When Rock Mechanics Met Drilling

7/29/2019 When Rock Mechanics Met Drilling

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Copyright 2000, IADC/SPE Drilling Conference

This paper was prepared for presentation at the 2000 IADC/SPE Drilling Conference held inNew Orleans, Louisiana, 2325 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 the

paper, 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 or

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AbstractA new concept and process for real-time monitoring andcontrol of wellbore stability establishes the drilling parameters

required to optimize the drilling process and thereby reduce

the potential for wellbore instability and subsequent

unscheduled events or lost rig time. Surface and downholemeasurements, recorded while drilling, are used to make

regular updates to a model of the wellbore and to revise thedrilling plan accordingly.

The first step in the process is the generation of a

mechanical earth model (MEM) using information obtained in

offset wells and field and regional data. The proposed well

trajectory for a new well is projected into the MEM and a set

of stability parameters is generated for a given initial drilling

plan. The product identifies potential danger zones within a

well plan.

During drilling, real-time data, including logging-while-

drilling (LWD), measurements-while-drilling (MWD), surface

mechanical measurements, and fluids and solids monitoringinformation, are used to diagnose the state of the wellbore.

Any significant hole instability is detected and a warning is

given to the driller. The state of the wellbore is compared to

the model, and any revision required to align the predicted

with the actual state is made. This real-time update of the

mechanical model is then used to predict the future state of the

wellbore, in front of and behind the bit, for the given drilling

plan. If the drilling plan can be improved, a revision will be

recommended; for instance, reduction in the rate of

penetration, increase in mud weight and circulation, and

change in hole direction. The drillers can independentlyevaluate their own recommendations for changes to the

drilling plan and then decide on the best course of action. The

process also provides a record of wellbore stability

information that can be input to the field description for use infuture wells and continuous improvement of the drilling

process.Use of this concept was validated on the Valhall field in

the Norwegian sector of the North Sea. Extended-reach

drilling (ERD) to downflank targets has been problematic in

recent years; there is a high risk that wells will be suspended

or abandoned because of problems associated with wellbore

instability in this very weak overburden.The Real-Time Wellbore Stability Control (RTWBSC)

project team produced an MEM for the Valhall field, working

closely with the drilling engineers to develop a well plan for a

proposed ERD well. Implementation involved providing

wellsite 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 revise

the well plan. Through this process the team proposed and

implemented a strategy of drilling the well in controlled states

of failurenot a conventional drilling approach. The well

successfully reached its target ahead of schedule and a plannedstring of intermediate casing was not required, mud losses (a

previous problem contributing to instability and cost) were

minimal and the well was cased to below the unstable

overburden intervals.

Introduction

Wellbore instability is a major problem during the drilling ofmany oil and gas wells. Often quoted as costing the industry

between 0.6 and 1 billion dollars per year,1

it currently leads to

major 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 for

the Cusiana field operated by BP Amoco and partners in

Colombia. Wellbore instability was very severe there, leading

to costs per well of tens of millions of dollars. An integrated

approach 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


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

costs.4,5

A fundamental aspect of this approach was to accept

that wellbore instability was inevitable and to manage it ratherthan to eliminate it.

The drilling industry historically addresses wellbore

instability issues in two ways. The first approach treats the

problem on an ad hoc basis; for specific problem formations,

data and cores are collected and the drilling history is

analyzed, allowing the formulation of a set of empirical rules.In the Valhall field of the North Sea, for example, wells drilled

through the Middle Eocene formation at inclinations

exceeding 65 are at high risk. These rules do reduce

nonproductive time. However, they do not identify the

underlying instability mechanism and do not appropriatelyrelate it to drilling operations so that the full benefit of this

knowledge is realized. Furthermore, many of these empirical

rules apply to a well, and all need to be taken into account to

determine the drilling parameters (e.g., mud flow rate, rate of

penetration, pump pressure, trajectory). Techniques exist to

solve this type of problem, but these are often not applied andcan lead to an inadequate set of drilling parameters that can

trigger wellbore instability. The second approach is based onlog interpretation methods that estimate the safe mud weight

window using rock strength and in-situ stress state predictions

based primarily on sonic logging. The calculations are made,

however, within the framework of classical rock mechanicswhere it is assumed that the maximum and minimum mud

weights 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 to

this classical approach. The description of wellbore stability is,

therefore, generally incomplete.

