1st SPWLA India Regional Conference Formation Evaluation in Horizontal Wells

Mumbai, India. March 19-20th, 2007

Log-based Geo-mechanical Characterization for Openhole Stability Analysis of Horizontal Wellbores

Javier A. Franquet, Baker Hughes / Baker Atlas-GEOScience, Abu Dhabi, U.A.E.

ABSTRACT

This paper presents the methodology of a rock mechanics characterization from logging data in order to examine the stability of openhole horizontal wells. This methodology investigates the borehole stability under multiple production scenarios during the projected pressure decline of the reservoir.

The stability study includes the in-situ stress tensor description of the field and the mechanical property characterization of the formations based on a constitutive model describing the microscopic processes occurring in a rock sample during tri-axial loading. The micromechanical constitute model derives continuous profiles of log-based static mechanical properties and strengths at different confining conditions. An analytical program for borehole-wall failure prediction and in-situ stress estimation predicted the shear failure zone around the borehole wall under progressive production scenarios. The risk of borehole collapse increases with a growing shear failure zone around the borehole wall. In order to prevent borehole collapse, the maximum allowable drawdown at the borehole wall interface was computed under progressive depletion scenarios. In order to address the effect of grain-cement disintegration after acid stimulations, several compressive strength degradation scenarios were also considered. The results of this type of analysis provide assistance in optimizing the horizontal well completion i.e. open-hole, cased and cemented, slotted liners, or expandable screens in carbonate reservoirs.

INTRODUCTION

Knowledge of rock mechanical properties is important for the planning of both drilling and production strategies. Traditionally, rock mechanical properties are obtained directly from laboratory triaxial tests on core samples. Hence, mechanical properties are typically only defined at discrete depths where the core samples were taken and lab values can be skewed due to the core handling process. Accurate borehole stability prediction of openhole horizontal wells, however, requires a continuous characterization of the mechanical properties and in-situ

stresses along the well. A continuous profile of static mechanical properties can be obtained from logging data once a constitutive model which reproduces the stress-strain behavior of the formations is calibrated with laboratory results. The discrete data set from laboratory testing of core samples does not fully describe the horizontal variations of rock properties when drilling horizontal well-bores; therefore, it is most appropriately used as calibration data for continuous log derived properties measured under in-situ conditions.

Borehole stability analysis is invaluable for both drilling operations as well as for determining wellbore completion strategies. When borehole instabilities occur during drilling, LWD and MWD data are more suitable for geo-mechanical model generation and real-time borehole stability analysis. On the contrary, wire-line logging data is commonly used to obtain formation’s mechanical properties when the instability of horizontal well-bores may occur during production scenarios as the reservoir pressure depletes or the drawdown pressure is increased along the productive life of the field.

Barefoot completion is often an attractive and cost-effective option for horizontal wellbores drilled in carbonate reservoirs, thanks to the high compressive strength of these formations. However, for development purposes, the stability of the horizontal openhole needs to be analyzed for the whole productive life of the reservoir. Consequently, the openhole stability needs to be analyzed with different pore pressure and drawdown scenarios in order to safeguard hole integrity during reservoir depletion. This methodology will ultimately establish the maximum allowable drawdown which provides the maximum production flow-rate without collapsing the horizontal borehole as the pore pressure depletes.

In order to analyze the openhole stability under several production scenarios, the development of the shear failure zone around the borehole is predicted as the reservoir pressure depletes with multiple drawdown conditions. Additionally, formation strength sensitivity simulations are often required to account the effect of rock strength degradation due to acid injection treatments. The results allow well-completion engineers to select a suitable

completion strategy which ensures borehole integrity during the productive life of the field or plan for work-over operations at a predetermined time. The productive life of an oil field may end either when the abandonment reservoir pressure is reached or when the water cut reaches the threshold for commercial viability.

