Geomechanical Wellbore Stability Modeling

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85CT&F - Ciencia, Tecnologa y Futuro - Vol. 3 Nm. 3 Dic. 2007

GEOMECHANICAL WELLBORE

STABILITY MODELING OF

EXPLORATORY WELLS STUDY

CASE AT MIDDLE MAGDALENA

BASIN

Jenny-Mabel Carvajal Jimnez1*, Luz-Carime Valera Lara2, Alexander Rueda3,and Nstor-Fernando Saavedra Trujillo1

1Ecopetrol S.A. - Instituto Colombiano del Petrleo, A.A. 4185, Bucaramanga, Santander, Colombia2DTH Ltda., Calle 91 # 24-69, Bucaramanga, Santander, Colombia

3Ecopetrol S.A. Reservoir and Production Engineering Office, Calle 37 # 8-43, Bogot, Cundinamarca, Colombiae-mail: [email protected]

(Received May 30, 2006; Accepted Oct. 11, 2007)

This paper presents the geomechanical wellbore stability model of an exploratory well sited at MiddleMagdalena Basin (MMB), which is based on the validity of linear elastic deformational theory for porousmedia; the use of correlations and field tools such as well and image logs to indirect determination

of mechanical properties and stress state. Additionally, it is shown the model calibration and validation usingdrilling events which occurred at other previously drilled wells in the study area, at the exploratory well itselfand experimentally evaluated mechanical properties on outcrop and core samples from the basin formations.This application allowed the Instituto Colombiano del Petrleo (ICP) at Ecopetrol S.A. to formally performthe geomechanical modeling of Colombian formations and to accomplish a complete and appropriatemethodology to do so; such methodology has been standardized as part of the drilling support process atEcopetrol S.A., supplying the possibility for taking decisions that contribute to reduce drilling costs and risks

during operations.

*To whom correspondence may be addressed

Keywords:exploratory well, rock mechanics, modeling, stability, drilling, Middle Magdalena Basin, Cagi 1.

*To whom correspondence may be addressed


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CT&F - Ciencia, Tecnologa y Futuro - Vol. 3 Nm. 3 Dic. 200786

En este artculo se presenta el modelamiento geomecnico durante la perforacin de un pozo explo-ratorio, ubicado en la cuenca del Valle Medio del Ro Magdalena; el cual supone la validez de lateora elstica lineal para determinar el comportamiento deformacional de las rocas, soportado en

el uso de correlaciones para la obtencin indirecta de las propiedades geomecnicas de las formaciones yel estado de esfuerzos in situ, a partir de herramientas de pozo como los registros elctricos y de imgenes.

Adicionalmente, se presenta la calibracin de dicho modelo con los eventos de perforacin observados enpozos perforados previamente en el rea de estudio y con pruebas de laboratorio realizadas en muestras deafloramiento de la cuenca. La validacin del modelo extrapolado se bas en la experiencia de perforacin yen pruebas de laboratorio adicionales realizadas en el corazn extrado del prospecto perforado. Con esteejercicio, el Instituto Colombiano del Petrleo de Ecopetrol S.A. incursion de manera formal en el rea delmodelamiento geomecnico de las formaciones colombianas, lo que permiti el desarrollo de una meto-dologa robusta y apropiada para la regin de estudio y la estandarizacin de este proceso como apoyo ala perforacin en Ecopetrol S.A., brindando la posibilidad de establecer acciones que permiten reducir el

costo de perforacin y los riesgos inherentes a las diferentes operaciones desarrolladas.

Palabras clave:pozos exploratorios, mecnica de rocas, modelamiento, estabilidad, perforacin, cuenca del ValleMedio del Magdalena, Cagi 1.