Both approaches can be applied before or after, but not

during, drilling. Any lessons learned from data or experience

gathered on a well can therefore only be applied on subsequentwells in the same field. As a result, several wells can be drilled

before the minimum cost construction technique is found. This

significantly increases both the capital required for field

development and the cycle time. Managing borehole

instability in real time would potentially allow learning to be

implemented on the current well so that the optimal

construction technique is achieved over the minimum number

of wells. Such an approach has not, however, been possibleuntil recently because of technical constraints. The following

developments 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 of

different kinds influence drilling.

The RTWBSC concept uses real-time measurements and

interpretation to manage wellbore instability (real-time here

means essentially during drilling of the well; some real-time

data arrive immediately as a formation is being drilled, but

other 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 of

understanding of chemical and mechanical damage remains

problematic despite intensive efforts throughout the oil

industry. Accordingly, the RTWBSC process (a) determineswhether a particular drilling problem is mechanical or

chemical in origin, (b) deals with the mechanical aspects and

makes recommendations, based on known rules of thumb, if

the problem is chemical in origin.

The four main components of this process are described inthe next section. The first component is a wellbore model

consisting of the trajectory, in-situ stress state, rock

constitutive parameters and all types of instability

mechanisms, together with a description of the drilling

practices. It is constructed through the two approaches by

which wellbore instability is currently addressed, and it usesoffset well data, drilling experience and in some cases a

seismic survey to define the geological structures. Theaccuracy of the model depends on the information available,

but it always provides a framework against which real-time

observations and interpretations are judged. The second

component is the data acquisition program, which defines thetypes of data and sampling rate necessary to provide a reliable

diagnosis of the instability mechanisms, their severity and the

conditions under which they occur. The third component is a

software tool that accepts data from a wide range of sources

and manages the data flow, diagnoses the instability

mechanisms, and quantifies both their severity and corrective

drilling practices. A key part of this third component is the

refinement of the subsurface model. The fourth component is a

communication tool, such as an intranet Web site, that acts asa data repository and enables rapid dissemination of

information and recommendations.

The RTWBSC process was validated on an ERD well in

the Valhall field of the North Sea (see Valhall field test

section). This field, operated by BP Amoco Norge, is located

in offshore blocks 2/8 and 2/11 in the Central Graben area of

the southern part of the Norwegian North Sea. It was

discovered in 1975, when the exploration well 2/8-6encountered over 100 m of hydrocarbon-bearing section in

Late Cretaceous chalk formations. Production began in 1982

from the highly porous Tor and Hod chalk formations.8

Valhall was originally developed to recover reserves of

250 million barrels. There are ongoing projects to increase

recoverable reserves to 1000 million barrels.9

One projectinvolves accessing downflank reserves in the far northern and

southern parts of the field through ERD wells. Although this is

economically attractive, because of the potential for significant

gains in recoverable reserves over a relatively small cycle

time, wellbore instability is a major problem: There is a high

risk that wells will be abandoned or suspended before reaching

their target. This factor, together with the availability of a

comprehensive data set, meant that Valhall was suited to


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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 and

control of wellbore instability.

Real-time wellbore stability control processThe process uses four main components: the MEM, a data

acquisition program, data management software and a

communication system. The implementation of the process

and its components, for drilling optimization, is shownschematically in Fig. 1 and has three phases:

1. In the design phase, relevant data are gathered and theMEM is constructed. The wellbore stability and drilling

plans are then formulated: these are taken into account

during the design of the data acquisition program.2. In the execution phase, the drilling process is monitored

and 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 as

necessary and recommendations relating to drilling

practices are made to the rig crew. Interpretation of real-time data should be made within the context provided by

the MEM and the wellbore stability predictions:assessments of the validities of the interpretation and/or

the MEM will be more reliable.

