3-d Geomechanical Modeling and Wellbore Stability Analysis in Abu Butabul Field

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3-D Geomechanical Modeling and Wellbore Stability Analysis in AbuButabul FieldLi Qiuguo, Xing Zhang and Khalid Al-Ghammari, Schlumberger; Labib Mohsin, Fadi Jiroudi, Ahmed Al Rawahi,Oman Oil Exploration & Production LLC

Copyright 2012, Society of Petroleum Engineers

This paper was prepared for presentation at the Abu Dhabi International Petroleum Exhibition & Conference held in Abu Dhabi, UAE, 11 –14 November 2012.

This paper was selected for presentation by an SPE program committee following review of information contained in an abstract submitted by the author(s). Contents of the paper have not beenreviewed by the Society of Petroleum Engineers and are subject to correction by the author(s). The material does not necessarily reflect any position of the Society of Petroleum Engineers, itsofficers, or members. Electronic reproduction, distribution, or storage of any part of this paper without the written consent of the Society of Petroleum Engineers is prohi bited. Permission toreproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of SPE copyright.

AbstractAbu Butabul Field is located within onshore Oman Block 60 in the Western region of the Central Oman Desert (Figure 1).

Gas-condensate was discovered in the field in 1998. The main reservoir is the Cambro-Ordovician clastic Barik formation,

which is buried over 4200 m below sea level with very low porosity and permeability. Wellbore instability related drilling

problems were encountered while drilling most of the appraisal wells in the field. The problems were mainly in the shallower

Natih and Nahr Umr formations, Gharif formation and deeper Safiq, Ghudun and Mabrouk formations. A geomechanical

modeling study was conducted in the field to understand the causes of the wellbore instability problems and to provide

recommendations for drilling new wells.

Data from nine wells were analyzed and used for the construction of 1-D mechanical earth models. Rock mechanical testing

data on core samples and pressure and stress memasurement were integrated in the models. Wellbore stability analysis of

those wells provided insight into the causes of the wellbore instability problems. To predict wellbore stability at any location

in the field more efficiently and capturing the lateral formation property variation as indicated by the seismic data, a 3-Dgeomechanical model was constructed and subsequently used for predicting wellbore stability for new wells to be drilled in

the field and hydraulic fracturing pressures for fracturing stimulation of horizontal wells.

This paper describes the process of constructing the 1-D mechanical earth models, performing wellbore stability analysis for

the appraisal wells, , Integeration of 3D seismic Inversion, constructing the 3-D geomechanical model, predicting wellbore

stability for new wells using data contained in the 3-D model and post-drill wellbore stability analysis of the planned wells.

IntroductionThe Abu Butabul discovery is situated on a regional high that lies between the two Early Cambrian salt basins (Ghaba and

Fahud) that provided the principal source kitchens for northern Oman. The Barik Sandstone comprises a prograding, braid

delta plain/shoreface succession. The top seal is provided by marine mudstones of the Mabrouk Member (Lower Ordovician)

and the base seal is formed by marine mudstones of the Al Bashair Member (Upper Cambrian). Internal seals are likely to be

associated with intra-Barik marine flooding surfaces. The Abu Butabul structure is a gentally dipping 3-way dip closure on

the footwall of a down to the west throwing fault. Figure 2 shows the stratigraphy in the block. The Barik Sandstone is the

principal reservoir interval that has been appraised in the structure.

Wellbore instability related drilling problems accounted for a large portion of the non-productive time while drilling. Many

of these wellbore instabilities are induced by excessive stress concentrations at the borehole wall and inadequate mud

support. Geomechanical analysis of rock mechanical properties, pore pressure, stresses in the far field and around the

wellbore can help to determine the appropriate mud weight for maintaining wellbore stability. Rock mechanical properties,

pore pressure and in-situ stresses need to be determined based on data from existing wells before wellbore stability analysis

can be performed. Such analysis helps to understand the borehole failure mechanisms and predict wellbore stability for other

well trajectories. If wellbore stability prediction needs to be conducted for a new well location prior to drilling the well based

on the single well geomechanical model, formation properties at the new well location are typically assumed to be the same

as those at the offset well location. With the advancement of seismic interpretation and geostatistics technology, the lateral


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variation of formation properties can be characterized with seismic data and geological modeling. A 3-D geomechanical

model capturing such lateral variation will bring more accuracy in terms of formation properties to wellbore stability

analysis. Furthermore, stress variations due to formation property changes and geological discontinuities such as faults and

fractures can also be better characterized with a 3-D geomechanical model. The more accurate description of formation

properties and stresses will result in more accurate wellbore stability predictions for any locations in a field prior to drilling.