LOG-BASED GEOMECHANICAL

CHARACTERIZATION

The log-based static mechanical properties are predicted by using a Baker Atlas proprietary program based on the FORMEL model (Raaen et al., 1996) developed at IKU. The program utilizes the fundamental behavior of grain-to-grain contacts and micro-crack deformation (sliding along the surfaces of micro-cracks) to construct a constitutive relationship between stress and strain for a given rock material. Under a given stress condition, deformation due to solid rock (including grain-to-grain contact), sliding of micro-crack surfaces, and pore spaces are considered separately. The elastic deformations that can be expected under dynamic loading conditions (elastic deformation of the matrix and elastic opening or closing of existing cracks) are considered in combination with log measured acoustic velocities. Other deformations due to micro-crack sliding and pore deformation, as well as dilatancy, are considered to be the main contributors for the deformation of the rock material under static stresses. The difference between static and dynamic properties can be derived by distinguishing the processes associated with static deformation from those associated with dynamic deformation.

From theoretical analysis and experimental studies, the relationship among rock porosity, bulk density, mineral content, dynamic properties, grain contact parameters, sliding crack parameters, and dilatancy parameters has been established and documented in calibration tables. Using density, lithology, and acoustic logs, a representative “rock sample” for a given depth can be reconstructed from these calibration tables. Subsequently, the constitutive behavior of the rock sample can then be examined with simulated loading. Figure 1 shows the process flow of predicting rock mechanics properties from logging data.

Using logs to predict formation mechanical properties is an indirect technique. However, the method offers several benefits including cost, continuous estimations of mechanical properties with depths and prevalence of log data. Traditionally, the most commonly used method for deriving elastic mechanical properties from logs is based on relations expressing these properties in terms of acoustic velocities. These acoustically derived ‘dynamic’ properties differ significantly from their ‘static’ counterparts, introducing the potential for large errors if used for borehole stability analysis. The main cause for such errors is due to fundamentally different deformation mechanisms between

static and dynamic loading. Static loading has long duration and high magnitude of applied stresses; while, dynamic loading has low magnitude and short duration of pressure or sonic waves. Therefore, empirical relationships between static mechanical properties and dynamic properties for specific lithologies and compositional variations are required. The establishment of Static-Dynamic empirical correlations requires a large numbers of laboratory tests on core samples increasing the cost significantly. Further, the traditional method cannot be used in fields where such correlations do not exist or are limited in accuracy.

Logging input data. The log-based mechanical

property program simulates triaxial compression tests under any given confining pressure conditions. It generates stress-strain curves at different confining conditions from which static mechanical properties and strengths can be determined. These properties are mainly obtained from processed log-data and petrophysical description. The input data required to run the program are:

• Volumetric model: relative volume of shale, effective or total porosity, and water saturation. (Vsh, φ, and Sw).

• Acoustic compressional and shear wave slowness. (DTC and DTS).

• Formation density log. • Pore pressure, horizontal stress, and overburden stress. • Density and compressibility of pore fluids.

The program works under the following constraints:

1=+++ whcmatrixsh VVVV ………………..……….(1)

φ=+ whc VV ………….………………………..(2)

The program can use the petrophysical model either

from the total or effective porosity approach. When total porosity is used, Vsh is considered to represent the dry bulk volume of shale, but it represents the wet bulk volume of shale when the effective porosity is used. The rock mechanic property calculations for the borehole stability analysis under production scenarios in carbonate reservoirs are usually made using the total porosity model.

The in-situ stress profiles (overburden stress, horizontal stress, and pore pressure) are required for the program in order to associate the in-situ acoustic responses to the effective stress state. Commonly, fluid density and compressibility are obtained from PVT analysis on reservoir’s fluid samples.

The outcome of the log-based mechanical property program are continuous representations “curves” of formation compressive strengths, Young’s modulus, Poisson’s ratio, shear & volumetric modulii, cohesive strength, internal friction angle, and Biot’s poroelastic parameter at different confining conditions.

Log and core data correlation. The micromechanical model for deriving static mechanical properties from log responses had been calibrated and validated against an extensive data base of laboratory experiments, (Yi et al. 2005). Figure 2 shows the stress-strain curves derived from the micromechanical model and from triaxial loading of core samples in the laboratory. Particular reservoirs however may require an additional core-log tuning correlation of the static mechanical properties. This correlation is performed by a slightly modification of the micromechanical parameters of the calibration tables of the Model until the simulated stress-strain curves match the laboratory results of triaxial tests at different effective confining stress conditions. When this calibration is done for a particular field or reservoir, the operator company would obtain a customized micromechanical simulation of their reservoir mechanical behavior.