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CT&F - Ciencia, Tecnologa y Futuro - Vol. 3 Nm. 3 Dic. 2007 87

GEOMECHANICAL WELLBORE STABILITY MODELING OF EXPLORATORY WELLS

NOMENCLATURE

Sh Minimum horizontal stress, psi

SV Overburden stress gradient, psi/ftg Gravity

sea Sea water density

b Formation density

D Depth, ft

P/D Pore pressure gradient, psi/ft

S/D Overburden stress gradient or lithology pressure gradient, psi/ft

tn Normal transit time

to Observed transit time

VP Compressive seismic velocity

VS Shear seismic velocity Poisson ratio

K Bulk modulus

Rock density average

E Youngs Moduli

To Tensile strength

P Maximum stress

D Sample diameter

t Sample thickness

Vshale Shale content

GRread Gamma ray readingGRclean Clean gamma ray

GRshale Dirty gamma ray

V Overburden stress

e Lithology thickness

1 Maximum stress

3 Minimum stress

UCS Unconned compressive strength

Internal friction coefcient

Rock failure angle

pWT Mudweigth Pressure

h Minimun horizontal stress

H Maximum horizontal stress

P0 Pore pressure


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JENNY-MABEL CARVAJAL JIMNEZ et al.

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INTRODUCTION

Rock mechanics has become one of the supporttechnologies applied in order to obtain an efcient ex-

ploitation of hydrocarbons due to the changes inducedby petroleum industry activities performed on oil andgas reservoirs. Early applications of geomechanics inpetroleum industry were made to prevent and controlsanding and stability problems of wells (Roegiers,1995).

Wellbore stability is considered when the well di-ameter ts the bit sizes and it remains constant whiledrilling. In constrast to this, geomechanical instabil-ity refers to mechanical conditions such as wellborecollapse or failure. In general, wellbore instability is

related to drillpipe sticking, tight spots, caving produc-tion, wellbore collapse and unscheduled sidetracks,these conditions are mostly caused by unknown rockmechanics and lead to increased costs during drillingand completion operations.

Hubbert and Willis (1957) developed a mechani-cal wellbore stability model in which the primary as-sumption was the linear elastic stress pathway aroundthe borehole. From this model, Geertsma (1966) andother researchers stated methodologies to couple porepressure effects and improve the stress prediction on

the borehole using non linear elastic hypothesis andanelastic strain. Most recent models include wellboreinstability numerical simulation (Vsquez, Castilla, &Osorio, 2004). A comparative outline of current insta-bility models available is shown in Table 1.

The proposed methodology assumes the validityof linear elastic theory for porous media in order topredict geomechanical rock behavior. In addition tothis, to reduce solution uncertainties in the model a setof data is used, which is obtained from drilling reports,well logging, laboratory tests, well tests such us LOT

(Leak off Test), FIT (Formation Integrity Test) andmicrofracturing. The main goal of this method is toobtain representative models to be used while drilling,so it would be possible to prevent instability problemsand to reduce non productive time and drlling costs.

This paper describes the methodology used by Eco-petrol S.A. to perform and calibrate the geomechanicalwellbore stability models and its application to an

exploratory well sited at MMB Colombia, calledCagi 1. This application demonstrated the validityof hypothesis such as: I) applicability of linear elastictheory in mechanical stability simulation while drill-ing, II) applicability of well logging to rock mechanicfeatures modeling, III) applicability of laboratorytest to calibrate rock mechanical modeling and iv)applicability of drilling reports to calibrate wellbore

stability models.

FUNDAMENTAL THEORY

Wellbore instability modeling

According to rock mechanics, drilling generateschanges in the stress eld of the formation due tosupporting material losses. In fact, drilling inducesradial and tangential stresses that result in additionalshear stresses. At certain point induced stress may be

higher than the rock strength and rock will fail caus-ing borehole collapse and stuck piping. This mechanicbehavior can be addressed to know the rock feature soit would be possible to prevent and to reduce instabil-ity problems.