The four components are discussed further in the following

paragraphs. The implementation of the design-execute-evaluate cycle is discussed in the Valhall field test section

and is illustrated using events that occurred during the drilling

of the well.

Planning. Before drilling, the optimal, or least damaging, well

construction techniques are identified through prognoses of the

geology and instability mechanisms likely to be encountered

and estimates of the conditions, including the stress state, that

trigger the mechanisms.In areas where drilling has occurred, the geology can be

characterized using offset well data such as logs and

geological reports, perhaps combined with a seismic survey. In

areas where no exploration has occurred (the case in Cusiana),

it is necessary to rely on a geological prognosis only, albeit

one now aided by geological modeling software tools.10,11

The process of analyzing the likely instability mechanisms

and estimating their trigger conditions is described in thefollowing paragraphs.

Review of offset well construction. This review should

include the drilling phase, with trips and casing runs. Attention

is typically focused on (a) mud losses, cavings rates and

morphology, geological reports and any (partial or full) stuck

pipe incidents and (b) relating instability issues to theoperation (tripping, backreaming) and comparing the mud

density and/or equivalent circulating density (ECD) to the

predicted stable mud weight window.

The product of this review includes the instability

mechanisms 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 or

bedding 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, mobile

formation, permeable formation or chemical activity. This list

is not exhaustive; further categories can be envisaged. The

severity of the instability is categorized as low, medium or

high.1. A low severity problem is one for which symptoms exist,

but no remedial action is required.

2. An instability of medium severity has noticeablesymptoms; minor action is required either to inhibit the

problem 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 cleaning

could be emphasized (to deal with breakout debris without

stopping breakouts) or the mud weight could be increased

by 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 total

loss of borehole integrity is highly likely and will result ina sidetrack or abandonment.

Density, sonic and gamma ray logs. Data can be

constructed using logs from several offset wells. The sonic log

should ideally consist of compressional and shear slownesses.In many cases, however, only compressional slowness is

available: an empirical correlation is then needed to derive the

shear wave speed. These data form the primary input for the

MEM, which consists of the in-situ stress state, the formation

constitutive parameters and the failure mechanisms. The

accuracy of the MEM can be enhanced by correlating (a) the

log-derived results to point data, such as information from

cores or leakoff tests, and (b) quantities such as sonic

velocities to constitutive parameters such as formationstrength.

13The MEM and proposed well trajectory may then

be used to predict the safe mud weight window.14

The instability evaluation must be combined with other

factors considered during well planning, such as mud

hydraulics, hole cleaning, torque and drag calculations, and

casing programs. A discussion of how the relevant factors are

integrated 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 is

dominated by a central uplift, elongated about a North-

Northwest axis.8 Otherwise, the stratigraphy is relatively

uniform, with formations varying a little in thickness anddipping away from the center of the field at an angle of

approximately 5o

to the horizontal. Figure 2 shows a generic

stratigraphic column.

Owing to the relatively uniform geology, 1D mechanical

earth models (where the properties are only a function of

TVD) are adequate for wellbore stability purposes in this field.

The structure of Valhall is not, however, entirely

axisymmetric, so it was necessary to construct MEMs that are


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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 is

restricted in the remainder of this paper to the MEM

constructed for northern Valhall. The MEM derived prior to

drilling is shown in Figs. 5 and 6.15

The associated mud

window, derived using an undrained linear elastic-brittle

model, is shown in Fig. 7.16 It is important to note, however,that in cases where the geological structure and/or rock

behavior is more complicated (e.g., a salt diapir), fully

numerical techniques, such as finite element analyses, are

necessary to model the in-situ stress state and derive the mud

weight window.The classical rock mechanics approach just described

determines the risk of breakouts and mud losses. It is,

however, increasingly recognized that many wellbores,

especially those drilled at higher inclinations, fail because of

instability mechanisms that are not amenable to this approach.