Once a 3-D geomechanical model is constructed, all geomechanical data required for wellbore stability analysis are available

at any location within the 3-D volume. This will also reduce the amount of time required for predicting wellbore stability for

well planning at new well locations.

The objectives of the Abu Butabul geomechanical modeling study were to characterize rock mechanical properties, pore

pressure and in-situ stresses based on nine appraisal wells, understand wellbore intability mechanisms in those wells with

wellbore stability analysis, construct a 3-D geomechanical model for the field so that wellbore stability prediction can be

performed for any future well in the field with data contained in the 3-D model.

1-D Mechanical Earth ModelA 1-D Mechanical Earth Model (MEM) is a description of rock elastic and strength properties, in-situ stresses and pore

pressure as a function of depth, referenced to a stratigraphic column (Plumb et al, 2000). In order to determine formation

mechanical properties, pore pressures, in-situ stress orientation and magnitudes, 1-D MEMs were constructed for nine

appraisal wells in the Abu Butabul field. The MEM consisted of continuous profiles of the following rock mechanical data

and parameters along the well trajectories:

Mechanical stratigraphy, the differentiation of clay-supported rock from grain-supported rock. Formation elastic properties, including dynamic and static Young’s modulus and Poisson’s ratio.

Rock strength parameters, including unconfined compressive strength (UCS), friction angle and tensile strength.

Pore pressures.

In-situ stress state, including the azimuth of the minimum horizontal stress, magnitudes of vertical stress, minimumand maximum horizontal stresses.

Wireline logs including compressional slowness, shear slowness, bulk density, gamma ray etc. were used to compute log-

derived rock elastic and strength properties and in-situ stresses. Rock mechanical testing data were available from six of the

nine appraisal wells. These tests include triaxial compression tests, unconfined compression tests, rock scratch tests and

Brazilian tensile strength tests. The core samples tested were taken from Mabrouk, Barik and Al Bashair formations. Static

Young’s modulus were measured on both vertical and horizontal core plugs, the values obtained were similar which

demonstrated that the rock samples were relatively isotropic. Correlations for calculating static Young’s modulus, static

Poisson’s ratio, UCS and friction angle were established based on the rock mechanical testing data and wireline logs as

shown in Equations 1 to 4. Figure 3 shows the Comparison between log-derived elastic and strength properties and those

measured on core samples for Well ABB-7.

8531.00251.2 dyn sta E E (1)Where Esta is static Young’s modulus, Mpsi, Edyn is dynamic young’s modulus, Mpsi and is total porosity.

1.0 dyn sta (2)

Where sta

is static Poisson’s ratio and dyn is dynamic Poisson’s ratio.

2026.21808.0 GUCS (3)

Where UCS is unconfined compressive strength, MPa and G is dynamic shear modulus, GPa.

215.6814.355.29 VCLVCL FANG (4)

Where FANG is friction angle, degrees, is total porosity and VCL is volume of clay, fraction.

Vertical stress at any point in the formation is equivalent to the weight of the formation materials above and it was computed

by integrating the bulk density log. The pore pressure was computed based on measured pore pressures and the mud weight

used while drilling the well. The minimum horizontal stress azimuth was determined based on the observation of borehole

breakout and drilled induced fractures from FMI* borehole image and dual caliper logs. It ranges from N140°E to N160°E

across the field, which is consistent with the regional stress direction. The magnitudes of the minimum and maximum

horizontal stresses were determined using the poro-elastic horizontal strain model as shown in Equations 5 and 6 (Fjaer et al,


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2008). The minimum horizontal stress was calibrated based on data from leakoff tests, extended leakoff tests and closure

pressures interpreted from hydraulic fracturing pressures. The magnitude of the maximum horizontal stress cannot be directly

measured. It was inverted from analyzing fracture initiation pressures and verified by conducting wellbore stability analysis

and matching simulated shear failures and tensile failures with observed borehole breakout and drilling induced fractures.

