The Impact of Shale Anisotropic Strength and Stiffness on Borehole Stability
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ABSTRACTBorehole instabilities is an expensive problem for the petroleum industry and most of the time, problems occur in shale. Shale is an anisotropic rock but most often it is assumed to be isotropic during borehole stability analysis due to complexity of obtaining some of the data required for the analysis of anisotropic rocks. The main result of borehole stability analysis is the mud weight window. Using the mud weight window obtained by assuming the rock is isotropic in drilling anisotropic rock can result to severe borehole instability problems since the margin between wellbore collapse limit and fracturing limit decreases with increase in degree of anisotropy. This study has investigated the significance of considering the impact of strength and elastic anisotropy in determination of mud weight window, as a means to minimize borehole instabilities problems in shales. The impact of strength anisotropy has been investigated by parametric sensitivity study through SINTEF s PSI code where different degree of strength anisotropy was generated by using the ratio of strength value obtained in bedding planes to the strength value of intact parts of the rock. The difference in mud weight window observed by assuming rock is isotropic to that of anisotropic case was significant and the effect was increasing with the increase in degree of anisotropy. Stability prediction where strength anisotropy is taken into consideration, displayed up to 17% higher minimum mud weight limit compared to prediction based on strength isotropy while the upper mud weight was almost constant, a slight decrease of 1% was observed, meaning that the strength anisotropy mainly affect the prediction of minimum mud weight.The impact of stiffness anisotropy has been analysed in terms of how it affects the calculations of borehole stresses. The radial and axial stresses is the same for both isotropic and anisotropic case, the effect of elastic anisotropy will be studied on the tangential stress only. The influence of elastic anisotropy on tangential stress has been demonstrated by comparing the solutions for anisotropic cases with the commonly used isotropic approach. Both magnitude and direction of stresses around the boreholes were found to be affected by the degree of elastic anisotropy. It can be concluded that, any engineering applications that do not consider the impact of anisotropic behavior of rock produce errors of differing magnitudes depending on the extent of rock anisotropy. To minimize borehole instabilities in laminated formations (shales), the effect of strength and stiffness must be incorporated into borehole stability analysis.