Multifracture response to supercritical CO ₂ ‐EGS and water‐EGS based on thermo‐hydro‐mechanical coupling method

Abstract

Summary Hydraulic‐fracturing treatments have become an essential technology for the development of deep hot dry rocks (HDRs). The deep rock formation often contains natural fractures (NFs) at micro and macroscales. In the presence of the NF, the hydraulic‐fracturing process may form a complex fracture network caused by the interaction between hydraulic fractures and NF. In this study, analysis of carbon dioxide (CO2)‐based enhanced geothermal system (EGS) and water‐based EGS in complex fracture network was performed based on the thermo‐hydro‐mechanical (THM) coupling method, with various rock constitutive models. The complexity of the fracture geometry influences the fluid flow path and heat transfer efficiency of the thermal reservoir. Compared with CO2‐based EGS, water‐based EGS had an earlier thermal breakthrough with a rapid decline in production temperature. CO2 can easily gain heat rising its temperature thus reducing the effect of a premature thermal breakthrough. Both CO2‐based EGS and water‐based EGS are affected by in‐situ stress; the increase in stress ratio improved the fracture permeability but resulted in an early cold thermal breakthrough. When the same injection rate is applied to water‐based EGS and CO2‐based EGS, water‐based EGS displayed higher injection pressure buildup. Water‐based EGS had higher reservoir deformation area than CO2‐based EGS, and thermoelastic constitutive model for water‐based EGS showed larger deformed area ratio than thermo‐poroelastic rock model. Furthermore, higher values of rock modulus accelerated the reservoir deformation for water‐based EGS. This study established a novel discussion investigating the performance of CO2‐based EGS and water‐based EGS in a complex fractured reservoir. The findings from this study will help in deepening the understanding of the mechanisms involved when using CO2 or water as a working fluid in EGS.

Combination of double and single cyclic pressure alternation technique to increase CO 2 sequestration with heat mining in enhanced geothermal reservoirs by thermo‐hydro‐mechanical coupling method

“…[24][25][26][27][28] Depending on the amount and content of the rock-forming mineral, the rock effective strength is affected under pore-pressure or external stress influence, which further affects the fluid flow and rock deformation degree. 21 The thermo-poroelastic constitutive model captures the effect of pore-pressure on rock strength in simulation analysis. 21,29,30 EGS reservoirs consist of the complex fracture network, propagation and fracture roughness 31 that were formed during the fracturing process.…”

“…21 The thermo-poroelastic constitutive model captures the effect of pore-pressure on rock strength in simulation analysis. 21,29,30 EGS reservoirs consist of the complex fracture network, propagation and fracture roughness 31 that were formed during the fracturing process. The morphology of the fracture void 32 and the matrix pores 33 influence the flow behavior and inaccurate prediction of the CO 2 sequestration.…”

“…Various models of thermo‐hydro (TH), hydro‐mechanical (HM) and thermo‐hydro‐mechanical (THM) describing the coupled quantities were extensively studied . Depending on the amount and content of the rock‐forming mineral, the rock effective strength is affected under pore‐pressure or external stress influence, which further affects the fluid flow and rock deformation degree . The thermo‐poroelastic constitutive model captures the effect of pore‐pressure on rock strength in simulation analysis .…”

“…Reducing production pressure while maintaining a constant injection flow rate was shown to have a significant impact in increasing CO 2 sequestration and improving heat mining efficiency during the initial period of heat extraction from an EGS reservoir . Furthermore, Bongole et al performed the numerical investigation using water or CO 2 in the hydraulically multi‐fractured thermal reservoir at fixed injection and production pressures. The authors showed that CO 2 has a significant heat extraction rate than water.…”

Experimental and Numerical Modelling of Nonlinear Flow Behavior in Single Fractured Granite

“…Gas has the properties of low viscosity and easy diffusibility, so when gas flow through rock fractures, a higher Reynolds number occurs and nonlinear flow regimes, including transition and turbulent flow, generate more easily. Gas flow through rock fracture research can also be beneficial to understand supercritical carbon dioxide fracturing technology used in an Enhanced Geothermal System (EGS) [35,36], since supercritical carbon dioxide has the same property of low viscosity and easy diffusibility [37]. In the present study, nitrogen flow tests were conducted through granite rock fractures under different loading stresses and pressure gradients.…”