. Thesebeds tend to avalanche down the well at inclinations around60º, causing pipe and BHAs to stick.

In Section AB, it was found, unfortunately, that holdingangle was difficult. Drilling was therefore halted at 4306 mMD (Point B) for the following reasons.1. If drilling had continued, there was a risk the well would

have penetrated a partially drained section of thereservoir, which is to the left of the fault shown on Fig. 8.

2. The wellsite engineer observed a caving produced throughdestabilization of the naturally fractured zone.

The proximity of the planned trajectory to the fault (Fig. 8),made it necessary to trip out of hole to change out the BHA.The cavings analysis dictated the trip should occur withoutfurther drilling so as to limit damage to the key fractured zone.

During the trip back into the hole, 12 bbl of mud were lostwhen the ECD exceeded 15.35 lbm/gal at 4120 m MD. Theminimum horizontal stress in the MEM was therefore revisedto 15.35 lbm/gal from 1610 to 4306 m MD. The refined modelof the in-situ stress state is shown in Fig. 16. Figure 17 showsthe strength profile of the overburden (to Point B) updatedusing LWD compressional slowness data. This data verifiedthe rock strength profile constructed using offset well data(Fig. 5) and therefore no significant changes were made in thedrilling strategy. The updated mud window is shown in Fig.18.

In Section BC, the necessity to maintain the ECD at 15.35lbm/gal or less meant that breakouts were a severe problem(Fig. 18). The difficulty of cleaning hole with such severebreakouts can be seen in Fig. 12. The cavings rate variedgreatly and, in particular, there were sudden bursts of solidsover the shaker. The reservoir was, however, penetrated (PointC) ahead of schedule.

Limestone stringers were again encountered at 4000, 4075,4150, 4700, 4740, 4780, 4830, 4930, 4985, 5024, 5160, 5170,and 5310 m MD.

Tripping procedures. To prevent problems associated withswabbing as the downhole assembly was pulled out of hole,the mud weight was increased from 14.6 to 14.8 lbm/galduring the first two trips out from Points A and B (Fig. 8). Theprocedure required the heavier mud to be circulated into thewell after pulling 10 stands. The increase in mud weight wasdeduced from an analysis of pressure while drilling data fromoffset wells. Conversely, during the trips in to Points A and B,the mud weight was reduced from 14.8 to 14.6 lbm/gal tominimize problems associated with surging. The procedurerequired mud gels to be broken, after tripping 10 stands intothe hole, by increasing the revolutions per minute. Lightermud was then circulated into the well. It was also found that

increases in ECD during trips into hole were reduced byshearing the mud on the surface prior to circulating itdownhole.

The trip out of hole following the reservoir penetration(Point C) required the mud weight to be increased from 14.6 to14.8 lbm/gal at the start of the 12.25-in. hole (Point A). Thisensured the mud had sufficient carrying capability in the 14-in.hole section while keeping the effective mud weight in theentire open-hole section to a minimum. This was an importantconsideration for the casing operation; as the casing is run,large surge pressures destabilize the naturally fractured zones.

During the trip out of the hole after the reservoirpenetration, the hole was accidentally swabbed, causing thewell to collapse at 3500 m MD. The wiper trip to clean thisdamage unfortunately initiated a sidetrack from around 3600m MD. This sidetrack also penetrated the reservoir using thesame wellbore stability strategy described previously in the“Planning” section, with further emphasis on hole cleaning.The casing string then re-entered the original track, which hadbeen open through the fractured zone for several weeks, andwas landed below the fractured zone. It could not, however,quite reach the bottom of the hole. The well as a whole cannot,therefore, be called a success. However, since the reservoirwas penetrated ahead of schedule and the casing could still beinstalled in the troublesome fracture zone after several weeksof open-hole exposure to drilling fluid, the original wellborestability strategy and the real-time approach can still beconsidered a success.