Since the log-based algorithm can reproduce the stress-strain curves obtained during triaxial compression tests by modifying the micromechanical parameters of shale and matrix, this methodology is more robust than using empirical acoustic correlations for estimating the elastic mechanical properties and strengths. Empirical correlations have limited use since they are obtained from a reduced number of samples and many experimental factors influence the lab results especially in weak sands. Rock’s mechanical deterioration from core retrieval, plug handling, and drying exposure tends to underestimate reservoir strength in poorly consolidate sandstones, (Franquet et al. 2005). However, laboratory results in consolidated sandstones or carbonate reservoir samples are less affected by these factors.

If acoustic measurements of compressional and shear wave velocities are available in core samples under triaxial tests, the log-based micromechanical model can also be calibrated to reproduce the acoustic response as the shear stress is increase in the laboratory. Figure 3 show a comparison of the simulated acoustic response from the micromechanical model and from acoustic measurements under triaxial loading.

The final output of the micromechanical model is a set of curves (as logs) of the elastic properties and strengths at different effective confining conditions. A log example of simulated formation compressive strength from the micromechanical model is presented in Figure 4 and compared with few UCS values from lab.

Far-field in-situ stress estimation. The overburden stress is preferably obtained from the integration of a density log from surface to reservoir depth as it is shown below:

=TD

v dzg0

ρσ � ………………………………………...(3)

The magnitude of the horizontal stress can be estimated from the fracture closure pressure of Minifrac tests or breakdown pressure observed in leak-off test. Thereafter, the minimum horizontal stress curve “σh,min” would be recalculated with Poisson’s ratio “ν” , reservoir pressure, and Biot’s poroelastic parameter “α” once these properties are obtained from the log-based micromechanical model. The relationship of stresses, pore pressure, and linear-elastic properties was presented by Voight (1974) assuming the uniaxial strain compaction:

[ ] rrvh PP αασν

νσ +−��

����−

=1min, ………………...(4)

where: σv is the overburden stress and Pr is the reservoir pore pressure.

The above equation does not incorporate strain-tectonic effects and thermal induced stresses which need to be considered in high-tectonic areas where the stresses are not relaxed or in abnormally pressurized basins, (Prats, 1981).

The magnitude of the maximum horizontal stress is not required in the micromechanical model to derive formations elastic properties and strengths based on logging data. However, it will be necessary for the borehole stability analysis of horizontals wellbores. Does not exist a direct way to quantify the magnitude of the maximum horizontal stress; therefore, it has to be estimated through back-analysis from breakouts widths and hydraulic induced fractures observed in acoustic image logs. The maximum horizontal stress is then calibrated as more well information and drilling evidences are reported in the field such as mud losses, tight holes, kicks, etc. The geomechanical model is calibrated and verified by using a Borehole Integrity Analysis Software which is explained in detail in the following section.

The directions of the horizontal stresses are commonly deduced from breakouts and hydraulically induced fractures identified on acoustic logs of vertical wellbores. Figure 5 illustrates an example of breakouts and induced fractures at the borehole wall. The orientation of breakouts corresponds to the direction of minimum horizontal stress while the induced fractures occur in the direction of maximum horizontal stress.

Acoustic image logs are particularly more suitable for breakout and induced fractures identification because it has full borehole circumferential coverage and thus illustrates all features on the borehole wall. In contrast, resistivity images do not have full 360 degrees coverage and measure the electric conductivity into the formation.

Pore pressure depletion effect. When the reservoir pore pressure decreases as a result of production related drawdown, porous media try to contract and the attempted decrease in strain is converted to a reduction in the total

horizontal stress. Cleary (1978) and Geertsma (1978) described the effect of pore pressure depletion on horizontal stresses in reservoir rocks by the following poroelastic relationship:

( )( ) rh P∆

−−=∆ν

νασ1

21 .………………………………(5)

Therefore, the magnitudes of horizontal stresses need to be recalculated during the borehole stability analysis of the openhole horizontal wells as the pore pressure depletion is simulated. Pore pressure depletion has two detrimental effects in openhole stability of horizontal wells:

• The effective stresses in the formation increase as the pore pressure decreases, as is represented in the Biot’s poroelastic relationship – Equation 6.

