Simulating fracture initiation from inclined wellbore in layered reservoir using analytical model

Abstract

ABSTRACT For the layered reservoirs such as shale, the anisotropic mechanical properties exhibit variations in elastic modulus and tensile strength, both of which are factors influencing fracture initiation pressure (FIP) from the wellbore. The previous works are mainly concentrated on analyzing the influence of elastic modulus anisotropy but the anisotropic tensile strength is usually ignored. In this study, we extended the previous work by considering the effect of both elastic modulus and tensile strength anisotropy to predict FIP in layered reservoir. The analytical solution of stress concentration around wellbore in anisotropic reservoir is adopted, and a layered tensile strength formula applied to 3-D stress conditions is developed. We verified the model with numerical simulation and experimental data. Results show that both elastic modulus anisotropy and tensile strength anisotropy are important factors affecting FIP, and the influence of anisotropic modulus is greater than anisotropic tensile strength. FIP reduces monotonously with the degree of Young’s modulus anisotropy and tensile strength anisotropy increasing. Besides, when the Young’s modulus anisotropy ratio (the ratio between the Young’s modulus in the layered planes and normal to the layered planes) is larger than 3, there is a drastic reduction of FIP by 100% at high wellbore inclination (≥75°). Furthermore, the effect layered planes occurrence is tremendous when the wellbore is close to parallel to the layered planes, and the reduction of FIP can be as much as 17%. Moreover, the influence of anisotropy on FIP is significant under normal fault and strike slip, and the maximum FIP reduction is 19%. This work is meaningful for developing unconventional reservoir.