The following items help to state a stability model(Adam, Bourgoyne, Keith, Martin, & Young, 1986):

Table 1. Comparative outline of instability models

ProposedModel

RepresentativeSolution

ModelingData

Availability

SolutionAvailability

Linarelasticmodel

Low HighComercialsoftware

Bilinearelasticmodel

Middle LowComercialsoftware

Poroelasticmodel

Middle - High Very highComercialsoftware

Non linearelasticmodel

Middle - High Very lowComercialsoftware

Numeric

model High Minimum Yes


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1. To evaluate the acting forces on objective area (grav-ity and tectonic stresses).

2. To assess the rock strength.

3. To calculate resulting stress eld.

4. To use the constituve laws to relate stress eld andstrain.

5. To state boundary conditions for stresses andstrains.

6. To identify the failure mode.

7. To determine the mudweight window and to calcu-late the best mud density.

8. To dene activities to control of instability.

Calculating overburden stress. To do so, the densitywell logging is integrated with respect to vertical depthusing the following equation:

(1)

Minimum horizontal stress assessment (Sh). Thisstress is read from LOT and FIT results, Figure 1 showsa type curve of these tests (Aird, 2001) and the pointcorresponding to Sh.

sure, minimum horizontal stress, UCS, tensile strengthand failure conditions observed at previously drilledwells. To draw the stress polygon (Figure 2) it must becalculated the strike-slip, normal and thrust regime limits,then the polygon area is closed using the vertical stress

and pore pressure and nally with rock properties it is ap-proached the maximum horizonta stress for each depth.

Figure 1. LOT Type Curve (from Aird, 2001 )

Figure 2. Stress Polygon (modified from Zoback et al., 2003)

Maximum horizontal stress calculation (Zoback et al.,2003). In order to quantify this stress the stress polygondenition is used, in wich is possible to identify variousmagnitude ranges based on overburden stress, pore pres-

Horizontal stress orientation. Finally to completethe horizontal stress determination the orientationof them must be known. This can be achieved fromimage well logging (UBI or FMI), geometry well log-ging (four arms caliper or higher calipers), regional

or structural studies. When well logging are availablethe current orientation can be obtained. On the otherhand, when regional and structural studies are used theorientation will depend on tectonic conditions (Muozet al., 1996).

Pore pressure calculations. To estimate the pore pres-sure prole the Eatons correlation (1969) is used:

(2)

(3)

Geomechanical properties modeling of rock forma-

tions

The geomechanical properties can be modeled fromrock features such as composition (shale content), den-sity and acoustic velocities based on well logging tools


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90

Gamma Ray, Neutron Density and Sonic log andexperimentally evaluated petrophysic properties on wellsamples. Rock composition, porosity and density can beobtained from well logging or laboratory tests, howeveracoustic velocities require special data treatment, which

is accomplished using the equations proposed by Nielsen,and Kohlhaas (1979).

Elastic Moduli assessment (Biot, 1956). Accordingto elastic theory it is possible to obtain all the elasticmoduli from two of them (as it can be seen at Table 2,published by T. Bourbi, Coussy, & Zinszner, 1987).A common array is the Poisson ratio and Volumetricmoduli which can be calculated from acoustic velocitiesas shown inEquations 4and 5).

(4)

(5)

and Mohr-Coulomb failure envelope. The parametersfrom geomechanic tests are representative of rockbehavior and can be used to calibrate geomechanicalmodels built from well logging. Table 3 lists the teststo evaluate wellbore stability parameters.

E G

K, v

Table 2. Mathematical expressions for elastic moduli

The values obtained by the earlier equations aregreater than pseudostatic measurements of mechani-cal properties. The dynamic measurements need to beupscaled in order to t the corresponding pseudostaticproperties since those are the data used in geomechani-cal modeling.

Mohr-Coulomb failure envelope and rock strengths.These parameters are computed from Tixier, Loveless,and Anderson (1975) equations which are based onexperimental correlations.