ConclusionsReal-time monitoring and control of wellbore stabilitysystematically reduce the drilling risks associated withwellbore instability and other geological hazards. This real-time process treats such instabilities and hazards as conditionsthat impose constraints on the drilling parameters (mudweight, ROP, revolutions per minute, etc.) and then providesrecommendations on the drilling practices most likely toensure the entire hole section is maintained in the best, or leastdamaged, state.

The concept and process discussed here have beenvalidated on an ER well drilled in the Valhall field of theNorth Sea. The well reached its target ahead of plan and withmuch lower mud loss to the formation than usual (around 10%of the typical value for a well such as this on Valhall) andnegligible activation of the fracture zones. The well was casedto below the unstable overburden intervals.

NomenclatureUCS = Rock uniaxial compressive strengthφ = Rock friction angle

hσ = Total in-situ minimum horizontal stress

Hσ = Total in-situ maximum horizontal stress

Vσ = Total in-situ overburden stress

PP = Pore pressure

8 BRADFORD, ALDRED, COOK, ELEWAUT, FULLER, KRISTIANSEN, WALSGROVE IADC/SPE 59121

AcknowledgmentsThe Real-Time Wellbore Stability project was partly fundedby the European Commission, under the THERMIE initiative(contract number OG-0199-95).

During the Valhall field test, Schlumberger personnellocated offshore (Paul Benoit, Ruth Bertelsen, Gael Boche,Andy Foster, Caroline Hatch, Vidar Haugen and Al Pattillo)were responsible for the data acquisition program. Theycontributed greatly to the success of the field test through theirinitiative and dedication. The assistance provided by CharlesJenkins of Schlumberger Cambridge Research is alsogratefully acknowledged.

Appendix A - Cavings monitoringAn analysis of cavings can provide a signal that the borehole isfailing and indicates both the nature of the instability and thetroublesome formations. Cavings dimensions range from a fewmillimeters to 10 cm or more, with larger examples rising tothe surface while lodged in the BHA.

There are four main types of caving: tabular, angular,splintered and those that cannot be characterized. Examples ofthe first three types are shown in Figs. 19 to 21. Tabularcavings, shown in Fig. 19, are the result of natural fractures orweak planes. In the case of natural fractures, the fluid pressurein the annulus exceeds the minimum horizontal stress,resulting in mud invasion of fracture networks surrounding thewellbore. This can result in severe destabilization of the near-wellbore region (resulting from movement of blocks of rock),leading rapidly to high cavings rates, lost returns, stuck pipeand tools lost in hole. The blocks of rock are bounded bynatural fracture planes and therefore have flat, parallel faces(Fig. 19). The other characteristic is that bedding, if any, willnot be parallel to the faces of the caving. In the case of weakplanes, the combination of low mud weight and a boreholeaxis that is within approximately 15o of the bedding directioncan induce massive failure along the planes of weakness,leading to the symptoms described above.12 Cavings resultingfrom weak planes are characterized by having flat, parallelfaces. The bedding direction is also parallel to the faces.Figure 20 shows angular cavings, which are a consequence ofbreakouts. These cavings are characterized by curved faceswith a rough surface structure. The surfaces intersect at acuteangles (much less than 90o). Splintered cavings are shown inFig. 21. These cavings have two nearly parallel faces withplume structures. This type of caving is due to tensile failureoccurring parallel to the borehole wall and commonly occursin overpressured zones drilled with a small overbalance.

The cavings rate can indicate the severity of failure,coupled with the efficiency of hole cleaning. It is measuredevery 30 min by the time required to fill a bucket placedunderneath the shakers. This method may seem crude, but it isversatile (in terms of the number of different models of rig thatit can be applied to) and reliable; more sophisticated solidsmeasuring devices have been tried on a number of rigs, butvery few have been satisfactory.

Micropalaeontological analyses determine the geologicalage of cavings. During the field test, an analysis of tabularcavings indicated that they originated from the upper sectionof the open hole, where the exposure time was longest, ratherthan from the dangerous naturally fractured zone.

