-
SPE 37088
Geomechanical Design and Evaluation of a Horizontal Wellbore in
Maracaibo Lake,Venezuela: Real-DriIling-Time ApplicationMarisela
Sanchez D., Intevep, S.A., Jose R. Cabrera S., Intevep, S.A.,
Carolina Coil, Maraven, S,A.
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nacassanly mlhai lnypaition d h Sac48q of PaImlaum En@wara, b
omcefa, of mambwa Papafa prcsantad ntSPE maetings am wbjact la
pubhcabon Wcw by Ed(tcoal Commmees of m+ .SocIety ofP@trolaum
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3Mwxdt Illustrsfmm msy IKA ba CC.PIad Tha lbstract should contain
cmspuuouslcknohwd~rn.nt d +am snd by WAOM th ~paf was pre8entad
Wr!te Ltbrmmn, SPE, P O.Sox S.X?&S, Rlchardaon, TX 7S0S3-2S26
U. S A fax 01 .214.952.S4S5
AbstractA horizontal well was planned for a location in
Maracaibo
Lake, Venezuela, and it was required to asses its stability.
Apilot vertical hole was drilled including an oriented core,Several
laboratory and field techniques such as anelasticstrain relaxation
(ASR), differential strain analysis (DSA),acoustic anisotopy in the
cores, breakouts, sonic logs andfocal mechanisms were used to
estimate the in-situ stressfield. A cased-hole minifrac was
available from a nearby wellin the reservoir sand. While finishing
the drilling of the pilothole, running logs and cement-plugging,
laboratory static anddynamic tests were performed for both, cap and
reservoirsandstones. Linear elastic theory and Mohr-Coulomb
failurecriteria were used for the borehole stability
analysis,Information from the rig site about mud weight used
inunstable sections of the pilot was used to calibrate the
linearelastic model and bound the maximun horizontal
stresscomponent. As a result of the analysis, safe mud weight
limitsto prevents collapse and lost of circulation were released
tothe drillers for both, the inclined and horizontal sections.
Thewell was drilled without any stability problems. In this case
itwas shown that simple linear elastic models, when calibratedwith
field data, give reliable, first approximation values,specially
useful when real-drilling-time answers are needed.
The stability during production was studied using a 2-Dfinite
element model with a generalized plasticity constitutiveequation. A
safe drawdown to prevent failure wasdetermined. The well was
completed open hole without anyliner. It is producing above the
riginal potential without anysanding or stability problems.
Introduction
Horizontal well activities in Venezuela started aroundthe 80s
and now the companies have agressive horizontaldrilling programs.
Even though there have been succesfulcases, there have been some
horizontal well failures, as well.llse problems have @n related to
geological andpetrophysical uncertainties, formation damage in
maturereservoirs and collapses due to borehole instability.
Programsconsidering vertical pilot holes with oriented coring
havebeen deveioped to increase the chances of success inreservoirs
in which very Iittie information has been gathered[1]. This
approach has allowed to make timely evaluation ofthe reservoir rock
and in situ stresses before the horizontalhole is drilled and
therefore minimize risks fromgeomechanical and mud drilling design
points of view.
An example of such case was the planning of a horizontalwell in
a reservoir in the Maracaibo Lake area, Venezuela.This was the
first horizontal well in this field and it wasdrilled with the
purpose of draining the atic oil accumulatedagainst an adjacent
normal fault. The well was placed sub-parallel to the normal fault
(with strike N36W approximately)as shown in Figure 1. The
horizontal well was planned to beapproximately parallel to the
fault.
It was difficult to asses the geomechanical risk for ahorizontal
well in this area without having an idea of the rockstrength and
the in situ stress field. Moreover, the regionalstress environment
in the Maracaibo Lake area is verycomplicated and it was not known
if the horizontal well wasgoing to be drilled in the most risky
direction. Trying todecrease as much as possible the uncertainties,
a verticalpilot hole with 60 feet oriented coring was
drilled.Additionally, three hundred feet conventional coring
wasrecovered for sedimentological studies, The pilot-hole
andhorizontal weli plan is presented in Figure 2.Sonic logs were
added to the conventional petrophysicallogging program to asses a
strenght profile for the rock alongthe pilot hoie. A complete, but
fast rwk mechanics testingprogram including static and dynamic
measurements was
457
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2 M. SANCHEZ, J. CABRERA, C. COLL SPE 37088
developed for mechanical characterization. Several laboratoryand
field techniques were used to estimated the in-situ stressfield
including breakout orientation [2], anisotropic sonic logs[3],
Anelastic Strain Relaxation (ASR) [4], Differential StrainCurve
Analysis (DSA) [5], Acoustic Shear Wave AmplitudAnalysis (SWAA) and
Acoustic Velocity Anisotropy (AAA)[6] from samples and logs from
the pilot hole. Informationfrom a minifrac [7] and density logs
were gathered fromnearby vertical wells. The overall regional
tectonicinformation was considered at the time of interpretation
andintegration of the in-situ stress data. A borehole stability
studywas carried out to define the mud-weight operational
window.For the stability during perforation a linear elastic
analyticalmodel was used [8]. The time available for this study was
theone corresponding to the completion of coring,
cementing,logging, and other operational procedures to be done
beforethe kick-off of the inclined section, so this project
wasdeveloped as a real-drilling-time application.
