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Introduction

Reservoir geomechanics has become a relevant subject for the petroleum industry during the last decades. There are many benefits on improving geomechanics understanding, including better characterization of reservoir volume deformation and impact on rock permeability (compaction and dilation), prediction of surface subsidence, reducing risks of fault reactivation and out-of-zone hydraulic fracture propagation during water flooding, or other improved oil recovery process. Due to the complexity of the problems and coupled interactions between production, injection, and stress change, a comprehensive analysis requires numerical modelling involving the coupling of geomechanics with porous media fluid flow, injection and fault behavior. All of these factors make it very difficult to predict the state of stress associated with a well before drilling and make it even harder to predict stress changes over time. Those changes can have important and unexpected consequences if the well is to be stimulated, left as an open- hole completion, and if the environs faults will be reactivated.

Summary

 The study field has been an oil-producing area in southern Iran for nearly 50 years. Complex geological structure and varying levels of depletion scenarios require geomechanical analysis of the reservoir to enhance its production and mitigate geomechanical risks. This paper describes creating a time-lapse (4D) integrated geomechanical model by generating 3D maps of mechanical properties and a 3D stress state that can be altered over time as pore pressure changes, then explores pressure depletion and related stress changes effects on faults and fractures reactivation. The first phase of the study was an integrated stress analysis using Image logs and sonic anisotropy interpretation. 1D–3D Mechanical Earth Model was built by gridding the reservoir and populate the model with mechanical properties. The third phase provided a distribution of stresses and associated strains under initial conditions using finite element calculations. Ultimately, stress and strain changes associated with depletion simulated by the reservoir flow model were determined during the fourth phase of study. In the resulting model, different critical coordinates points from the initial year (1992) to 2045 were selected five time-steps. Results show no critical faults reactivation but by increasing production time the instability of fractures gradually rises by stress regime changes.

Conclusions

The studied field is a structurally and geomechanical challenging field in which a 3D stress modelling approach significantly help in recognizing the relationships between the variety of factors that control the reservoir’s mechanical behavior and overburden status. 3D maps of the mechanical properties and stress were developed for the area and were applied in several different aspects of well design. The key conclusions reached for the area analyzed in this analysis are:

(1) The studied field includes a wide variety of structural and geological complexity, such as different kinds of fractures (major, medium, minor and hairline open fracture) and giant thrust faults. One of the different aspects of this study is to include all geomechanical procedures from 1D to 3D geomechanics and finally focus on faults reactivation in the reservoir. It was tried to integrate all general approaches with the geomechanical study as a new method to optimize reservoir management.

(2) The modelling process described here has utilized a variety of data (static model, logs, drilling parameters, test data, . . .) to generate calibrated estimates of 3D geomechanical parameters and in-situ stresses that have been used in time-lapse (4D) geomechanics. For this purpose, mechanical properties extracted from logs demonstrate fair consistency between various studied field wells which were modelled as 1D MEM. The VISAGE measured 3D stress field is compatible with the 1D stress profiles at wells calibrated using breakouts, mud losses and other drilling events. The in-situ stresses were matched using different Sh_min magnitudes and SH_max/Sh_min ratios in all intervals and All stress characterization such as direction and magnitude in 3D were inline by 1D MEM and regional stress in the studied field.

(3) A fault and fracture potential reactivation study were conducted in well #4 for Gurpi formation of the studied field. To observe stress changes on the vicinity of the major faults from initial time (1992) to 2045, different critical coordinates points were selected on the time-lapse (4D) geomechanical model which includes five steps time in 1992, 2002, 2017, 2018 and 2054. Possible reactivation of the studied faults was investigated by Mohr’s circles’ plot for all mentioned points in different step times. Results show that there is no critical faults reactivation based on Mohr’s circles’ results in points near to main studied faults and accordingly, the production phase could be done safely without any faults slips in the studied field. On the other hand, it can be concluded, over time and with oil production operation, the instability of fractures gradually rises and some of the fractures which were stable in the initial condition, show some instability behavior after depletion of the reservoir which is affected by changes in the studied field stress regime.

(4) Fault and fracture stability analysis was the final purpose of this time-lapse (4D) geomechanical study which in each time step, matches the results and can be almost sure about results accuracy. The 3D geomechanics model could be improved and completed by involving pressure data due to different depletion and gas injection scenarios.