• The difference between the total vertical stress and the total horizontal stress gets bigger with production, leading to higher tangential stress concentration in the formation near the borehole wall.

rijijij Pαδσσ −=′ ….…………… …………………(6)

BOREHOLE STABILITY MODEL

The borehole stability model, coupled with the log-based mechanical property algorithm, provides a foot-by-foot analysis of the induced stresses and the size of the shear failure zones around the horizontal borehole. Continuous profiling allows quick identification of potential catastrophic shear failure zones and provides guidelines for selecting the appropriated horizontal well completion without having high risk of borehole collapse.

The Borehole Integrity Model is an algorithm that estimates formation failure and in-situ stress from borehole measurements. It uses closed-form elasticity solutions to describe the distribution of stress around a borehole drilled in any given stress state. The stress distribution around a borehole drilled in an arbitrary stress field has been described by many researchers, Aadnoy and Chenervert (1987) and Bradley (1979), from the original Kirsch’s elastic solutions. Kirsch (1898) derived for the first time the elastic stress distribution around a circular hole under two principal stresses acting orthogonal to the axis of the hole. The stress tensor components in cylindrical coordinate system are presented as follows: Jaeger and Cook, (1979).

( ) ( )w

hHhHrr P

r

a

r

a

r

a +������ +−−+��

���� −+= ϕσσσσσ 2cos3412

12 4

4

2

2

2

2

( ) ( )w

hHhH Pr

a

r

a −���� +−−��

�� ++= ϕσσσσσ ϕϕ 2cos312

12 4

4

2

2

( ) ϕσστ ϕ 2sin3212 4

4

2

2 ������ −+−−=

r

a

r

ahHr

....(7), (8),and (9)

Under elastic conditions, the maximum stress concentration occurs on the borehole wall, and shear failure will occur when the maximum effective principal stress on the tangential plane of the borehole exceeds the effective strength of the rock. Zoback et al. (1985) and Zheng et al. (1987) investigated the breakout creation and growth applying the Mohr-Coulomb shear failure criterion presented in Equation 10. Borehole breakouts are created from continuous failures of the borehole wall at particular locations where the stress concentration overcomes the strength of the rock.

ifanS φστ tan0 += ….………… …………....…(10)

The ensuing rock failure leads to a change of geometry and induced stress redistribution around the borehole. The new distributed stresses can cause further failure thereby increasing the shear failure zone even more. In most vertical boreholes, the shear failure ultimately evolves into a stable geometry producing breakouts along the borehole. However, when the shear failure zones around the borehole circumference get too large (more than 140 degrees in each breakout wing), the borehole may collapse since a stable geometry cannot be obtained, Zheng (1998).

The Borehole Integrity Model predicts the size, location, and extension of the shear borehole failure zone by incorporating the Mohr-Coulomb failure criterion, the Biot’s poroelastic relationship for effective stress, and the 3D tensor calculation of the stress distribution around a borehole drilled into an arbitrary stress state. The arbitrary stress state is obtained from the tensor rotation of the far-field in-situ stress state into the cylindrical coordinate system aligned to the borehole orientation and inclination, in which the z’ axis is parallel to the drilling direction, as it is illustrated in Figure 6.

The elastic equations that describe the stress distribution around a borehole drilled into an arbitrary stress field in term of effective stress are presented in the following set of relationships:

( ) ( )

θσσσσ

σ 2cos3412

12

'4

4

2

2''''

2

2'''' �

���� +−−

+� ���� −

+=

r

a

r

a

r

a yyxxyyxxrr

rwyx P

r

aP

r

a

r

a αθτ −+� ���� +−+

2

2

4

4

2

2

'' 2sin341 ……(11)

( ) ( )

θσσσσ

σ θθ 2cos312

12

'4

4''''

2

2'''' �

���� +−

−� ���� +

+=

r

a

r

a yyxxyyxx

rwyx P

r

aP

r

a αθτ −−� ���� +−

2

2

4

4

'' 2sin31 ……….(12)

( ) ryxyyxxzzzz Pr

a

r

a αθντθσσνσσ −−−−= 2sin42cos2'2

2

''2

2

''''''''