The compatibility of the shales with drilling fluids
wasdetermined, showing poor reactivity. Therefore, the water-based
mud used for the inclined section would not affect thechemical
stability of the shales. However, the mechanicalstability of the
shales was not investigated. For the producingzone (horizontal
section) a saline mud was designed andoptimized to minimize
formation damage and ensure a highlyefficient mud-cake. The size
distribution of pore throats wasdetermined by microscopic analysis
and proper particle sizedistribution in the saline mud was used to
prevent excessivefiltrate [9]. Tests of mud-cake efficiency on core
from the payzone were also performed to verify the effectiveness of
themud design.
An open hole completion without any liner was suggestedand
therefore, a study of wellbore stability during productionwas
developed to determine a safe drawdown allowable forthis horizontal
well. The model used for this estimations wasa 2D finite element
model, plane strain, with several stress-strain constitutive laws
such as linear elasticity, ehstic-perfectly plastic material,
generalized plasticity [10].
Coring and Measurement of Mechanical Propertiesof the Rock
The core for geomechanical testing was taken usingaiuminum
barrei in order to minimize mechanical damageand it was preserved
with conventional procedures. Ageological visual description was
made at the rig-site and thecore was transported directly to the
lab. Sandstone sampleswere taken at the depth correspondhg to the
inclined section(cap rock) and at the pay zone. Figure 3 shows the
depth ofcoring, together with the gamma ray iog, indicating the
KOPand target point for the horizontal well. The results for
theuniaxial and triaxial static testing are presented in Table
I.The Mohr-Couiomb failure envelope was represented by a
straight line for the rocks at the pay zone. This was
notpossible for the rock at the KOP in which several envelopeswere
drawn at different stress levels as is presented in Figures4 and 5.
The line corresponding to the appropriate stress ieveiwas chosen
(Fig 5) for the failure parameters for the caprock. The overbalance
expected for this well was about 2000psi so the initial iinear
envelope was considered to beapplicable to the conditions around
the wellbore duringdrilling. However, it is known that for weaker
rocks it isconvenient to consider a non-linear criterion [11].
Dynamicmeasurements were made under hydrostatic conditions [12]and
during triaxial compression [6]. Comparison betweendynamic and
static values for Young Modulus are presentedin Figure 6 together
with the acoustic vaiues inferred from themechanical logging [13].
Laboratory dynamic vaiues werehigher than sonic log vaiues and
static values by a factorranging from 1.2 to 2, respectively, at
the measured points.
In situ Stress Determination
Most of the uncertainties in borehole stability problemsare
reiated to the in-situ stress determination and rockstrength [14].
In this case a great effort was made to obtain areasonable estimate
of magnitude and orientation of in-situstresses within the time
limitations. Going from the regionaltectonic scenario to the iocal
measurements was the approachin order to aid in the interpretation
of local field and coretesting results. The structural history of
the Maracaibo Basinis characterized by a complex structural system
dominated byregional fauits. Repeated tectonic events and
reactivation offaults are reported [15]. A perspective of the
structural systemand results of focal mechanism studies for the
region arepresented in Figure 7. The focai mechanisms allowed
todetermine an orientation of the compressional principal axiswhich
rotates from an E-W direction beiow 10 degrees ofNorth latitude to
a transitional direction NW-SE and then toa NNE-SSW direction, The
E-W orientation of thecompresional axis corresponds to the
compression of theNazca piate and the NNE-SSW orientation resuits
from themovements of the Caribbean plate agaist the South
Americanplate [15], However, on a local basis the stresses may
beredistributed depending on the proximity to iocal fauitssystems,
making the interpretation difficult. The focalmechanism study also
shows that at great depths principaistresses are not horizontal and
vertical as is typicallyassumed. Whether this situation prevails at
the upper layerswas difficult to asses at the time, but certainiy
somesensibility analysis shouid be done in the models to
considerthis possibility.
