利用热-孔隙流体耦合有限元数值模拟研究干热岩开发温度下降过程——以青海共和盆地恰卜恰地区干热岩开发为例

Thermal-hydraulic finite element simulation of temperature decrease process during hot dry rock exploitation:A case study in the Qiabuqia area,Gonghe Basin,Qinghai Province

水力压裂是干热岩(HDR)开发最常用到的压裂方式,水力压裂形成的裂缝网络为增强地热系统的运行(EGS)提供了高渗透率的人工储层.本文在系统总结前人关于共和盆地水力压裂实验、数值模拟资料和现场水力压裂监测结果的基础上,引入了离散裂缝网络(DFN),利用多物理场模拟软件COMSOL Multiphysics建立了共和盆地恰卜恰地区干热岩开采过程中的二维裂缝-基质热-孔隙流体耦合模型,并讨论了裂缝开度和基质渗透率对干热岩开采温度下降过程的影响.结果表明,裂缝网络是流体运移的主要通道.温度下降和早期压力变化范围沿着裂缝延伸,并向周围被裂缝分割的基质扩展.裂缝开度和基质渗透率是影响干热岩地热开采过程中温度下降的重要因素.当裂缝开度越大时,流体运移范围就越大,储层温度和产出水温下降就越快,储层下降范围就越广,热突破时间和运行寿命就越短.当基质渗透率越大时,越有利于流体进入基质进行热量交换,越容易从干热岩中提取热量,产出水温下降越快,运行寿命越短.

Hydraulic fracturing is widely employed for hot dry rock(HDR)exploitation,providing a fracture network that facilitates high-permeability of artificial reservoirs in Enhanced Geothermal System(EGS).This study establishes a 2D thermal-hydraulic coupling model for fracture-matrix media to simulate the extraction of HDR geothermal energy in the Qiabuqia area,Gonghe Basin using COMSOL Multiphysics software.The model incorporates insights from previous hydraulic fracturing experiments,numerical simulation,and on-site hydraulic fracturing monitoring data in the Gonghe Basin.This study investigates the influence of fracture aperture and matrix permeability on the temperature decrease process of HDR exploitation.The simulation results demonstrate that the fracture network is the main channel for fluid migration.The temperature decrease and early pressure change range extend along the fractures and then extend to the surrounding matrix divided by the fractures.The fracture aperture and matrix permeability significantly influence the temperature decrease process of HDR exploitation.When the fracture aperture is larger,the wider the fluid migration range,the faster the temperature decrease of the reservoir output water,and the shorter the thermal breakthrough time and the operating lifetime.Higher matrix permeability enhances fluid entry into the matrix for efficient heat exchange,leading to faster output water temperature decrease and earlier operating lifetime.