References1. Santarelli, F.J.: “Rock mechanics characterization of deep

formations: a technico-economical overview,” paper SPE 28021presented at the 1994 Eurock Rock Mechanics in PetroleumEngineering Conference, Delft, August 29-31.

2. Charlez, P.A., Bathellier, E., Tan, C. and Francois, O.:“Understanding the present in-situ state of stress in the Cusianafield – Columbia,” paper SPE/ISRM 47208 presented at the1998 Eurock Rock Mechanics in Petroleum EngineeringConference, Trondheim, July 8-10.

3. Charlez, P.A. and Onaisi, A.: “Three history cases of cases rockmechanics related stuck pipes while drilling extended reachwells in North Sea,” paper SPE/ISRM 47287 presented at the1998 Eurock Rock Mechanics in Petroleum EngineeringConference, Trondheim, July 8-10.

4. Last, N., Plumb, R.A, Harkness, R., Charlez, P., Alsen, J. andMcLean, M.: “An Integrated Approach To Evaluating andManaging Wellbore Instability in the Cusiana Field, Colombia,South America,” paper SPE 30464 presented at the 1995 AnnualSPE Techical Conference and Exhibition, Dallas, Oct 22-25.

5. Last, N., Plumb, R.A and Harkness, R.: “From theory to practice:evaluation of the stress distribution for wellbore stability in anoverthrust region by computational modelling and fieldcalibration,” paper SPE/ISRM 47209 presented at the 1998Eurock Rock Mechanics in Petroleum Engineering Conference,Trondheim, July 8-10.

6. Rosthal, R.A., Best, D.L. and Clark, B.: “Borehole caliper whiledrilling from a 2-MHz propagation tool and borehole effectscorrection,” paper SPE 22707 presented at the 1991 Annual SPETechical Conference and Exhibition, Dallas, Oct 6-9.

7. Bradford, I.D.R. and Cook, J.M.: “A semi-analytical elastoplasticmodel for wellbore stability with application to sanding,” paperSPE 28070 presented at the 1994 Eurock Rock Mechanics inPetroleum Engineering Conference, Delft, Aug 29-31.

8. Munns, J.W.: “The Valhall field: a geological overview,” Marineand Petroleum Geology (1985), February, p. 23-43.

9. Kristiansen, T.G., Mandzuich, K., Heavey, P, and Kol, H.:“Minimizing drilling risk in extended-reach wells at Valhallusing geomechanics, geoscience and 3D visualizationtechnology,” paper SPE 52863 presented at the 1999 SPE/IADCDrilling Conference, Amsterdam, March 9-11.

10. Bryant, I.: “Cybergeologist: 3D reservoir modelling using digitalgeological analogs,” GasTIPS, Spring 1998, GRI-98/0144-001.

11. Bryant, I., Kaufman, P.S., McCormick, D.S. and Tilke, P.G.:“Knowledge capture and reuse in geological modelling,” paperpresented at Gulf Coast Section of Society of EconomicMineralogists and Paleontologists Annual Meeting, December1999.

12. Okland, D. and Cook, J.M.: “Bedding-related instability in high-angle wells,” paper SPE/ISRM 47285 presented at the 1998Eurock Rock Mechanics in Petroleum Engineering Conference,Trondheim, July 8-10.

13. Plumb, R.A.: “Influence of composition and texture on the failureproperties of clastic rocks,” paper SPE 28022 at the 1994Eurock Rock Mechanics in Petroleum Engineering Conference,Delft, August 29-31.

IADC/SPE 59121WHEN ROCK MECHANICS MET DRILLING: EFFECTIVE IMPLEMENTATION OF REAL-TIME WELLBORE STABILITY CONTROL 9

14. Bradford, I.D.R., Fuller, J., Thompson, P.J. and Walsgrove, T.R.:“Benefits of assessing the risk of solids production in a NorthSea reservoir using elastoplastic modelling,” paper SPE/ISRM47360 presented at the 1998 Eurock Rock Mechanics inPetroleum Engineering Conference, Trondheim, July 8-10.