Examples of such mechanisms include fractured zones, mobileformations and faulting. Practical quantitative or

semiquantitative modeling of these instabilities requiresdevelopment. Currently, issues pertaining to them are handled

in a soft manner: drilling histories are analyzed to identify

the location and severity of nonclassical failure. The

dominant instability mechanisms for the discussed well areshown in Fig. 8. Medium and high severity instabilities are

denoted by the thick vertical dotted and solid lines on the right

side of the figure, respectively. Experience indicates that the

naturally fractured zone lying between 2000 and 2200 m TVD

[4160 and 4570 m measured depth (MD)] poses the most

severe risk, particularly if the well inclination through this

zone exceeds 65o. The region from 1510 to 1850 m TVD

(2370 to 3680 m MD) contains rock with weak bedding

planes; 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 was

impossible to drill the well without continuous rock failure

because simultaneous remedies to all the instabilities did not

exist:

1. The mud weight needed to be high to avoid bothbreakouts and underbalanced drilling.

2. The mud weight needed to be less than the minimum in-situ horizontal stress to prevent fluid loss, particularly into

the fractured zone between 2000 and 2200 m TVD.

To formulate a strategy for drilling the well, it was necessary

to assess the risk posed by each instability:

1. Breakouts are a controllable failure. This type of failure iseither self-stabilizing (breakouts tend to stop growingafter reaching a certain size) or can be controlled by

remedial actions (increasing mud weight prevents

breakout development), or both.

2. Destabilized fractured zones are an uncontrollable failure.This type of failure, once initiated, cannot be stopped

easily and is expected to become ever more severe.

Thus, the strategy for the well was to prevent destabilization of

the fractured zone between 2000 and 2200 m TVD. This

approach is contrary to conventional drilling practices, which

emphasize breakout control. This strategy involved thefollowing:

1. A relatively low mud weight. It was accepted that thiswould induce breakouts. The resulting cavings were dealt

with using hole-cleaning procedures and rate of

penetration (ROP) control. The mud weight was increased

in 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 a

warning of a destabilized fracture zone.

Recommendations for drilling parameters, such as ROP, couldonly be quantified as drilling progressed and trends for

parameters such as ECD became established.

Data acquisition. A reliable diagnosis of the instability

mechanisms, their severity and their trigger conditions

requires a combination of MWD and LWD measurements,mud analysis, geological/micropalaeontological analysis and

other 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, together

with 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 all

available information. It is also important that the sampling

rates are such that interpretations can be provided on an

appropriate timescale.

Clearly, data acquisition programs are designed on an

individual well basis, taking into account the nature and risk

posed by the anticipated hazards, together with other factors

such as budget constraints, formation evaluation requirements

and contingency plans. The benefits provided by acquiringspecific types of data and desirable sampling rates are

summarized below: use and flow of the data are discussed in

the following paragraphs.

LWD measurements can include annular pressure, caliper,

gamma ray, resistivity (phase and attenuation; i.e., shallow and

deep, respectively) and compressional slowness:

1. Annular pressure is an important measurement. It can beused 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 assessed

using the appropriate time-lapse data.

MWD and surface measurements must include deviation,

inclination, ROP, pump pressure, rotation rate in revolutions

per minute, downhole torque, downhole weight on bit, surface

torque and hookload, possibly combined with turbine

revolutions per minute. The data are principally used to

determine the risk of stuck pipe and hole-cleaning


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effectiveness. The combination of ROP and ECD data enables

annular cuttings and/or gas loading to be managed.Mud logging data should, for safety reasons and loss

control, consist of mud flow rate in and out, total active tank

volume, change in the total active tank volume, average

background gas and maximum background gas. Periodic mud

measurementssuch as rheological parameters and fluid loss,

and the percentages of oil, water and solidsare alsodesirable, not least to aid interpretation of annular pressure

data.

