ABSTRACT:

Longmaxi shale rock is a typical anisotropic rock, the most common used strength criterion is the single plane of weakness (SPW) criterion for wellbore stability analysis of anisotropic formations. However, is this SPW criterion the optimal strength criterion for Longmaxi shale rock? To answer this question, the Longmaxi shale rock was collected from the Silurian formation in Sichuan Basin of China, the specimens were cored with different angles (0°, 15°, 30°, 45°, 60°, 75°, and 90°), then, the triaxial compression tests were conducted. Five most commonly used criteria, such as the SPW criterion, weak plane patch (WPP) criterion, Walsh-Brace (W-B) criterion, McLamore-Gray (ML-G) criterion, and Ramamurthy-Rao (R-R) criterion, were identified to evaluate which one is the optimal strength criterion for Longmaxi shale. The grid search algorithm was used to quickly match the material parameters of anisotropic criteria, the correlation coefficient (R2) and absolute average relative error (AARE) were used as the standard to evaluate the fitting degree of five criteria. The results indicated that the optimal one is the WPP criterion, followed by the W-B criterion, SPW criterion, ML-G criterion, and R-R criterion. The correlation coefficient (R2) and absolute average relative error (AARE) of the WPP criterion are 0.9905 and 5.4477\%. Therefore, the WPP criterion is recommended as the optimal strength criterion for wellbore stability analysis of Longmaxi shale.

1. Introduction

With the impact of the successful development of shale gas in North America, the world is also taking adequate measures to increase the exploration and development of unconventional oil and gas, such as shale oil and gas, tight sand oil and gas, coal-bed methane (Ma et al., 2015; Soder, 2018; Jiang et al., 2020; Wang et al., 2020; Wu et al., 2020; Nie et al., 2021). However, the reservoir rocks of unconventional oil and gas are rich in bedding, fissures, joints, cleavage, and other weak planes, resulting in the strength anisotropy of such rocks. The existence of strength anisotropy has brought significant challenges to oil and gas exploration and development. For example, strength anisotropy makes the borehole more likely to collapse and become unstable (Fjar et al., 2008; Aadnoy and Looyeh, 2019). According to statistics, the cost of dealing with borehole instability accounts for more than 15\% of the entire investment and development (Fjar et al., 2008).

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