( ) ���

����� −+� ���� +

−=

4

4

2

2

'''''' 3212cos2sin

2 r

a

r

ayx

yyxxr θτθ

σστ θ

..(14)

[ ] �������� −+=

2

2

''''' 1cossinr

azxzyrz θτθττ …………………...(15)

[ ] �������� ++−=

2

2

''''' 1cossinr

azyyxz θτθττθ

………………...(16)

The shear failure zones obtained by the wellbore integrity model along the horizontal section are graphically displayed as dark-brown areas from 0 to 360 degrees of the borehole circumference (e.g., see Figure 7). On the contrary, the yellow area illustrated in the output modeling plots represents borehole stable condition. Several of these plots were generated considering different pore pressures and equivalent borehole fluid densities, and they were then combined in order to visualize the openhole stability evolution as the reservoir pressure is depleted and the drawdown is increased. Figure 8 shows a composite of the shear failure development around the borehole as the drawdown pressure is increased in the horizontal wellbore for three reservoir pressure scenarios.

When the equivalent fluid density is decreased, the bottom-hole flowing pressure “Pwf” decreases and the drawdown pressure increases as shown below:

wfrdd PPP −=∆ …………………………..…….....(17)

The borehole stability analysis under production scenarios addresses two adverse effects in the openhole stability in comparison with the standard overbalance drilling situation. The first effect is that the reservoir pressure decreases in the near wellbore area as a result of production drawdown thereby leading to a decrease in reservoir pressure around the wellbore wall. In turn, these lead to higher effective stresses in the near wellbore area and a reduced stability of the hole. Secondly, the drag forces generated by the fluid flow in the radial direction may reduce stability but their effect is considered negligible in carbonate reservoirs due to the high tensile strength which prevents solid production and grain disintegration from occurring. In clastic reservoirs, however, drag forces may create significant sand production mainly in loosely consolidated formations.

RESULTS

The borehole stability analysis under production scenarios in a horizontal well was done for three reservoir pressure conditions: 10,500 8,000 and 5,500 psi corresponding to initial, intermediate, and abandonment pressures respectively. The results of the stability analysis are presented in Figure 8 where breakouts can be identified as terracotta shading around the borehole wall. The shear failure zones around the borehole were calculated for each pore pressure scenario and for multiple drawdown conditions, ranging from 16 to 6 ppg of equivalent borehole fluid density (EMW). Each vertical strip indicates a 0 to 360 degrees image around the borehole. According to the borehole integrity analysis, the well appears to be competent enough to sustain a 3,000 psi drawdown (the right-most strip) created by a borehole fluid density of 6 ppg EMW when the reservoir pressure gets depleted to 5,500 psi (the bottom-most plot). Low risk of borehole collapse is predicted on this well if no acid stimulation is planned. The critical drawdown may be lower if the formation strength gets reduced by acid treatment.

The breakouts occur in the intervals with low compressive strength which are associated with layers that contain higher porosity values in formation. The stability of the borehole may look slightly better as the reservoir pressure depletes where smaller breakout widths are predicted for the same equivalent borehole fluid density. But in reality, the stability looks better because the drawdown is lower for the same fluid density as the reservoir pressure depletes. The shear failure zone around the hole is plotted clockwise and from the low side of the borehole. Figure 9 shows how the critical drawdown for a stable borehole decreases as the reservoir pressure depletes over time. A formation strength sensitivity analysis was done in the openhole stability simulations of this well by reducing the formation strength by 25% and 60%.

Morita (2002) stated that the borehole failure in a limestone formation is induced by shear failure after pore collapse and acid treatments slightly lowered the borehole stability due to formation strength degradation.

CONCLUSIONS

Static mechanical properties are traditionally obtained by conducting triaxial compression test in the laboratory. Such measurements may not be routinely carried out due to cost and/or core material availability. The mechanical property derivation presented here offers an alternative way of obtaining static mechanical properties mainly from logging data.

Additionally, this program provides continuous representations of the formation’s mechanical properties with depth, which is the ultimate objective in geomechanical characterization especially when these

properties will be used for determining the stability of horizontal open-holes under production scenarios.