A cavings analysis greatly reduces the ambiguity in

instability diagnoses; rate (i.e., volume), size range, average

size, 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 become

detached from the wellbore wall). This is discussed in

Appendix A.

LWD, MWD and surface information should be monitored

continuously during drilling and also while tripping, providedthe driller is pumping out of hole at a sufficiently high flow

rate. It is advisable to conduct cavings analyses at 30-minintervals, with periodic mud logging data gathered every few

hours. All data should be indexed to date, time, hole depth and

bit 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 the

basis of any data acquisition program that is part of a real-time

wellbore stability control process. It is not an exhaustive list;

other key data may be required depending on the nature of the

instability. For example, if swelling shales are a severe

problem, further mud analysis may be required. It is also not a

must have list; the approach to real-time detection and

control must be flexible so that no measurement is critical.The data acquisition program for the Valhall field test

consisted of surface measurements, mud and cavings analyses,

and extensive MWD and LWD measurements. The benefits

provided by this program are discussed in the Valhall field

test section.

Decision support software. The process summarized in Fig. 1

is embodied, to a significant extent, in the decision supportsoftware shown in Fig. 9 and is designed for use on a Pentium

laptop computer. This package contains data manipulation,

evaluation and visualization algorithms that help the user

make efficient, effective real-time decisions. It is not intended

to 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, updating

the earth model to ensure consistency between the predicted

and the diagnosed states, providing recommendations to the

driller, and visualization.

Predicting the instability mechanisms and their trigger

conditions has been discussed. Algorithms enable users to

build trajectories and MEMs; safe mud weight windows are

calculated with an undrained elastic-brittle theory.

Diagnosing the wellbore state using real-time data involvesthe integration of a number of disciplines; namely, geological

analysis, drilling mechanics, formation evaluation, wellbore

stability and mud logging (mud analysis and palaeontology).

This is a complex process requiring human judgment,

particularly to distinguish wellbore instability and poor hole

cleaning. Diagnoses made within the context provided by theMEM and the planning analysis are more reliable than those

made using only the real-time data.

After the diagnosis is completed, the current wellbore state

is compared to the model; human judgment determines if the

two are consistent. If inconsistencies exist, it is necessary toupdate the MEM.

When the predicted and diagnosed wellbore states agree

adequately, recommendations either to suppress the

instabilities or minimize their consequences can be made to

the driller. For example, increasing mud weight will reduce the

amount of breakouts, whereas decreasing the ROP will reducethe rate at which breakouts are exposed, resulting in less debris

in the annulus given constant flow and rotation rates. Therecommendations should apply over the entire open-hole

interval or a specified subsection of it. The aim is to optimize

the condition of the complete open-hole section and not to

focus on remedial actions required just at the bit.Visualization is a key component of the support tool; the

quality of the real-time decisions depends strongly on the

ready and unambiguous assimilation of the output of the

RTWBSC process. For example, Fig. 10 shows the predicted

damage zone around a borehole resulting from shear failure. It

is immediately evident that the failure is extensive enough to

warrant increasing the mud weight to suppress the failure;

hole-cleaning procedures would not be able to cope with the

debris that would fall into the annulus.

Communications. Decisions on well construction are made at

the wellsite and in the office. The influence exerted by each

location varies according to the operator, the level of

actual/anticipated risk and the maturity of the field

development program.

The distribution of wellsite data and the procedures for

implementing decisions resulting from the RTWBSC analysismust be compatible with working practices; there should be

particular attention on communication.

During the Valhall field test, the RTWBSC process was

managed in the office by wellbore stability specialists working

with an existing team of drilling engineers. A Schlumberger

engineer trained in drilling risk management was at thewellsite to ensure (a) the necessary measurements were taken

correctly and (b) the data flowed efficiently to the relevant

people at the wellsite and in the office. This engineer was also

responsible for communicating recommendations for wellbore

stability at the wellsite and for conducting the cavings

analysis. Although these recommendations are usually made

by office-based personnel, a suitably trained engineer can

make recommendations independently in some situations.