Experimental geomechanic evaluation

Experimental geomechanic evaluation is used to de-termine straightforward elastic moduli, rock strengths

Test Name Evaluated Parameter

Uniaxialcompressive test

Young Moduli

Poisson ratio

UCS

Strains: Axial, Circumferencial,Volumetric

Velocities: Compressive and Shear

Brazilian test Tensile strength

Triaxialcompressive test

Young Moduli

Poisson ratio

CCS

Strains: Axial, Circumferencial,Volumetric

Multiple failureenvelope

Cohesion, Internal friction coefficient

Table 3. Geomechanic tests for wellbore stability study

Uniaxial compressive test. This experimentalevaluation consists in applying axial force on therock in a continous way increasing the stress until thesample shows shear failure. The strains of the rockare measured during the test so Young moduli andPoisson ratio can be calculated from them (Charlez,1991; ASTM D2938 95 (Reapproved 2002), ASTMD 3148, 2002).

Indirect tensile test. It is also called Brazilian Testand it implies the progressive increase of compressiveforce applied transversely to the axial axis of the sampleat one point of the diameter until the rock fails on indi-rect tensile mode (ASTM D 3967 05, 2002).

Triaxial compressive test. At the beginning of thistest the sample has conning pressure and axial forceapplied on it the value of this stress is similar toaverage in situ stress state. Next to this, axial force isincreased until the rock fails in a shear mode (ASTMD 2664 04, 2004; ASTM D 5407 95 Reapproved2000).


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In addition, to obtain the Mohr-Coulomb failureenvelope, a series of triaxial compressive tests mustbe carried out by applying different initial stress stateson the specimens. This is possible to achieve usingeither as many samples as stress states are evaluated

or using just one specimen in a special triaxial knownas multiple failure envelope test.

EXPERIMENTAL PROCEDURE

This methodology consists in three stages. First stepis to build up the model based on the correlation wellsavailable (Figure 3) which are sited next to the studiedwell and have similar behavior respect to it. Further-more, correlation wells must have as much information

as possible to obtain the geomechanical model. Thismeans well logging (density log, sonic log, gamma raylog, resistivity log, porosity log), drilling reports andformation tests.

Geomechanical model building up

Primary assumptions during this stage are thevalidity of linear elastic theory for porous media, theuniformity of rock formations, the representativity offormation test and well logging. Based on these, thegeomechanical model building up consists of the nextitems:

1. To choose the correlation wells.

2. To determine the lithologies at wells.

3. To assess shale content at different lithologies(Vshale).

4. To calculate shear and compressive acoustic velocityon wells.

5. To compute elastic moduli, rock strength and Mohr-Coulomb faillore envelope.

6. To get well in situ stresses (overburden stress,minimum horizontal stress, maximum horizontal

Figure 3. Wellbore stability model building up

The second stage is model calibration (Figure 4) whichis the replication of drilling conditions on the correlationwells, based on the following information: formation testsdata, daily drilling reports, and experimental tests data

(both petrophysics and geomechanics data).At last, the model is extrapolated to the exploratory

well (Figure 5) by taking into account geologic features,well congurations and the different drilling condi-tions occured at correlation wells. In order to updatethe model, the behavior of rocks is observed whileexploratory well drilling. This may help to support thedecision making on the operation.

Figure 4. Wellbore stability model calibration

Figure 5. Wellbore stability model extrapolation


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92

stress, orientation of horizontal stresses and porepressure).

7. To evaluate the mudweight window, which has threeparameters: I) minimum mudweight, II) maximum

mudweight and III) optimum mudweight whiledrilling.

Correlation wells are sited in the surrounding area ofthe exploratory well, to choose the main correlation wellit is assumed that formations are uniform so wells near tothe prospect will behave similarly. Once the correlationwell is chosen, the lithology of it must be determinedfrom core and cutting samples description, compositewell log and gamma ray log. Based on this information,the shale content can be calculated using the value ofclean gamma ray which corresponds to gamma ray

reading at non shaly zones and dirty gamma ray read-ing at shaly zones applyingEquation 6.

