A Thermal-Mechanical Coupled DEM Model for Anisotropic Deep Shale Reservoir Rocks

Abstract

ABSTRACT With the development of reservoir stimulation technology, more and more deep shale reservoirs (>4000m) have become the targets of shale gas exploitation. In order to better understand the performance of shale in deep reservoir environment, we carry out a series of high-temperature and high-pressure experiments on the Longmaxi shale. A thermal-mechanical coupled model for anisotropic rocks is then proposed based on the discrete element method (DEM). The thermal behaviors are implemented by introducing thermal expansion algorithms, while the anisotropic characteristics are reproduced by replacing any linear parallel bonds dipping within a certain angle range with smooth-joint contacts. The proposed theory is validated by comparing the numerical results with experimental data under different thermal and stress conditions. The influence of the high temperature and high pressure conditions is then analyzed. The numerical results show that high confining pressure conditions lead to an increase of peak strength, elastic modulus and residual strength. High temperature treatment could induce thermal damage in the model, result in a degradation in macro mechanical properties. The proposed model exhibits obvious ductile characteristics under high temperature and high pressure conditions. INTRODUCTION Over the past few decades, with the development of reservoir stimulation technology, more and more deep shale reservoirs (>4000m) have become the targets of shale gas exploitation, which led to an increasing attention on the mechanical and thermal response of shale in deep reservoir environment. On this behalf, considerable efforts have been devoted to the study of shale behavior under high temperature and high pressure conditions. Rybacki et al. (2017) experimentally examined the creep behavior of shale, subjected to temperature up to 200°C and confining pressure up to 200 MPa, simulating elevated in situ depth reservoir conditions, and their samples showed that the semi-brittle creep of shale presented high deformation rates under high temperature, high stress, and low confinement (Rybacki et al., 2017). Guo et al. (2020) conducted uniaxial compression test and acoustic emission monitoring on shale specimens with different bedding plane inclinations after thermal treatment of different temperatures. They found that the P-wave velocity decreases with the increase in bedding plane inclination at each thermal treatment temperature showing anisotropy (Guo et al., 2020). Chandler et al. (2017) measured the fracture toughness of shale along all three principal crack orientations at elevated temperatures, and the fracture toughness was seen to change only very little as a function of temperature in the range below 120°C, a modest increase was then observed at temperatures between 120°C and 200°C (Chandler et al., 2017). Wang et al. (2019) studied the anisotropic permeability of shale at in-situ reservoir conditions, their experimental results showed that the threshold temperature of permeability in the direction perpendicular to bedding is 450°C and in the direction parallel to bedding is 400°C, and the formation of thermally induced cracks as well as the connected macropores that result from pyrolysis are the main reason for the increase of shale permeability at elevated temperature (Wang et al., 2019). Although experiments mentioned above have revealed important characteristics of shale under different environmental conditions, the inherent mechanism of the internal thermal damage generated in deep reservoir shale due to the different expansion coefficients of mineral grains is difficult and costly to clarify experimentally. Therefore, in order to better understand the thermal and mechanical behaviors of confined shale, which is a key factor guiding the development of deep shale reservoirs, experimental and numerical studies are both carried out in our research.