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The acquired data are generally analyzed by personnel of

differing disciplines (geologists, drilling engineers, mudloggers, formation evaluation and wellbore stability

specialists), both at the site and in the office. This joint

evaluation requires a reliable link with sufficient bandwidth

between rig and office and a readily accessible repository for

the information. The experience gained from Valhall and from

the BP Amoco ETAP field has shown that a Web site canfulfill this requirement.

Training classes on wellbore stability in general and

cavings monitoring in particular were given to all drilling and

mud logging crews going offshore on Valhall. The crews

responded 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 the

reservoir was penetrated at 5602 m (Point C). The sectionbetween the casing shoe and Point A was drilled using a rotary

steerable assembly with a 12.25-in. bit and a 14-in. three-armstabilized reamer. Sections AB and BC were drilled with

conventional 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 weight

of 14.2 lbm/gal; a leakoff indicated that fluid loss occurred at

pressures exceeding 15 lbm/gal. During drilling, ECD data

indicated that a safe lower bound to the minimum horizontal

stress 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 2200

m MD to reduce background gas levels from 20% (gas peaks

of 35% were observed). These high gas levels were consistent

with the drilling hazards prognosis (Fig. 8) and resulted frommatrix gas being released into the annulus as rock was crushed

beneath the bit. The necessity for further mud weight

increases, which would have led to the destabilization of the

critical fractured zone between 4160 and 4570 m MD, was

eliminated by slowing the ROP to below 30 m/h (Fig. 11).

This action reduced the rate at which gas was released into the

annulus and, combined with the mud weight increase of 0.4

lbm/gal, eventually led to background gas levels decreasing toless than 5%.

Wellbore stability in this section was controlled following

the strategy outlined previously. A mud weight of 14.2 lbm/gal

prevented significant breakouts after the shoe was drilled out

(Fig. 7). Subsequent mud weight increases resulted solely

from the overpressure problems described, as hole cleaningcoped with the levels of debris in the annulus caused by

breakouts.

Cavings analysis indicated no failure had occurred as a

result 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 not

exceed 30 m/h: this rate controlled the cuttings loading and

gas levels in the annulus. The ROP limit was deduced by

correlating 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 as

the cuttings loading in the annulus increased. Partial packoffs

then occurred, causing the ECD to become highly erratic.

Subsequently, the ROP was reduced to below 30 m/h and the

hole was cleaned more effectively by increasing both therevolutions per minute and flow rate. The ECD became more

stable and decreased gradually to 15.1 lbm/gal, indicating the

ECD effects were a result of inadequate hole cleaning rather

than continued wellbore instability.

As drilling proceeded, mud weight rose to 14.6 lbm/gal

and the ECD increased above the estimated minimumhorizontal stress (Figs. 6 and 7) to between 15 and 15.2

lbm/gal, without mud losses. The minimum horizontal stresswas therefore assumed to be 15.2 lbm/gal in the section 1610

to 3832 m. Although this value is a lower bound ofh , it is

more accurate than the previoush estimate. Figure 14 shows

the refined model of the in-situ stress state.

A severe problem occurred at 3649 m, where a fault was

encountered. This fault was diagnosed using resistivity,

gamma ray and mud loss data, as shown on Fig. 15. It can also

be inferred from this data that a packoff occurred below the

LWD resistivity tool where the ECD sensor is housed. The

surface pump pressure increased significantly while the ECD

remained constant. The reason for the packoff is uncertain, but

it is due to either fault movement or rubbilized rock, whichcan occur around faults, blocking the annulus. This incident

caused seal failure on the rotary steerable system, leading to

lubricant loss. The assembly had to be pulled out of hole after

drilling to 3832 m MD (Point A on Fig. 8). Specific

procedures for wellbore stability control were developed forthese trips and are discussed separately.

