Numerical Simulation of Hydraulic Fracturing Damage Evolution in Geothermal Reservoirs with Natural Fractures Based on THMD Coupling Model
Authors: Wang J. W. , Qu Z. Q. , Guo T. K. , Chen M. , Zhang B. 
Published: October 2022

ABSTRACT:
The key to develop the geothermal energy economically and effectively is forming a high quality enhanced geothermal system (EGS) with complex fracture network. Clarifying the interaction between hydraulic fracture (HF) and natural fracture (NF) is of positive significance to study how to establish an efficient EGS. In this paper, based on meso damage mechanics, elastic thermodynamics and Biot’s classical seepage mechanics, a thermo-hydro-mechanical-damage (THMD) coupling model of hydraulic fracturing in geothermal reservoir with randomly distributed NFs is established. Then the evolution of thermal field (T), seepage field (H), solid field (M) and damage (D) during hydraulic fracturing is simulated, and the effects of injection flow rates, rock thermal expansion coefficients and the NFs parameters (including length, dip angle and distribution density) on HF propagation are analyzed. The results indicate that under the action of water pressure and thermal stress, the normal stress on the NF surface will increase, resulting in the fracture damage. High injection flow rate would create longer fracture and significantly increase the fracture width, which is conducive to the development of deep thermal reservoirs. And high thermal expansion coefficient of rock is more conducive to form complex fractures. NFs will guide the HF to turn along the dip angle of NFs ,and the longer the NFs are, the stronger their guiding effect on HF. When the angle between the dip angle of NFs and the expansion direction of HF is large, the longitudinal expansion of HF is hindered, but the HF could communicate and activate more NFs. The NFs density directly determines the degree of hydraulic fracturing damage, and the damage area of HF increases with the increase of NFs density.

1. INTRODUCTION
Geothermal energy is a widely distributed, low-carbon, environment-friendly and sustainable clean energy (Potten and Thuro 2017; Zhao et al. 2020). Hot Dry Rock (HDR) refers to the high-temperature rock mass with general temperature greater than 180 °C, buried depth of several kilometers, no fluid or only a small amount of underground fluid (Hofmann et al. 2014; Xu et al. 2012). As a new geothermal resource, it has advantages in geothermal energy development and application, which has attracted more and more attention. However, the HDR found at present is mainly composed of granite, showing low porosity and permeability (Lu and Wang 2015). Hydraulic fracturing technology is a mature and effective method to reconstruct low quality reservoir in oil and gas field development projects (Qu et al. 2021). This technology can be applied to HDR geothermal reservoir to realize efficient development of geothermal energy, too. Through the circulation of cold water in high-temperature rock formations with fractures, the geothermal energy is efficiently exploited. And then the high-temperature fluid will be used for step applications such as power generation or heating, which is the so-called enhanced geothermal system (EGS) (Bujakowski et al. 2015; Guo et al. 2019; Olasolo et al. 2016). Studies have shown that in the production process of EGS, the number of fractures or the complexity of fracture network will significantly affect the exploitation efficiency of geothermal energy (Guo et al. 2020b; Zhang et al. 2021; Jin et al. 2020). In order to form EGS with complex fracture networks for efficient development of geothermal energy, it is necessary to clarify the interaction between artificial fractures and natural fractures, and find out the evolution law of fracture during hydraulic fracturing.