(6)

The sonic well log is used to get the compressiveacoustic velocity and after this calculation it is possibleto assess the shear acoustic velocity according to thewell lithologies. Once these velocities are computed,the elastic moduli can be determined from the expres-sions in Table 2. Finally, rock strength must be dened

for each well lithology.To determinate the in situ stress state it is required

to calculate rst of all the overburden stress (Equation1). If there is not enough density information to useEquation 1, it is possible to build up this stress fromlithological description, core petrophysics data andcuttings using the following equation:

(7)

Whereas 0,433 is the unit conversion constant fromg/cc to psi/ft. Table 4 lists various lithology densitiesassuming 100 % of purity on each.

After calculating overburden stress, the minimumhorizontal stress magnitude can be dened from LOTand FIT at correlation wells. To extrapolate the mini-mum stress gradient it is assumed that its magnitud isconstant at each rock formation. Once the previousstresses are computed, it is possible to approach maxi-

mum horizontal stress magnitude using stress polygon(Figure 2). The orientation of horizontal stresses isdened based on well logs or regional studies. Finally,the stress state is completed when pore pressure proleis obtained from Eaton method (Equations 2 and3).

According to geomechanical denitions mudweightwindow is the value or range of values that might beused to keep a safe operation while drilling. Minimumvalues of this window correspond to the minimummudweight required to avoid collapse formation in the

borehole; maximum values address to prevent hydraulicfracturing while drilling and optimum mudweight is thesuggested mud density to perform drilling.

The minimum mudweight is calculated from Mohr-Coulomb failure criteria using the following equations

(Muoz et al., 1996): (8)

(9)

Taking into account the relative magnitude of in-duced stresses (failure regimes) and the Mohr-Coulombparameters (Equations 8and 9) there can be six dif-ferent possible conditions for borehole collapse. theseconditions are listed in Table 5.

The maximum mudweight for a vertical well can be

dened considering hydraulic failure condition (tensilefracture), which is calculated upon the next equation(Muoz et al., 1996):

(10)

The mudweight window is favorable when thecollapse density is lower than the fracture density inany other case such mudweight window will be non

Table 4. Lithology densities (modified from Schlumberger, 1972)

Lithology Apparent Log Density (Kg/m3)

Shale 2200 2750

Sandstone 2485Bituminous Coal 1300 1500

Limestone 2540

Dolomite 2683


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favorable. In the Figure 6 is shown a skematic drawingof these cases.

When the mudweight window is favorable the op-timum mud density is the range average. In the othercases the optimum mud density corresponds to either

fracturing or collapse values, taking into account whichis the most critical situation under drilling conditionsand trying to minimize mechanical failure.

Geomechanical model calibration

In order to calibrate a geomechanical model, it isnecessary to identify the drilling conditions on correla-tion wells. To do so, the primary information is obtainedfrom daily drilling reports, operative reports and naldrilling reports. After this information is extracted themodel is tested and tted so it accurately reects the

observed behavior.

Extrapolating the model to the exploratory well and

updating the model while drilling

The geomechanical model can be extrapolated oncethe initial model is calibrated. This can be addressedfrom the geomechanical and petrophysical behavior ofthe rocks to be drilled, their width and their structural

Table 5. Conditions for borehole collapse on vertical wells

CaseStress Regime

on the Wellbore WallFailure Conditions

1

2

3

4

5

6

Figure 6. Mudweight window

conditions on the new well. Here, the primary assump-tion is the lithological and mechanical uniformity of the


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94

formations since this allows to calculate the necessaryparameters to obtain the new model.

While drilling it is necesary to update the model.This issue can be accomplished using drilling param-

eters, well logging, lithological description of therocks from cuttings and LOT/FIT performed on theexploratory well. Once this information is recordedthe geomechanical model and its parameters can beupdated. Finally, the initial model is compared to theupdated model and the model parameters can be vali-dated with the actual data to obtain an accurate modelthat represents the drilled zone behavior.

