Impact of Anisotropy Induced by Shale Lamination and Natural Fractures on Reservoir Development and Operational Designs
- Ming Gu (West Virginia University)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Evaluation & Engineering
- Publication Date
- November 2018
- Document Type
- Journal Paper
- 850 - 862
- 2018.Society of Petroleum Engineers
- orthorhombic formation, natural fracture, geomechanical logging, shale anisotropy, transverse isotropy
- 4 in the last 30 days
- 145 since 2007
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The laminated nature of shale rocks leads to different mechanical properties parallel and perpendicular to the bedding plane, which is known as transverse isotropy (TI). If natural fractures (NFs) are present, the additional elastic anisotropy is introduced within the bedding plane, making shale an orthorhombic (OB) medium. Most current geophysical log interpretations ignore such shale anisotropies, resulting in erroneous estimates of the elastic moduli, brittleness, and stress gradient, which consequently causes problematic or suboptimal drilling, completion, and hydraulic-fracturing design. The objective of the current study is to investigate the effects of the two shale anisotropies on geomechanical-property characterizations, and, hence, on operational designs.
In this paper, synthetic OB-rock stiffness tensors are built by introducing NF sets in a vertical symmetric-axis (VTI) background. Then, the VTI model, which ignores the NF effect, and the isotropic model, which ignores both anisotropies, are applied to the synthetic OB rock to interpret the stiffness coefficients, elastic moduli, and stresses. The results are compared with the “true” values interpreted from the original OB-rock stiffness tensors. The impacts of the two types of shale anisotropies on shale-rock geomechanical-property characterizations—and, hence, operational designs—are examined. According to the modeling results, the lamination-induced anisotropy is more significant than the NF-induced anisotropy when predicting Young’s modulus. Besides, ignoring the two anisotropies can lead to an overestimation of both the minimum-horizontal-stress magnitude and the stress contrast; the overestimation is larger for stiffer zones and less-tectonically-active zones. As a result, ignoring two anisotropies can lead to an overoptimistic design of the mud-weight window, resulting in higher risks of borehole tensile failure and shear failure. Ignoring the two anisotropies will not alter the brittleness-index (BI) trend observed in the OB rock. However, the stress contrast is overestimated, which will lead to shorter stage-spacing design and suboptimal selections of perforation locations. If one is ignoring the anisotropy induced by NFs, the fracture width is underestimated, leading to insufficient proppant size or pumping-amount design and, hence, to suboptimal low fracture conductivity.
|File Size||1 MB||Number of Pages||13|
Chertov, M. 2012. Closed-Form Solution for Vertical Fracture Width in Anisotropic Elastic Formations. International Journal of Rock Mechanics and Mining Sciences 53: 70–75. https://doi.org/10.1016/j.ijrmms.2012.04.006.
Far, M. E. 2011. Seismic Characterization of Naturally Fractured Reservoirs. PhD dissertation, University of Houston.
Far, M. E., Thomsen, L., Sayers, C. M. et al. 2013. Seismic Characterization of Reservoirs With Asymmetric Fractures. Geophysics 78 (2): N1–N10. https://doi.org/10.1190/GEO2012-0319.1.
Far, M. E., Buller, D., Quirein, J. et al. 2015. A New Integrated Data Analysis Algorithm and Workflow for Optimizing Horizontal Well Completion in Unconventional Reservoirs. Presented at the SPWLA 56th Annual Logging Symposium, Long Beach, California, 18–22 July. Paper CCCC, Trans.
Far, M. E., Quirein, J., and Mekic, N. 2016. Geomechanics of Orthorhombic Media. Petrophysics 57 (6): 588–596. SPWLA-2016-v57n6a2.
Fjar, E., Holt, R. M., Horsrud, P. et al. 2004. Petroleum-Related Rock Mechanics, second edition, Vol. 53, p. 135. Elsevier.
Grieser, W. V. and Bray, J. M. 2007. Identification of Production Potential in Unconventional Reservoirs. Presented at the Production and Operation Symposium, Oklahoma City, Oklahoma, 31 March–3 April. SPE-106623-MS. https://doi.org/10.2118/106623-MS.
Gu, M., Gokaraju, D., Chen, D. et al. 2016a. Shale-Fracturing Characterization and Optimization by Using Anisotropic Acoustic Interpretation, 3D Fracture Modeling, and Supervised Machine Learning. Petrophysics 57 (6): 573–587.
Gu, M., Quirein, J., Murphy, E. et al. 2016b. Method for Acoustic Anisotropy Interpretation in Shales When the Stoneley-Wave Velocity is Missing. Petrophysics 57 (2): 140–155.
Khan, S., Williams, R., Ansari, S. et al. 2012, Impact of Mechanical Anisotropy on Design of Hydraulic Fracturing in Shales. Presented at the Abu Dhabi International Petroleum Conference and Exhibition, Abu Dhabi, 11–14 November. SPE-162138-MS. https://doi.org/10.2118/162138-MS.
Nye, J. F. 1985. Physical Properties of Crystals. Oxford: Oxford University Press.
Quirein, J., Eid, M., and Cheng, A. 2014. Predicting the Stiffness Tensor of a Transversely Isotropic Medium When the Vertical Poisson’s Ratio Is Less Than the Horizontal Poisson’s Ratio. Presented at the SPWLA 55th Annual Logging Symposium, Abu Dhabi, 18–22 May. SPWLA-2014-OOOO.
Rickman, R., Mullen, M. J., Petre, J. E. et al. 2008. A Practical Use of Shale Petrophysics for Stimulation Design Optimization: All Shale Plays Are Not Clones of the Barnett Shale. Paper presented at the SPE Annual Technical Conference and Exhibition, Denver, 21–24 September. SPE-115258-MS. https://doi.org/10.2118/115258-MS.
Sayers C. M. 2009. Seismic Characterization of Reservoirs Containing Multiple Fracture Sets. Geophysical Prospecting 57 (2): 187–192. https://doi.org/10.1111/j.1365-2478.2008.00766.x.
Schoenberg, M., Muir, F., and Sayers, C. 1996. Introducing ANNIE: A Simple Three-Parameter Anisotropic Velocity Model for Shales. Journal of Seismic Exploration 5 (1): 35–49.
Thomsen, L. 1986. Weak Elastic Anisotropy. Geophysics 51 (10): 1954–1966. https://doi.org/10.1190/1.1442051.