Acknowledgements

References

• Alford, R.M., 1986. Shear data in the presence of azimuthal anisotropy: dilley, Texas. In: Society of Exploration Geophysicists (SEG) technical program expanded abstracts 1986, November 1986 SEG Annual Meeting, Houston, TX. Anis, A.S.L., et al., 4D geomechanical simulation in fractured carbonate reservoir for optimum well construction and reservoir management, case study in offshore east java area. In: International petroleum technology conference,March 2019 Beijing, China.
• Chen, H.-Y., Teufel, L.W., and Lee, R.L., 2019. Coupled fluid flow and geomechanics in reservoir study - I. Theory and governing equations. In:SPE annual technical conference and exhibition, October 1995 Dallas, TX.
• Cook, J., et al. 2007. Rocks matter: ground truth in geomechanics. Oilfield Review, 19, 36–55.
• Correa, A.C.F., et al., 2013. Integrated modeling for 3D geo-mechanics and coupled simulation of fractured carbonate reservoir. In: OTC Brasil, October 2013 Rio de Janeiro, Brazil.
• Dutta, N., 2002. Geopressure prediction using seismic data: current status and the road ahead. Geophysics, 67 (6), 2012–2041. doi:10.1190/1.1527101.
• Fredrich, J.T., et al., 1996. Three-dimensional geomechanical simulation of reservoir compaction and implications for well failures in the Belridge diatomite. In: SPE annual technical conference and exhibition, October 1996 Denver, CO.
• Fredrick, J.T., et al., 1998. Reservoir compaction, surface subsi- dence, and casing damage: a geomechanics approach to miti- gation and reservoir management. In: SPE/ISRM rock mechanics in petroleum engineering, July 1998 Trondheim, Norway.
• Gutierrex, M. and Lewis, R.W., 1998. The role of geomecha- nics in reservoir simulation. In: SPE/ISRM rock mechanics in petroleum engineering, July 1998 Trondheim, Norway.
• Gutierrez, M., 1994. Fully coupled analysis of reservoir com- paction and subsidence. In: European petroleum conference, October 1994 London, UK.
• Heffer, K.J., Koutsabeloulis, N.C., and Wong, S.K., 1994. Coupled geomechanical, thermal and fluid flow modelling as an aid to improving waterflood sweep efficiency. In: Rock mechanics in petroleum engineering, August 1994 Delft, Netherlands.
• Herwanger, J. and Koutsabeloulis, N., 2011. Seismic geome- chanics: how to build and calibrate geomechanical models using 3D and 4D seismic data. In: 73rd EAGE conference and exhibition workshop, May 2011 Vienna, Austria.
• Khan, M. and Teufel, L.W., 1996. Prediction of production-induced changes in reservoir stress state using numerical model. In: SPE annual technical conference and exhibition, October 1996 Denver, CO.
• Koutsabeloulis, N. and Zhang, X., 2009. 3D reservoir geome- chanical modeling in oil/gas field production. In: SPE Saudi Arabia section technical symposium, May 2009 Al-Khobar, Saudi Arabia.
• Koutsabeloulis, N.C. and Hope, S.A., 1998. “Coupled” stress/ fluid/thermal multi-phase reservoir simulation studies incorporating rock mechanics. SPE/ISRM Rock Mechanics in Petroleum Engineering. Trondheim, Norway: Society of Petroleum Engineers, 6.
• Lacy, L.L., 1997. Dynamic rock mechanics testing for opti- mized fracture designs. SPE Annual Technical Conference and Exhibition. San Antonio, Texas: Society of Petroleum Engineers, 14.
• Molenaar, M.M., et al., 2004. Applying geo-mechanics and 4D: “4D In-Situ Stress” as a complementary tool for optimizing field management. In: Gulf rocks 2004, the 6th North America Rock Mechanics Symposium (NARMS), June 2004 Houston, TX.
• Osorio, J.G., Chen, H.-Y., and Teufel, L.W., 1997a. Fully coupled fluid-flow/geomechanics simulation of stress-sensitive reservoirs. In: SPE reservoir simulation symposium, June 1997 Dallas, TX.
• Osorio, J.G., Chen, H.-Y., and Teufel, L.W., 1997b. Numerical simulation of coupled fluid-flow/geomechanical behavior of tight gas reservoirs with stress sensitive permeability. In: Latin American and caribbean petroleum engineering con- ference, August 1997 Rio de Janeiro, Brazil.
• Plumb, R., et al. 2000. The mechanical earth model concept and its application to high-risk well construction projects. Plumb, R.A., 1994. Influence of composition and texture on the failure properties of clastic rocks. Rock Mechanics in Petroleum Engineering. Delft, Netherlands: Society of
• Petroleum Engineers, 8.
• Price, D., Angus, D., and Parsons, S., 2016. Understanding a 4D geomechanical model for time-lapse seismic calibration. SEG Technical Program Expanded Abstracts, 2016, 5425–5429.
• Qiu, K., et al., 2012. Evaluation of fault re-activation potential
• during offshore methane hydrate production in Nankai Trough, Japan. In: Offshore technology conference, April 2012 Houston, TX.
• Qiu, K., et al., 2014. Well integrity evaluation for methane hydrate production in the deepwater Nankai Trough. In: International petroleum technology conference, January 2014 Doha, Qatar.
• Sayers, C.M., et al., 2006. Predicting reservoir compaction and casing deformation in deepwater turbidites using a 3D mechanical earth model. In: International oil conference and exhibition in Mexico, August 2006 Cancun, Mexico.
• Settari, A., et al., 2002. 3D analysis and prediction of micro- seismicity in fracturing by coupled geomechanical model- ing. In: SPE gas technology symposium, April 2002 Alberta, Canada.
• Stone, T., et al., 2000. Fully coupled geomechanics in a commercial reservoir simulator. In: SPE European petro- leum conference, October 2000 Paris, France.
• Sulak, R.M. and Danielsen, J., 1989. Reservoir aspects of ekofisk subsidence. Journal of Petroleum Technology, 41 (7), 709–716. doi:10.2118/17852-PA.
• Yang, X., et al. 2018. Case study: 4D coupled reservoir/geo- mechanics simulation of a high-pressure/high-temperature naturally fractured reservoir. SPE Journal, 23 (5), 1518–1538. doi:10.2118/187606-PA.
• Younessi, A., et al., 2018. Evaluation of reservoir compaction and geomechanical related risks associated with reservoir production using a two-way coupled 4D geomechanical model for a deep-water field in South East Asia. In: Offshore technology conference Asia, March 2018 Kuala Lumpur, Malaysia.
• Zhang, X., Koutsabeloulis, N., and Heffer, K., 2007. Hydromechanical modeling of critically stressed and faulted reservoirs. AAPG Bulletin, 91 (1), 31–50. doi:10.1306/08030605136.