STUDY CASE: EXPLORATORY WELL ON

MMB

The methodology presented in this paper was appliedon one exploratory well sited at MMB named Cagi 1.

Geomechanical model building up

This exploration well is sited in the north depressionof MMB, in the surroundings to Playon town San-tander, Colombia. According to the geological studies(Surez, 1998), this sector has three structural andstratigraphic features which are:

Triassic-Jurassic. Constituted by Girn Formation,this sector shows normal faulting systems enclosedby subvertical faults.

Cretaceous-Paleocene. It is composed by Tambor,Rosablanca, Paja, Tablazo, Simit, La Luna, Umirand Lisama Formations. this region is conformed

by inverse strike-slip faulting system.

Middle Post-Eocene. The formations found in thissector are: La Paz, Esmeralda, Mugrosa, Colorado,Real Inferior, Medio y Superior. These formationsdip to east and present inverse faulting systemcaused by Lebrija faulting system.

Correlation wells.Based on the structural and stra-tigraphyc information the correlation wells chosen toaccomplish the geomechanical model were Puntapiedra1 and 2 and Bosques 3. Puntapiedra 1 was chosen asthe primary correlation well since it has the largestamount of best quality information, however due to the

nal drilled depth of this well it was necesary to modelPaja and Rosablanca Formations from Bosques 3 well.Puntapiedra 2 well was used to correlate drilling eventsand validate the geomechanical model.

Lithostratigraphic description of correlation wells(Ortega and Ramrez, 2002, Rubiano et al., 2001).While drilling of Puntapiedra 1 well next formationswere found (from base to top): Simit, La Paz, Es-meralda, Mugrosa, Colorado and Grupo Real. Theseformations are primary shales and sandstones buriedon river channels which have certain content of calcare-ous material.

Shale content and acoustic velocities assessment. TheEquation 6was used to obtain shale content. After that,the compressive acoustic velocity prole was computed.

Finally, using this two data the shear acoustic velocity wasaddressed. In Figure 7 the Vshale, Vpand Vsproles ob-tained for Puntapiedra 1 well are shown. From this gureit is possible to state that velocities behavior is consistentto shale content in rocks and its lithological features. As aresult rocks with low shale content have higher velocitiesvalues than rocks with high shale content.

Elastic moduli, rock strength and Mohr-Coulombfailure envelope. In order to build up the geomechanicalmodel it is necessary to assess the elastic rock featuresin the correlation well. This can be accomplished us-

Figure 7. Puntapiedara 1 shale content, compressive seismic velocity andshear seismic velocity logs


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ing theEquations 4and 5and the expresions on Table2. On the other hand to compute the Mohr-Coulombfailure envelope and rock strength it was necessary toknow in advance the shale content and compressive andshear acoustic velocities. Figure 8 shows the proles

obtained for these properties. It can be observed that forPuntapiedra 1 well the Grupo Real y Colorado forma-tions are weaker than the other well formations, thisformations could be the most problematic ones duringdrilling. It also can be seen that Mugrosa Formationhas a higher strength than the previous formations andEsmeralda Formation has a decrease in strength as itsdepth increases, in the other hand La Paz Formationshows opposite behavior to Esmeralda Formation,nally Simit Formation has a mixed behavior, rstit decreases it strength down to a minimum value atabout 11300 ft and then starts increasing its strengthup progressively until the end of the well.

Figure 8. Puntapiedra 1 mechancial properties logs (from left to right ucs,cohesion, tensile strength and young moduli)

Stress eld calculation.This issue was accomplished

by rst calculating the overburden stress using the den-sity log from Puntapiedra 1 well, the lithology densitiesfrom Table 4 and Equations1and 7. In gure 9 thedensity log (left side) and the overburden stress rightside for this well are presented. The densities fromthe log vary between 2,45 and 2,68 gr/cc which aretipical values for drilled formations in the well. Theoverburden gradient obtained from density log varies

Figure 9. Puntapiedra 1 ROHB log (left) and overburdenstress gradient (right)

between 0,983 and 1,01 psi/ft which is also very com-mon value for this stress magnitud.