Breakouts (3 inch size) were observed from the 4-armcaliper log
run in the piiot hole together with an image log.The maximum
elongation azimuth was 118 degrees (N62W)at 9500 feet. Therefore,
the direction of the maximum
458
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SPE 370W GEOMECtiANICAL DESIGN AND EVALUATION Of A HORIZONTAL
WELBORE IN MARACAISO LAKE, VENEZUEU REAL-DRILLING-llME APLICAllON
3
principal stress component is interpreted as N28E. A soniclog
with shear velocity anisotropy capability [13] showed amaximum
stress direction between 15 and 20 degreesazimuth. This tool
indicated a rotation in the direction ofmaximum stress to E-W at
upper layers (depth). ASR testingwas also performed over three
pieces of the oriented coreimmediately after the recovery [16]. The
quality of the datawas not excellent and only one of these tests
were consideredreasonably fair, resulting N135E. Anisotropic
acousticvelocities and directional amplitude of acoustic shear
wavesand differential strain curves [17] were obtained for this
core.A summary of these results is shown in Figure 8. Note thatthe
breakouts and anisotropic sonic log are showing a NNE-SSW trend.
The ASR and differential strain analysis werecapable to see the
maximum tectonic event in the area, whichis the E-W compression
from the Nazca plate, The laboratorysonic tests @AA, SWAA),
however, are showing thetransitional trend.
In the interpretation of the set of data available for
in-situstress direction it is important to make an assessment of
thequality of the information. A general quality indicator is
thestandard deviation of the stress direction for a single
test.Particularly for the ASR, art absolute magnitude of
relaxationabove 100 microinches is necessary for a
reasonableinterpretation[ 18]. For a strong stress anisotropy
thedifferential strain analysis should be showing the
threeprincipal stresses at 90 degrees apart. Breakouts should
bedistinguished from washouts and wall geometry induced bynatural
fractures or rock heterogeneity. Depending on thequality, a weight
can be given to the data at the time ofintegrating conflicting data
as is presented in Figure 8. Afterall these considerations, for
this case a higher weight wasgiven to the breakouts, the
anisotropic sonic log (DSI) and thelocal focal mechanism closer to
the Lake area.
It was evident that it is necessary to gather moreinformation
about in situ stresses and also clarify the featuresthat each of
the tests are capable of measure to explain thedifferent
results.
Magnitude of in situ stresses:
A cased hole minifrac was performed at a nearby well. Amtdtirate
tests was done before and downhole pressuremeasurements allowed to
determine breakdown and closurepressures as 7650 psi and 5560,
respectively. A back-analysis of the minifrac data was considered,
ignoring theeffect of casing, cement and perforations, using the
followingwell known equation :
~=%,-K@b-p.)-2p.This gave a first rough idea of maximum
horizontal stressmagnitude. The resulting gradient was 0.78
psi/foot. Frombreakouts and from an instability event in which mud
weight
had to he raised from 66 to 72 pcf to enhance stability, it
wasalso possible to obtain another bound by assuming shearfailure
at the borehole. Another bound for the maximumhorizontal magnitude
was determined by assuming shearfailure, such that the Mohr-Coulomb
failure equation wasapplied and the maximum horizontal stress
obtained by back-anal ysis:
a,= co+or@$r)
That is, knowing the strength of the rock, the pore pressureand
the minimum in-situ stress magnitude and orientation,the maximum
horizontal stress can be back-calculated. The Jfunction was solved
using an elastic approach with theBORE3D model. The resulting
gradient was 0.84 psi/foot.
The vertical stress component was estimated from densitylogs of
nearby wells resulting in a gradient of 0.95 psi/feet.The density
logs covered part of the total depth; however, anuniform gradient
was assumed because of lack of betterinformation.
The results of measurement of in-situ stress field indicatedat
this location a stress regime characterized by SvXSH>Sh,which is
characteristic of a passive basin with typical featuresof normal
faults. If the set of normal faults were responsiblefor the stress
state at this point, then the minimum horizontalstress component
would be perpendicular to the normal fault.However, it seems as a
first approximation from the datagathered, that the minimum
horizontal component isparallel to the normal fault.