The stress regime in majority of the formations was found to be strike-slip with the vertical stress being the intermediate

principal stress. Figure 4 shows the 1-D MEM and wellbore stability analysis for Well ABB-2. The mud weight window and

the agreement between the predicted borehole breakout and the borehole enlargement as can be seen from the caliper log

indicate that wellbore instability related drilling problems were due to borehole failures induced by inadequate mud weight.Borehole failures mainly occurred in Natih, Nahr Umr, Gharif, the bottom part of Al Khalata, Safiq, Ghudun and Mabrouk

formations. This is consistent with the drilling observations.

H h pV h

E E P

22111

21

1

(5)

H h pV H

E E P

22111

21

1

(6)

Whereh

is minimum horizontal stress, psi, H

is maximum horizontal stress, psi, V is vertical stress, psi, is static

Poisson’s ratio, is Biot coefficient, p P is pore pressure, psi, E is static Young’s modulus, h is tectonic strain in the

minimum horizontal stress direction and H is tectonic strain in the maximum horizontal stress direction.

Formation mechanical properties and in-situ stress magnitudes were also obtained for the other appraisal wells. Wellbore

stability analyses for these wells revealed similar borehole failure characteristics.

Integeration of 3D seismic InversionThe Simultaneous Inversion workflow was applied on Abu Butabul Field in order to map out the good porosity sand

distribution as well as getting the elastics property volumes to be integrated in the 3D geomechanical model.

Elastic rock properties P-Impedance, S-Impedance, density, and Vp/Vs can be detrmined from Simultaneous inversion by

applying the full Zoeppritz equations to the angle of incidence range response in seismic data. Intensive QC steps and

parameters testing were applied through intermediate stages to insure that the output volumes are verified with blind testwells.

Editing and conditioning well logs is essential to start the workflow with, so detailed petrophysical analysis and rock physics

modeling was applied to correct for all kind of problems well logs might have such as cycle skipping, washouts, missing

intervals, etc. these edited well log were calibrated with seismic to ensure good sonic and density logs that are tied with

appropriate events on the seismic data. Then, wavelets are simultaneously extracted from angle gather stacks at multi-well

location. After that, the low frequency elastics property volumes for P-impedance, S-impedance and density are built using

horizons, stratigraphy information and filtered log data. Eventually, the simultaneous AVO inversion is run on the angle

gather stacks with their respective wavelets and as a result P-Impedance, S-impedance and Vp/Vs property volumes are

generated and verified by direct comparison with the well logs.


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3-D Geomechanical Model ConstructionThe 3-D geomechanical model construction consisted of three main steps, model geometry construction, model properties

definition and initial stress modeling. The geometry of the 3-D geomechanical model was based on an existing geological

model which was built from the surface to the base of Miqrat formation. Two bounding faults which intersect both

overburden and the reservoir and twenty six other faults which only exist in the Barik reservoir were included in the model.

The existing model contained approximately 1.7 million grid cells and spanned 17.2 km in the east-west direction and 46.7

km in the north-south direction. The depth ranged from 4453 m to 5002 m sub-sea level with a sea-water depth range of 54.1

m to 198.6 m. Figure 5 shows the geometry of the existing geological model.

The finite element mesh was constructed based on the existing grid with 8-noded brick elements. To reduce the impact of

boundary constraints on simulated stresses in the 3-D model, twenty additional rows of cells were added to each lateral

direction and 25 additional layers were added underneath the existing model. The base of the embedded model reached 145

km in depth and the total number of elements in the model was approximately 4.9 million.