In second instance, the minimum horizontal stressgradient was calculated using three LOT and three FITfrom Puntapiedra 1 and Puntapiedra 2 wells. The dataobtained in these test are listed in Table 6. The valueobtained for the minimum horizontal stress gradientvaries from 0,84 psi/ft to 1,01 psi/ft.

In third place, to determine the maximum horizontalstress magnitude the stress polygon was evaluated ateach FIT from Puntapiedra 1 well. The results from thesecalculations are shown in Table 7. It can be noticed thatthe maximum horizontal stress gradient varies between1,61 psi/ft and 1,08 psi/ft. It can be notice that for SimitFormation (11 475 ft and 11 790 ft) the maximum stressgradient is lower than for the Grupo Real Formation(3000 ft), the reason for this behavior is the lower valuesof tensile strength in Simit Formation.

Fourthly, the horizontal stress orientation was de-ned based on the regional study Levantamiento de


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96

True VerticalDepth (ft)

Mudweight (LPG) Gradient (psi/ft)Pump Pressureat leak off (psi)

CalculatedPressure (psi)

Well

3000 13 0,676 2000 4028 Puntapiedra 1

11475 16 0,832 1000 10547,2 Puntapiedra 1

11790 17,9 0,931 1000 11976,5 Puntapiedra 1

3091 14,7 0,764 2361,5 Puntapiedra 2

3106 15 0,78 2422,7 Puntapiedra 2

11153 18,3 0,952 10617,7 Puntapiedra 2

Table 6. Formation tests data

Table 7. Maximum horizontal stress for Puntapiedra 1

Test Integrity 1 Integrity 2 Integrity 3

Depth (ft) 3000 11475 11790Pore pressure gradient (psi/ft) 0,433 0,617 0,522

Pore pressure (psi) 1299 7076,6 6153,2

Minimum horizontal stress gradient (psi/ft) 0,84 0,84 0,84

Mudweight (lb/gal) 13 16 17,9

Maximum horizontal stress (psi) 4837,5 12354,8 12627,9

Maximum horizontal stress gradient (psi/ft) 1,612 1,077 1,071

Secciones Estratigrcas, Control de Cartografa Ge-olgica y Medicin de Fracturas, Pliegues y Fallas en

el Bloque Torcoroma (Technical Report), in which themaximum horizontal stress orientation is inferred fromnatural fracture orientation. According to this study themaximum horizontal stress orientation resulted to beabout N85E - S85W.

Finally, the pore pressure was calculated from theEaton method. Figure 10 shows the results obtained forthis property. In general, most of the correlation wellpresents normal pore pressure or slightly overpres-sured gradient (0,433 psi/ft to 0,502 psi/ft) exceptingthe Simit Formation in which pore pressure gradientgoes up to 0,82 psi/ft.

Mudweight window. To model the geomechani-cal behavior while drilling of Puntapiedra 1 well, thesoftware AGE was used. The results of this modelingare shown in Figure 11. From this information it can

be stated that there was a risk during drilling due tooverpressure on the Simit formation and that mud

density necessary to prevent wellbore collapse failurein the lower part of Real Inferior Formation was very

close to pore pressure, so it would be necessary to drillthe well with a high density mud. In fact, Puntapiedra1 well was geomechanically stable while drilling andthe drillers used high density muds which prevent bothkickings and collpse during the drilling operations.

Geomechanical model calibration

To do the model calibration, information from thenext sources was used: well drilling reports, well testsand experimental tests.

From experimental geomechanical data the Mohr-Coulomb failure envelope was obtained (evaluatedat the Rosablanca formation which was the reservoirtarget formation). This failure envelope was used tocalibrate geomechanical properties in the wellborestability model. In a related fashion, the well testsavailable supply enough data to calibrate pore pressureprole as shown in Figure 10. From this calibration the


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The performed application demonstrated the linear elas-tic theory validity for wellbore instability modeling.