The log-based program has the capability of modifying the micromechanical parameters of matrix constituent particles in order to reproduce the stress-strain curves obtained during triaxial compression tests in the laboratory.

The openhole stability of horizontal wells is reduced as the reservoir pressure depletes leading to gradual reduction of the maximum allowable production drawdown to maintain a stable barefoot hole. This explains why barefoot completed horizontal wellbores are drilled and put into production without any stability problems in early production life; whereas, borehole collapse is observed after several years of production.

A qualitative assessment of the effect of formation strength reduction through acid treatment of carbonate reservoirs is also attempted to understand its impact on well bore stability.

The stability analysis under production scenarios can predicts shear failure and borehole collapse in horizontal borehole as the reservoir pressure decrease, as the drawdown pressure increase, or as the formation strength gets degraded due to acid treatment. Thus, this analysis can provided valuable insight in selecting the most suitable drilling, stimulation, and completion scheme in order to ensure the well integrity during the productive life of the reservoir.

NOMENCLATURE

DTC = Compressional wave slowness, [τ/L] DTS = Shear wave slowness, [τ/L] E = Young’s Modulus, [m/Lt2] G = Shear Modulus, [m/Lt2] K = Volumetric Modulus, [m/Lt2] Pc = Confining pressure, [m/Lt2] Pr = Reservoir pore pressure, [m/Lt2] Pw = Wellbore pressure, [m/Lt2] Pwf = Bottom-hole flowing pressure, [m/Lt2] USC = Unconfined compressive strength, [m/Lt2] S0 = Cohesive strength, [m/Lt2] Sw = Water saturation, [L3/ L3] TD = Total reservoir depth in TVD, [L] Vhc = Bulk volume of hydrocarbon, [L3/ L3] Vmatrix = Bulk volume of sand, [L3/ L3] Vp = Compressional wave velocity, [L/ t] Vs = Shear wave velocity, [L/ t] Vsh = Bulk volume of shale, [L3/ L3] Vw = Bulk volume of water, [L3/ L3] a = Radial distance from the borehole’s center, [L] r = Borehole radius, [L] g

�

= Gravitational acceleration, [L/t2]

∆Pdd = Drawdown pressure, [m/Lt2] α = Biot’s poroelastic parameter

δij = Kronecker’s delta φ = Porosity, [L3/ L3] φifa = Mohr-Coulomb internal friction angle, [degrees] ϕ = Counter clockwise angle measured from the

maximum horizontal stress direction, [degrees] θ = Angle around the borehole wall measured from

the bottom or low side of the hole, [degrees] ν = Poisson’s ratio

ijσ ′ = Effective stress tensor, [m/Lt2]

ijσ = Total stress tensor, [m/Lt2]

σH = maximum horizontal stress magnitude, [m/Lt2] σh = minimum horizontal stress magnitude, [m/Lt2] σn = normal stress, [m/Lt2] σv = vertical stress magnitude, [m/Lt2] σrr = radial normal stress, [m/Lt2]

σϕϕ = tangential normal stress, [m/Lt2] τ = shear stress, [m/Lt2]

τrϕ = shear stress in cylindrical coordinates, [m/Lt2]

REFERENCES

Aadnoy, B. S. and Chenervert, M. E., 1987, Stability of highly inclined boreholes: SPE Drilling Engineering, December, p. 364-374.

Biot, M.A. and Willis, D.G., 1957, The elastic coefficients of the theory of consolidation: Journal of Applied Mechanics, v. 24, p. 594-601.

Bradley, W. B., 1979, Failure of inclined boreholes: Journal Energy Res. Tech. Trans. AIME, v. 101, p. 232-239.

Cleary, M.P., 1978, Rate and structure sensitivity in hydraulic fracturing of fluid-saturated porous formations, in Proc., 20th U.S. Symposium on Rock Mechanics, Austin, TX, June, p. 127-42.

Franquet, J.A., Stewart, G., Bolle, L., and Ong, S.H., 2005, Log-based Geomechanical Characterization and sanding Potential Analysis on Several Wells Drilled in Southern Part of Oman, paper presented in SPE/PAPG Pakistani Annual Technical Conference.