Material elastic and strength properties need to be defined in all the model elements before initial stress modeling can be

performed. The Mohr-Coulomb constitutive model was adopted to simulate formation yield and plastic deformations. The

required elastic and strength properites include Young’s modulus, Poisson’s ratio, cohesion, friction angle, dilation angle a nd

tensile strength. All these properties were already available at the nine well locations from the 1-D MEMs. Population of

material properties throughout the entire 3-D model was carried out using a co-kriging algorithm. Acoustic impedance from

a seismic inversion study was used to guide the distribution of those material properties. The variation of formation

properties between well locations were therefore characterized with the seismic information as well as well data in majority

of the model area because the size of the seismic inversion area was slightly smaller than that of the geological model. Figure

6 shows the distribution of Young’s modulus in the top Barik layer.

The material properties of faults were defined separately based on the mechanical propertis of the intac rock surrounding the

faults and our data base. The properties include normal stiffness, shear stiffness, cohesion, friction angle and dilation angle.

The elements which were intersected by at least one fault were defined as fault elements and the material properties of those

elements were calculated using both intact rock properties and fault properties based on the equivalent material methodology

(Zhang et al, 2011). With the equivalent material methodology, the deformational behavior of both the intact rock and the

faults can be accounted for in the fault elements. Relative displacement is allowed to develop within the fault plane.

Pore pressures in the 3-D model were calculated using pressure gradients estimated from 1-D MEMs. In order to simulate the

initial in-situ stresses in the 3-D model, horizontal stresses were applied to the boundaries of the model. These horizontal

stresses were oriented in the appropriate directions based on the estimated minimum horizontal stress azimuth. The horizontal

stress gradients were determined from the horizontal stresses contained in the 1-D MEMs. The applied stresses at the base ofthe model were based on the gravity loading computed using the density in all the elements.

Initial stresses were simulated in all the elements after all the required data were provided. In order to validate the initial

stresses in the 3-D model, principal stresses in the elements along the trajectories of the nine appraisal wells were extracted

and compared with the stresses contained in the 1-D MEMs. Agreement between the two solutions was achieved at all nine

well locations. Figure 7 shows an example of the initial stresses comparion between the 1-D and 3-D solutions for Well

ABB-5. The agreement indicated that a representative stress state in the 3-D geomechanical model had been established.

Figure 8 shows the magnitude of the minimum principal stress at the top layer of the Barik formation. It was noted that faults

have significant impact on the distribution of stresses, both stress magntidues and direction were altered in the vicinity of

faults. Figure 9 shows such changes of stress tensor in two faulted regions.

Since there was no production in the field yet, the initial stress state was also the present day stress state and this allows the 3-

D geomechanical model to be used for wellbore stability analysis for additional appraisal wells and development wellsdrilling.


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Wellbore Stability PredictionAfter the 3-D geomechanical model was constructed, wellbore stability prediction was performed for several development

wells to be drilled in the field. For each well, rock mechanical properties, pore pressure and the six components of the stress

tensor for cells intersected by the planned well trajectory were extracted from the 3-D geomechanical model. The mud weight

windows comprising of pore pressure, fracture gradient (from minimum principal stress), borehole breakout pressure predient

and fracture initiation pressure gradient were then calculated based on the extracted data for the planned well. Figure 13

shows the predicted mud weight windows for one of the planned development wells. Comparing to Figure 13, it can be seen

that the formations that require higher mud weight for preventing borehole breakout are similar to those as observed in theappraisal wells, mainly Khuff, Gharif and Safiq. The mud weight of 1.3 to 1.35 g/cc is required to drill without significant

borehole breakout. The fracture initiation pressure gradient is as low as 1.5 g/cc in the Al Khalata formation, hydraulic

fractures might be induced if the Equivalent Circulating Density (ECD) exceeds that while drilling or tripping. However,

since the fracture gradient is 1.65 g/cc, significant losses were not expected if the ECD is less than that, even hydraulic

fractures could be induced near the wellbore. With the designed mud weight following the mud weight windows, the drilling

of this development well proved to be successful in terms of wellbore stability. No significant borehole instability related

drilling problems were encountered.