The other key problem encountered in this zone was the

presence of limestone stringers at 2943, 3258, 3290, 3305,

3330, 3350, 3546, 3508, 3550, 3596, 3645, 3650, 3668 and

3795 m MD. When the bottomhole assembly (BHA) was

pulled back through these stringers, there was a tendency topack off. It is thought that while the limestone stringers

remained in gauge, hole enlargements either side of themresulted in lower mud velocities, which led to the formation of

cuttings beds. Accordingly, during circulation periods the

BHA was positioned away from these stringers. At the same

time, 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.


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Wellbore stability control in this section consisted of ensuring

the ECD did not exceed 15.2 lbm/gal (this initial constraintwas later relaxed to 15.35 lbm/gal) to avoid destabilizing the

naturally fractured zone. This was difficult given the large

amount of sliding that occurred, and there was a strong

emphasis on hole cleaning and ROP control procedures. The

stability of caving beds was also a source of concern. These

beds tend to avalanche down the well at inclinations around60, causing pipe and BHAs to stick.

In Section AB, it was found, unfortunately, that holding

angle was difficult. Drilling was therefore halted at 4306 m

MD (Point B) for the following reasons.

1. If drilling had continued, there was a risk the well wouldhave penetrated a partially drained section of the

reservoir, 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 without

further drilling so as to limit damage to the key fractured zone.During the trip back into the hole, 12 bbl of mud were lost

when the ECD exceeded 15.35 lbm/gal at 4120 m MD. The

minimum horizontal stress in the MEM was therefore revised

to 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 shows

the strength profile of the overburden (to Point B) updated

using LWD compressional slowness data. This data verified

the rock strength profile constructed using offset well data

(Fig. 5) and therefore no significant changes were made in the

drilling strategy. The updated mud window is shown in Fig.

18.

In Section BC, the necessity to maintain the ECD at 15.35

lbm/gal or less meant that breakouts were a severe problem(Fig. 18). The difficulty of cleaning hole with such severe

breakouts can be seen in Fig. 12. The cavings rate varied

greatly and, in particular, there were sudden bursts of solids

over the shaker. The reservoir was, however, penetrated (Point

C) 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 with

swabbing as the downhole assembly was pulled out of hole,

the mud weight was increased from 14.6 to 14.8 lbm/gal

during the first two trips out from Points A and B (Fig. 8). The

procedure required the heavier mud to be circulated into thewell after pulling 10 stands. The increase in mud weight was

deduced from an analysis of pressure while drilling data from

offset wells. Conversely, during the trips in to Points A and B,

the mud weight was reduced from 14.8 to 14.6 lbm/gal to

minimize problems associated with surging. The procedure

required mud gels to be broken, after tripping 10 stands into

the hole, by increasing the revolutions per minute. Lighter

mud was then circulated into the well. It was also found that

increases in ECD during trips into hole were reduced by

shearing 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 to

14.8 lbm/gal at the start of the 12.25-in. hole (Point A). This

ensured 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 important

consideration 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 reservoir

penetration, the hole was accidentally swabbed, causing thewell to collapse at 3500 m MD. The wiper trip to clean this

damage unfortunately initiated a sidetrack from around 3600

m MD. This sidetrack also penetrated the reservoir using the

same 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, and

was 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 reservoir

was penetrated ahead of schedule and the casing could still be

installed in the troublesome fracture zone after several weeksof open-hole exposure to drilling fluid, the original wellbore

stability strategy and the real-time approach can still be

considered a success.

ConclusionsReal-time monitoring and control of wellbore stability

systematically reduce the drilling risks associated with

wellbore instability and other geological hazards. This real-

time process treats such instabilities and hazards as conditionsthat impose constraints on the drilling parameters (mud

weight, ROP, revolutions per minute, etc.) and then provides

recommendations on the drilling practices most likely to

ensure the entire hole section is maintained in the best, or least

damaged, state.

The concept and process discussed here have been

validated on an ER well drilled in the Valhall field of the

North 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) and

negligible activation of the fracture zones. The well was cased

to below the unstable overburden intervals.