Stability analysis during drilling
The aim of this analysis is to provide minimum mudweight
required to prevent collapse and maximum mudweight required to
avoid exceeding the fracture gradient as afunction of inclination
and orientation of the well. Theanalysis consists in calculating
the stress concentration in thevicinity of the wellbore, which are
mainly function of rockproperties and in situ stresses (magnitude
and orientationrelative to the wellbore, and comparing these
stresses with afailure criterion for the rock. I%is analysis has
been doneusing a linear elastic model and Mohr-Coulomb
strengthcriterion. An impermeable borehole wall was considered,
i.e.100 % efficient mud cake. This is supposed to be a
gmdassumption is this case because the mud designed proved tomake
an efficient mud cake in laboratory tests.
By the time of the initiation of the inclined well,
theorientation of the in situ stresses was based on
preliminaryresults (maximum horizontal stress component
sub-parallel to
459
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4 M. SANCHEZ, J. CABRERA, C. COLL SPE 370ss
the fault and to the horizontal well). The input parameters
forthe inclined section and the target zone for the horizontalwell
are listed in Table II. The operational mud weightwindow for these
zones as a function of hole inclination arepresented in F@ures 9
and 10. This was the wome case forthe insitu stress regime
considered aod the lowest weight thatthe saline mud can preset
(typically around 80 PCO waswithin the safe values. Comparing these
curves, it is seen theinfluence of rock parameters in the
operational mud weightwindow. For inclinations higher than 40
degrees theoperational window is broader for the target zone.
Therefore,it would be misleading to apply this information for
mudweight values for the inclined section once the
higherinclinations are achieved. These may often happen becausethe
information is mostly recovered from the pay zone. SoNclogs can
help on this task by offering a strength profile rdongthe well, but
they should be properly calibrated to lead tomeaninfulg results. In
this experience the sonic loginterpretation was more conservative
than the linear elasticmodel, main] y because there was not time to
perform acareful calibration of the log.
The operational window for the mud weight was releasedto the
drillers by the time the pilot hole was being finished,which made
possible its application in the field. Thehorizontal well was
drilled with mud weight of 80 pcfwithout any stability problems.
There is not a caliper log toverifjf if the hole was exactly in
gauge, there is no evidence ofhole breakouts or washouts at both,
the inclined andhorizontal sections.
It is known that the linear elastic models can be
overlyconservative, unless carefully calibrated with field data[
14],and may overestimate the sensitivity of the hole stability
towell trajectory [18]. Whether lower mud weights could beused
safely and obtain in gauge holes in the field studied mustbe
investigated as the horizontal well activities increase. Moredata
for in situ stresses and post-analysis using other modelswill allow
to have a better calibration and get moreeconomic designs.
Stability during production
An open hole completion without any liner was anopportunity in
this well due the tehtively high strength of therock. Therefore, it
was needed to determine a safe drawdownto avoid sanding or collapse
of the hole. First attempt with thelinear elastic model predicted
failure for any drawdownconsidered. The geomecam cal module of the
program STARS[IO] was then used. This is a 2-D, plane strain
program whichcan consider several constitutive models of the rock
such aslinear elasticity, elastic perfectly plastic material and
ageneralized plasticity model. A generalized plasticityconstitutive
equation was used because it can model in a mom
realistic way the rock behavior. The constitutive model
isdescribe in reference [19].
The modeling was done as if the horizontal well wetedrilled in
the direction of the minimum in situ stress.Therefore, the
transverse section of the hole will have asboundary stresses the
vertical and the maximum horizontalstress components. Table II and
Table 111present the strengthinput data and the parameters that
define the constitutiveplasticity model [19]. These values were
estimated from thestress-strain curves obtained in the lab. Figures
11, 12 and 13represent the maximum principal stresses induced
around theborehole for three different drawdown values: 500 psi,
6(HIpsiand 800 psi. Note how the stress concentration and
thestressed zones increase as the drawdown increases. To assesif
these stress conditions lead to failure of the rock, the
mostcritical stress state at the borehole wali was plotted in
theMohr space to compare with the Mohr-Coulomb failurecriterion. As
shown in Figure 14, as the drawdown increases,the points grow
toward the failure, The analysis was carriedon up to a drawdown of
800 psi because this was a particularoperational restriction
related to gas lift. Ihe horizontal wellis now producing 2tM0 SBPD
steadily without sandingproblems or any stability problems.
Conclusions and Recommendation
Operational window for mud weight was given for thehorizontal
well drilling during drilling time. This studyallowed to make an
open hole completion without any linerfor a horizontal well in a
sandstone reservoir for the fwsttime in Venezuela. This type of
analysis should be done in anormal basis for horizontal wells to
study its integrity duringdrilling and during production.
