The Anisotropic Horizontal Stress Estimated Using Well Logs Sonic Data in Deep Longmaxi Shales

Abstract It is necessary to understand the geomechanical characteristics of deep Longmaxi shale for safety drilling and hydraulic fracturing. The horizontal stress in shale formations estimated using the vertical transverse isotropy (VTI) model requires five independent elastic stiffness coefficients. From velocity measurements, the five elastic stiffnesses can be obtained from five velocities with three plug samples at 0°, 90°, and 45° direction. But for well-log-based methods, only three out of five stiffness coefficients (C33, C44, and C66) can be estimated from acoustic well logs. To obtain remaining two stiffnesses C11 and C13, the empirical models such as the ANNIE, modified ANNIE (M-ANNIE) models are needed in well-log-based methods. On the other hand, the Biot's poroelastic parameter also exhibits anisotropy, the vertical αv and horizontal αh, respectively, which are not equal and neither is equal to 1. In addition, the formation temperature of deep shale is higher than that of shallow shale formation, which would have an effect on the horizontal stress prediction. Here, this paper estimates the anisotropic horizontal stress using well logs sonic data in a well of deep Longmaxi shales with consideration of poroelastic effect and temperature. Firstly, a series of ultrasonic velocity measurements and static triaxial tests are performed on core plugs of deep Longmaxi shale for obtaining correlations between the dynamic and static elastic properties. Then the well log-based as well as laboratory measurements are used to predict depth-by-depth horizontal stresses. The results show that the static stiffness coefficients are smaller than their corresponding dynamic results but the static Biot's coefficient is higher than dynamic value. And there are good linear regression correlations between above dynamic and static parameters. In the evaluation of log-based models, the ANNIE model is inapplicable here because ratio of νv/νh is not always greater than 1 while the M-ANNIE model improves the prediction of C13, which demonstrate that M-ANNIE model is applicable to shale. In the prediction of horizontal stresses, one hand the relative error significantly decreases after using static parameters, on the other hand the isotropic model underestimates the horizontal stresses compared with VTI model. The Biot's coefficient has a significant impact on predicting horizontal stresses while the effect of thermal stress on horizontal stress is extremely small. The outcomes of this work can potentially improve predictions of elastic properties and horizontal stresses for geomechanical elastic model analysis in anisotropic deep Longmaxi formation.