This application veries that well logging and for-mation tests can be used to obtain necessary datafor geomechanical formation modeling.

It was proved that the obtained data from laboratorytests and drilling reports can be used for wellborestability calibrations.

Continuous updating of geomechanical modelsleads to more accurate predictions, therefore bettermodels.

ACKNOWLEDGMENT

The authors are greatly indebted to the Cagi 1exploration well workteam (Exploration Department,Ecopetrol S.A.), to the Geomechanical and NaturallyFractured Reservoirs research workteam (ICP, Eco-petrol S.A.) and to the exploratory wells supportingtechnologies workteam (ICP, Ecopetrol S.A.) whocontributed to this study over an one-year period.

REFERENCES

Adam, T., Bourgoyne, Jr., Keith, K.M., Martin, E.C., & F.S.Young Jr. (1986). Applied drilling engineering, SPEtextsbook series(Vol.2). Richardson, Texas, USA, FirstPrinting SPE, 502 pp.

Aird, P. (2001). Extended leak off testing, kingdom drilling,Rev: 1.0 03/01/01, www.kingdomdrilling.co.uk, Fife,Scotland, Technical report, 15 pp.

Astm International (2002). Designation: D2938 95 (Reap-

proved 2002) standard test method for unconfined com-pressive strength of intact rock core specimens.ASTMInternational committee D18 on Soil and Rock, subcom-

mittee D18.12 on rock mechanics, West Conshohocken,Pennsylvania, USA.

Astm International (2002). Designation: D 3148 02: standardtest method for elastic moduli of intact rock core specimensin uniaxial compression.ASTM International committee

Rosablanca Formation and in the second one the PajaFormation was not buried at the exploratory well.

Case 1 Considering the Paja Formation overlyingthe Rosablanca Formation with a width to be about

one hundred feet. In this case it was necessary to useBosques 3 well information to model the exploratorywell geomechanical behavior. The primary assumptionto extrapolate this model was the connected hydraulicow between Paja and Rosablanca Formations. Thisresults in vertical transmissibility and so the RosablancaFormation will be overpressurized. According to this,there was an exploratory well geomechanical modelproposed which is shown in Figure 12, in this modelthe well is stable while drilling. It can be observedthat Real Formation is mechanically stable but themudweight window is narrower as it comes deeper in

to the well, Colorado and Mugrosa Formations are alsostable and they have a collapse density very close topore pressure which can represent some operationalsdifculties while drilling the well, it also can be no-ticed that Esmeralda, La Paz and Simit are the criticalformations since there is an important reduction ofmudweight window for these formations due to thereduction in tensile strength, besides of this, drillingcan be difcult since Simit is overpressured and muddensity to prevent kicks can be as much as 18 ppg.

Case 2 Considering the Paja Formation is not

present on the exploratory well. As in the earlier case inorder to model the Rosablanca Formation geomechani-cal behavior was used the information from Bosques3 well. Stress state, elastic moduli, rock strength andMohr-Coulomb failure envelope were modeled fromthe Tablazo Inferior Formation assuming connectedhydraulic ow between this formation and the Rosa-blanca Formation and geomechanical uniformity. Theresults from this modeling are shown in Figure 13. Asnoted earlier this exploratory well is also stable and thewellbore stability is reduced in the Simit Formation

even more than in the previous case.

CONCLUSIONS

The described metodology allows to predict, preventand reduce wellbore instability conditions in orderto reduce drilling costs and hazardous conditions.


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Figure 12. Cagi 1 mudweight window, case 1. left: well model; right up: Esmeralda and Simit Formation and down: Paja Formation


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Figure 13. Cagi 1 mudweight window, case 2. left: well model and right: Tablazo inferior Formation


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Geomechanical Wellbore Stability Modeling

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