The calibration of the linear elasticity model must berefined
including more field data as horizontal drillingactivities are
increased. This is important in order to be lessconservative and
maintain, at the same time, the advantagesof simplicity, little
computational and time effort of thesemodels. Calibration of the
mechanical logs will also enhancestrength predictions from this
source in order to obtain lessconservative designs for all the well
profile and not justsome discrete point in which cores are
available.
The magnitude of the maximum horizontal stresscomponent can be
estimated through back-calculations byconsidering plasticity models
and geometry of the breakouts.
It was evident in this horizontal well that the design
ofmechanical sealers for the drilling mud minimizedformation
damage, which is a vmy important issue in thistype of mature
reservoitx. IWs is also a way to enhancestability of the well as
the infiltration and extensive porepressure modification around the
wellbore is pmwented.
460
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SPE 37tma GEOMECHANICAL DESIGN MD EVALUATION G A HORIZWWM ~E IN
MARACNSO ME, VENEZUEIA: REAL-ORILUN&TIME APLICATKX 5
Nomenclature:
PoKm
OHcrhGrdco, 1$
PbEv
UcsToCp,crw. @Pn
a
m
= reservoir pore pressure.= poroelastic parameter (1S K< 2).=
vertical in situ stress coqtponent.= maximum horizontal in situ
stress component.= minimum horizontal in situ stress component.=
radial stress at the borehole wall.= tangential stress at the
borehole wall.= cohesion and fnction angle from A40hr-Coulomb
strenght criterion..= Breakdown pressure.= Youngs modulus.=
Poissons ratio.= unconfined compressive strength= tensile strength
of the rock.= peak and residual cohesion
= initial and peak fnction angles,= cohesion softening
parameter= friction hardening parameter= yieidfinction
curvature
AcknowledgmentsThe authors wish to thank the management of
INTEVEP,S.A. and MARAVEN S. A., for permission andencouragement to
public this paper, and also the manycolleagues who participated in
this study in the differentdisciplines: Alberto Munoz, Maria de
Blundum, GediGonzalez, Jose Luis Zkit, Gilberto Lopez, Maria
G.Faustino, Jose A. Natera, Jose A. Linarez, Lemniz Zerpa,Kerin
Urrecheaga, Jose Guzman, Ivanka Kancev, JesusBetancotrrt, Jose
Castillo and Entique Poleo.
References
1. Skopek, R.A. Rock Characterization in Reservoirs Targeted
forHorizontal Drilling JPT, December, 1993.
2. Zoback, M.D. , Moos D. , Mastin L. and Anderson, R. WellBore
Breakouts and In Situ Stress , Journal of GeophysicalResearch. Vol.
90, No. B7, pages 5523-5530, June 10, 1985.
3. Schrdmberger, Wireline & Testing, DSI SoNco dipolar
deCizathzrniento por lmagenes
4. El Rabaa, A.W.M. and Meadows, D.L. Laboratory and
FieldApplication of the Strain Relaxation Method, paper SPE15072
presented at the 56th California Regional Meeting of theSPE,
Oakland CA, April 2-4, t 986.
5. Strickland, F.G. and Ren N.K.; Predicting the lrr-Sittr
Stressfor Deep Wells Using Differential Strain Curve Anatysis,paper
SPE presented at the 1980 SPEJDGE on UnconventionalGas Recovery,
Pittsburgh, May 18-21.
6.
7.
8,
9.
10.
11.
12.
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14.
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16.
17.18.
19.
Ter-ratek, Static and Dynamic Properties and In-Situ
StressDirection, Unidad @o Treco Maraven, S.A., Caracas.February
1996.Economies, M., J., and Nolte K., G.fl Reservoir
Stimulation,Second Edition. Prentice Hall, Englewood Cliffs, New
Jersey07632, 1989.