NomenclatureUCS = Rock uniaxial compressive strength

= Rock friction angle

h = Total in-situ minimum horizontal stress

H = Total in-situ maximum horizontal stress

V = Total in-situ overburden stress

PP = Pore pressure


8/13


AcknowledgmentsThe Real-Time Wellbore Stability project was partly funded

by the European Commission, under the THERMIE initiative

(contract number OG-0199-95).

During the Valhall field test, Schlumberger personnel

located 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 their

initiative and dedication. The assistance provided by Charles

Jenkins of Schlumberger Cambridge Research is also

gratefully acknowledged.

Appendix A - Cavings monitoringAn analysis of cavings can provide a signal that the borehole is

failing and indicates both the nature of the instability and the

troublesome formations. Cavings dimensions range from a few

millimeters to 10 cm or more, with larger examples rising to

the 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. Tabular

cavings, shown in Fig. 19, are the result of natural fractures or

weak planes. In the case of natural fractures, the fluid pressure

in the annulus exceeds the minimum horizontal stress,resulting in mud invasion of fracture networks surrounding the

wellbore. 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 pipe

and tools lost in hole. The blocks of rock are bounded by

natural fracture planes and therefore have flat, parallel faces

(Fig. 19). The other characteristic is that bedding, if any, will

not be parallel to the faces of the caving. In the case of weak

planes, the combination of low mud weight and a boreholeaxis that is within approximately 15

oof the bedding direction

can induce massive failure along the planes of weakness,

leading to the symptoms described above.12

Cavings resulting

from weak planes are characterized by having flat, parallel

faces. The bedding direction is also parallel to the faces.

Figure 20 shows angular cavings, which are a consequence of

breakouts. These cavings are characterized by curved faces

with a rough surface structure. The surfaces intersect at acuteangles (much less than 90

o). Splintered cavings are shown in

Fig. 21. These cavings have two nearly parallel faces with

plume structures. This type of caving is due to tensile failure

occurring parallel to the borehole wall and commonly occurs

in 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 measured

every 30 min by the time required to fill a bucket placed

underneath the shakers. This method may seem crude, but it is

versatile (in terms of the number of different models of rig that

it can be applied to) and reliable; more sophisticated solids

measuring devices have been tried on a number of rigs, but

very few have been satisfactory.

Micropalaeontological analyses determine the geological

age of cavings. During the field test, an analysis of tabularcavings indicated that they originated from the upper section

of the open hole, where the exposure time was longest, rather

than from the dangerous naturally fractured zone.

References

1. Santarelli, F.J.: Rock mechanics characterization of deepformations: a technico-economical overview, paper SPE 28021presented at the 1994 Eurock Rock Mechanics in Petroleum

Engineering 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 the

1998 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 reach

wells 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 1998

Eurock Rock Mechanics in Petroleum Engineering Conference,

Trondheim, July 8-10.6. Rosthal, R.A., Best, D.L. and Clark, B.: Borehole caliper while

drilling from a 2-MHz propagation tool and borehole effectscorrection, paper SPE 22707 presented at the 1991 Annual SPE

Techical 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 in

Petroleum Engineering Conference, Delft, Aug 29-31.8. Munns, J.W.: The Valhall field: a geological overview, Marine

and 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/IADC

Drilling 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 1998

Eurock Rock Mechanics in Petroleum Engineering Conference,Trondheim, July 8-10.

13. Plumb, R.A.: Influence of composition and texture on the failure

properties of clastic rocks, paper SPE 28022 at the 1994Eurock Rock Mechanics in Petroleum Engineering Conference,

Delft, August 29-31.


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14. Bradford, I.D.R., Fuller, J., Thompson, P.J. and Walsgrove, T.R.:

Benefits of assessing the risk of solids production in a North

Sea 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 the

overburden 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 = m

3

ft x 3.048* E-01 = m

gal (U.S. liq) x 3.785412 E-03 = m3

in. x 2.54* E+00 = cm

lbm/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 fromMunns

8)

Fig. 4. Trajectory of the ER well.


10/13


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.


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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.


12/13


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.


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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.

When Rock Mechanics Met Drilling

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