3D Geomechanical Model Construction and Application of Deep Shale Gas in the Luzhou Block, Sichuan Basin

Abstract

ABSTRACT Deep shale gas with a burial depth of 3500–5000m became an important area for shale gas development in China. The characteristics of the rock's mechanics and the in situ stress have a significant impact on the drilling and fracturing effects of horizontal wells. Finite element simulation is used to build an accurate in situ stress model from the data of acoustic logging, diagnostic fracturing injection test, FMI imaging logging, and stress measurement. The results show that Young's modulus and Poisson's ratio in the Luzhou block change vertically with the deposition time. The Longmaxi reservoir has an abnormally high pore pressure gradient which ranges from 16.7 kPa/m to 21.7 kPa/m. The stress regime of the Luzhou block is primarily strike-slip type, with an overburden pressure gradient of 25.5kPa/m, and a minimum horizontal stress gradient range of 18.8 to 24.5kPa/m. According to wellbore breakouts and acoustic emission experiments of cores, the ratio between σhmax and σhmin is 1.165. The horizontal principal stress of the reservoir increases with the increase of Young's modulus and pore pressure. The conclusion is that high-precision geomechanical models can effectively serve to improve the efficiency of drilling operations, improve the production of single wells and benefit development. INTRODUCTION As an important unconventional energy source, shale gas has become a global hotspot for resource development. Shale gas deposits are abundant in China and are predominantly concentrated in the southern Sichuan Basin (Zou, et al. 2022; Ma, et al. 2021; Zhao, et al. 2020). Currently, deep shale gas deposits with a burial depth of 3500–5000 m have recently achieved a strategic breakthrough and have become an important area for shale gas development in China. However, deep shale gas development still faces numerous geological and engineering problems, though, as a result of the short exploration period, complex geological conditions, and inadequate engineering technology adaptability (He, et al. 2021). Shale is a typical "artificial gas reservoir" with nanoscale pore space and Nadasi grade permeability, and has no natural production capacity, therefore, commercial development of shale gas needs the use of horizontal wells drilling and large-scale volume fracturing technologies (Jiang, et al. 2017). The effectiveness of horizontal well drilling and large-scale volume fracturing is significantly influenced by reservoir characteristics and in situ stress conditions. During the drilling stage, high shear stress and pore pressure can lead to wellbore stability problems. Loss of circulation and sticking caused by the unreasonable design of drilling fluid density will negatively impact the drilling cycle (Mehrabian, et al. 2017). At the completion stage, casing deformation is a significant obstacle to shale gas production (Chen, et al. 2016).