Conclusions1. 1-D mechanical earth modeling and wellbore stability analyses for nine appraisal wells revealed that majority of the

borehole enlargement and wellbore instability related drilling problems were caused by shear failures of the

formation due to inadequate mud weight while drilling.

2. A 3-D geomechanical model which was constructed using 1-D MEMs, existing geological model and seismicinversion results is essential to characterize lateral variation of mechanical properties since it gives better prediction

throughout the full field.

3. Stresses that were simulated in the 3-D geomechanical model with finite element modeling have good agreementwith the stresses contained in the 1-D MEMs at the appraisal well locations, which indicate that the stresses in the 3-

D model were representative of the in-situ stresses in the field.

4. The presence of faults has a significant effect on the stress state, both stress magnitudes and stress direction arealtered in the vicinity of faults.

5. The 3-D geomechanical model enabled efficient wellbore stability analysis for planned wells in the field.

6. Wellbore stability analyses for planned wells provided mud weight recommendations on drilling these wells and itwas confirmed that those recommendations helped to maintain borehole stability and success of the drilling.


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AcknowledgementsThe authors wish to thank Oman Oil Exploration and Development LLC and Schlumberger for permission to publish this

work.

Nomenclature

= Biot poroelastic coefficient

= Poisson’s ratio h = Minimum horizontal stress, psi

H = Maximum horizontal stress, psi

v = Vertical (overburden) stress, psi

h,H = Horizontal strains

= porosity

E = Young’s modulus, Mpsi

ECD = Equivalent circulating density

FANG = Angle of internal friction, degrees

G = Shear modulus, Mpsi

MEM = Mechanical earth model

P p = Pore pressure, psi

UCS = Unconfined compressive strength, psi

VCL = Volume of clay

References

Fjaer E., Holt R.M., Horsrud P., Raaen A.M. and Risnes R., Petroleum Related Rock Mechanics, 2 nd Edition, Elsevier, 2008.

Plumb, R. A., Edwards, S., Pidcock, G. and Lee, D., The Mechanical Earth Model Concept and its Application to High-Risk

Well Construction Projects, Paper SPE 59128 presented at IADC/SPE Drilling Conference held in New Orleans, Louisiana,

USA, February 23-25, 2000.

Zhang X., Koutsabeloulis, N., Kristiansen T., Heffer K., Main I., Greenhough J. and Hussein A.M., Modelling of Depletion-

Induced Microseismic Events by Coupled Reservoir Simulation: Application to Valhall Field, Paper SPE 143378 presented at

the SPE EUROPEC/EAGE Annual Conference and Exhibition held in Veenna, Austria, 23-26 May 2011.


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Figure 1. Location map of Abu Butabul Field. Figure 2. Stratigraphy of Abu Butabul Field.


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Figure 3. Comparison between log-derived elastic and strength properties and those measured on core samples for Well

ABB-7.


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Figure 4. 1-D MEM and wellbore stabilty analysis for Well ABB-2.


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Figure 5. Simultaneous Deterministic Inversion Workflow

Figure 6. Inversion results

Figure 7. Cross-plot well vs. seismic inverted Elastic properties


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Figure 8. The existing geological model of Abu Butabul Field.

Figure 9. Distribution of Young’s modulus in the top Barik layer.


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Figure 10. Comparison of stresses at the location of Well ABB-5 from 1-D and 3-D solutions. TZSP is vertical stress, TXSP

is minimum horizontal stress, TYSP is maximum horizontal stress from 1-D MEM. Total P1 is minimum principal stress,

Total P2 is intermediate principal stress and Total P3 is maximum principal stress from the 3-D geomechanical model.

Figure 11. Minimum principal stress at the top Barik layer.


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Figure 12. Stress magnitude and direction alteration in the faulted region.

Figure 13. Predicted mud weight windows for a development well based on the 3-D geomechanical model. PP (blue) is pore

pressure gradient, Breakout (red) is the minimum mud weight required for preventing borehole breakout, P3 (light blue) is

the fracture gradient and Breakdown (black) is the maximum mud weight that can be applied before drilling induced fractures

are created on the borehole wall.

3-d Geomechanical Modeling and Wellbore Stability Analysis in Abu Butabul Field

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