15. Kristiansen, T.G.: “Geomechanical characterization of theoverburden above the compacting chalk reservoir at Valhall,”paper SPE/ISRM 47348 presented at the 1998 Eurock RockMechanics in Petroleum Engineering Conference, Trondheim,July 8-10.

16. Fjaer, E., Holt, R.M., Horsrud, P., Raaen, A.M., and Risnes, R.:“Petroleum related rock mechanics,” Elsevier, Amsterdam(1992).

SI Metric Conversion Factorsbbl x 1.589873 E-01 = m3

ft x 3.048* E-01 = mgal (U.S. liq) x 3.785412 E-03 = m3

in. x 2.54* E+00 = cmlbm/gal x 1.198264 E+02 = kg/m3

psi x 6.894757 E-03 = MPa* Conversion factor is exact.

Fig. 1. The design-execute-evaluate cycle for real-time wellborestability control. The starting point is at the top, with initial datagathering and construction of the first MEM in the planning phase.The remainder of the cycle occurs as the well is being drilled.

Fig. 2. Generic stratigraphic column for the Central Graben.(Extracted from Kristiansen et al.9)

Fig. 3. A plan view of the Valhall field and ER well. (Extracted fromMunns8)

Fig. 4. Trajectory of the ER well.

10 BRADFORD, ALDRED, COOK, ELEWAUT, FULLER, KRISTIANSEN, WALSGROVE IADC/SPE 59121

Fig. 5. Uniaxial compressive strength and friction angle in theValhall overburden, estimated before drilling the ER well.

Fig. 6. The in-situ stress state in the Valhall overburden, estimatedprior to drilling the ER well.

Fig. 7. Mud weight window, estimated prior to drilling the ER well.

Fig. 8. Anticipated instability mechanisms and their severities.The thick vertical dotted and solid lines on the right of this figuredenote medium and severe instabilities, respectively.

Fig. 9. The flow of information and decisions through theprototype system. Ellipses represent data input, diamonds aredecision or comparison points, and rectangles are processes. Thestarting points are the two upper ellispes, and the finish point isthe lower left corner.

IADC/SPE 59121WHEN ROCK MECHANICS MET DRILLING: EFFECTIVE IMPLEMENTATION OF REAL-TIME WELLBORE STABILITY CONTROL11

Fig. 10. A schematic of shear-induced borehole failure.

Fig. 11. The influence of mud weight and ROP on gas levels(shown as squares).

Fig. 12. Cavings data. The cavings rate is 1440 x the reciprocal ofthe time, in seconds, taken to fill a 4.5-L bucket placed under theshakers.

Fig. 13. Time-based data acquired during drilling of the intervalbetween the 13 3/8-in. casing shoe and Point A.

12 BRADFORD, ALDRED, COOK, ELEWAUT, FULLER, KRISTIANSEN, WALSGROVE IADC/SPE 59121

Fig. 14. The in-situ stress state, refined following the drilling of theinterval between the 13 3/8-in. casing shoe and Point A.

Fig. 15. Identifying a large fault at 3649 m MD. PUMP and TVCAdenote surface pump pressure and volume change in active mudtanks, respectively. TVCA takes account of the increase in holevolume during drilling.

Fig. 16. The in-situ stress state, refined following the trip into holeto Point C.

Fig. 17. Strength parameters calculated using LWD compressionalslowness data.

Fig. 18. Revised mud window calculation.

IADC/SPE 59121WHEN ROCK MECHANICS MET DRILLING: EFFECTIVE IMPLEMENTATION OF REAL-TIME WELLBORE STABILITY CONTROL13

Fig. 19. Tabular caving.

Fig. 20. Angular caving.

Fig. 21. Splintered caving.