Da Fontoura, S., Bore3D, Version 1.02, Release Alpha,
UsersManual, Rock Mechanics Consortium Report, RMC-92 13,University
of Oklahoma, 1992.Kancev, L, Guzman J.H.,Polco, E., Feranactez, D.,
Castillo J.,Betamcourt, J.S. Prrrebas de Dafio de Forrnacion con
IodoThixsat en el pozo VLC-1 184, INTEVEP Intemat Report,
inprogress, 1996.Rodriguez, H., Fung, L.S-K, Silva, R., zerp~ L.
and Wan,R.G.,Themral Simulation of Horizontal Wellbore Stability
inUnconsolidated Heavy Oil Reservoirs, paper SPE 37102presented at
1996 International Conference of Horizontal WellTechnology,
Calgary, Canada.Mclean, M.R. and Addis, M. A,: Wellbore Stability
Anatysis:
A Review of Current Methods of Anatysis and llseir
FieldApplication paper SPE 19941 presented at the 1990IADC/SPE
Drilling Inference, Houston, Feb.27-March.2.Sanchez D, M., Cabrera
J. R., Munoz R., A. , Faustino M.,Natera J. Diseno Integmdo del
Pozo Horizorrtat VLCI 184,hrtemal INTEVEP Internal Report, in
progress, 1996.Schhtmberger-GeoQuest., VICI 184 Bkque 111,Resumen
deProcesarniento e tnterpretacion, Maraven, S. A., Caracas.,Febmary
16, 1996.Morita, N. Uncertainty Anatysis of Borehole
StaMlityProblems, paper SPE 30502 presented at the AnnualTechnical
Conference & Exhibition, Datlas, October 22-25,1995.Malave G.
Deforrnacion Corticaf y Sisrnicidad SupeKlcial
Reciente en el Occidente de Venezuela lmplicacionesTectonics
Regionafes. INTEVEP Internal Report INT-03279,96.Halliburton., ASR
Testing at the Maracaibo Lake, Venezuela,Well VLC-I 184, Maraven S.
A., Ciudad Ojeda (LasMorochas), Zulia, Venezuela., January 14,
1996.Abass H. Personnal Communication , 1996.Law, N.C. and McLean,
M.R. Assessing the Impact of
Trajectory on Wells Drilled in an Overthrust Region pap?rSPE
30465 presented at the 1995 SPE Annual TechnicalConference and
Exhibition, Dallas Oct. 22-25.CMG, Wellbore Stability Simulation
for Vertical andHorizontal Wells, INTEVEP S.A. Dec., 1995.
461
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6 M. SANCHEZ, J. CABRERA, C. COLL SPE 37066
Depth(feet)
9483109483294846
9483 1095773957729579795801095772
Confining Peak Youngs Poissonpressure Stress Modulus Ratio
(psi) (Psi) (10 bpsi) (10 bpsi)
o258255259979
00
258255847527
551115415209492632141603995147522659032549
1,52 0,382,45 0,162,00 0,112,27 0,131,66 0,351,26 0,255,00
0,283,06 0,213,39 0,14
Table I. Uniaxial and Triaxial Testing Results.
Table II. Geomechanical Input Parameters
462
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SPE 37088 GEOMECHANICAL DESIGN ANO EVALUATION OF A HORIZONTAL
WELBORE IN MARACAIBO IAKE, VENEZUEIJ! REAL-DRILLING-TIME
APPLICATION 7
,
Fig. 1 Location of the Horizontal Well in theField
lm 1s,oOO
*MO I
l
Ak Imh 9.7W mm- B7ar
3-L...Fig 2. Well Profile and Completion Design.
+-=-+ CORINGrrzs=--l I ~ w~K.O.P
-d,!. .= em t TrmmalsJ
4-4ShaSecomD.s677 Tri.walsas HORIZONTALTARGET- muadcama.m
*am.
Fig
Fig 3. Depth of coring in the Vertical Pilot Hole.
463
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8 M. SANCHEZ, J. CABRERA, C. COLL SPE 37088
Fig 7. Focal Mechanisms Analysis.
-wA/iNosl
w DoL FOCALMm-l.
o
Fig 8. In situ Stress Analysis from Different Sources.
Fig 10. Operational Window the for Pay Zone (mudweight vs.
inclination).
. .......s . . .. m.,.,=.,,,,. . .. ,,.. .,, .... .
~.. ..,-. .-AZ-!-.,. ., ..w., !; .;. . ..-,, . . .
Fig 11. Maximum Effective Principal Stress atDrawdown = 500.
464
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SPE 37088 GEOMECHANICAL OESIGN AND EVALUATION W A HCfUZONTAL
WELSCX?E IN MARACN80 MI(E, VENEZllE~ REAL.DRKLIN5TIME APLICAllON
9
Fig 12. Maximum Effective Principal Stress atDrawdown = 600,
Ii
StKlu
00 swo MmrBma120u092soo 03mo 35ao
Noru181strc.s%psiFlg 14. Mohr-Coulomb Failure Envelop
Showing Sress State for IncreasingDrawdown Conditions.
Fig 13. Maximum Effeetive Principal Stress atDrawdown = 800.
465