Geertsma, J., 1978, Some rock-mechanical aspects of oil and gas well completions, paper EUR-38 in SPE European Off-shore Petroleum Conference and Exhibition, London, October, p. 24-27.

Kirsch, G., 1898, Die theorie der elastizitat und die bedurfnisse del festigkeitslehre, VDI Z v. 42: 707.

Morita, N, 2002, Stability of an openhole completed in a limestone reservoir with and without acid treatments, paper SPE-77776 in the SPE Annual Technical Conference and Exhibition, San Antonio, TX.

Prats, M., 1981, Effect of burial history on the subsurface horizontal stresses of formations having different material properties: SPEJ December, p. 658-62.

Raaen, A. M., Hovem, K. A., Joranson, H., and Fjaer, E., 1996, FORMEL: A step forward in strength logging, SPE paper SPE-36533, Proceedings of Annual Technical Conference and Exhibition, p. 439-445.

Voight, B., 1974, Stress history and rock stress: Proc., Third Cong. of the Intl. Soc. for Rock Mechanics, Natl. Academy of Science, Washington, DC. v.2, Part A, p. 580-82.

Yi, X, Ong, S.H., and Russell, J.E., 2005, Characterizing pore compressibility, reservoir compaction and stress path under uniaxial strain condition for nonlinear elastic rock, paper ARMA/USRMS 05-791 in 40th U.S. Symposium on Rock Mechanics: Rock Mechanics for Energy, Mineral, and Infracturure Development in the Northern Regions.

Zheng, Z., Kemeny, J., and Cook, N.G.W., 1989, Analysis of borehole breakouts: Journal of Geophys. Res., v. 94, p. 7171-7182.

Zheng, Z., 1998, Integrated borehole stability analysis – Against tradition, SPE-47282 in SPE/ISRM conference, Eurock ’98, July.

Zoback, M.D., Moos, D., and Mastin, L., 1985, Wellbore breakouts and in-situ stress: Journal of Geophys. Res., v. 90, p. 5523-5530.

About the author: Javier A. Franquet jointed the PDVSA’s Technological & Research Centre, Intevep, S.A., in 1996 as Researcher in Geomechanics after he finished a B. Sc. in Mechanical Engineering. He continued his education in Texas A&M University and IFP School by completing M. Sc. in

Petroleum Engineering and M. Sc. in RGE Reservoir Geosciences and Engineering, respectively. He returned to Intevep, S.A. in 2001 as a Professional in Geomechanics involved in multi-disciplinary projects related to Hydraulic Fracturing, Sand Control, Stress Field Characterization, and Wellbore Stability (including under-balance drilling). In 2004, Javier jointed Baker Atlas – Geoscience for the Middle East Area based in Abu Dhabi as a Senior Geomechanical Geoscientist. He had been involved in several rock mechanics related projects from North Sea, Arabic Peninsula, Egypt, India, and Pakistan. He has published papers in SPE, SPWLA, ARMA, and NARMS, and he serves as a director of MERock, the recently formed ISRM representation for the Middle East Region based in Abu Dhabi as a Geomechanical Geoscientist.

Figure 1: The process flow of the log-based micromechanical model program.

Figure 2: Comparison between the stress-strain curves obtained from laboratory triaxial test and the simulated mechanical behavior from the log-based micromechanical model.

Figure 3: Comparison between the compressional and shear wave velocities measured in the lab during triaxial loading and the simulated acoustic behavior from the log-based micromechanical model.

Figure 4: Log-derived compressive strengths at multiple confining conditions from the micromechanical model.

Figure 5: Breakouts and hydraulically induced fractures examples observed on an acoustic image log run in a vertical borehole.

Figure 6: Local cylindrical coordinate system aligned to the borehole trajectory and referenced to the geographic Cartesian coordinate system, where X is (North), Y is (East), and Z is (Down).

Figure 7: Shear failure zone obtained from the borehole stability analysis. Red dots represent the shear strength of the formation while the blue line is the shear stress around the horizontal borehole.

Figure 8: Reservoir pressure depletion sensitivity on the openhole stability analysis under production scenarios, and shear failure zone development around the borehole wall as the drawdown pressure is increased.

Figure 9: Openhole stability analysis under production scenarios of a horizontal well drilled in a carbonated reservoir.