Hydraulic and thermal fracturing techniques for successful stimulation in unconventional geothermal energy recovery

Abstract

Unconventional or enhanced geothermal systems (EGSs) have been recently identified as potential geothermal resources which can be utilized to extract the heat trapped in the deep geological formations. However, due to the low formation porosity in these systems, an underground heat exchanger must be artificially created to enhance reservoir permeability. A number of reservoir stimulation techniques are adopted in EGSs to enhance their permeability, including hydraulic fracturing and thermal fracturing. The aim of the present work is to provide an in-depth understanding of these reservoir stimulation techniques based on our current laboratory experimental work. Recent literature on the topic has been comprehensively reviewed, and advanced laboratory tests have been conducted to understand hydraulic and thermal fracturing techniques under reservoir conditions. Experiments were conducted utilizing the high-temperature high-pressure rock tri-axial apparatus and quenching treatments were performed by injecting cold water into granite rocks heated to different temperatures. Flow-through experiments were also conducted on intact and fractured granite rocks and the results were compared to predict production enhancement upon stimulation. CT scanning technology was employed to determine micro-scale characteristics following stimulation. Experimental work revealed that the propagation paths and apertures of hydraulic and thermal fractures are controlled by the in-situ stress and temperature and the heterogeneity of the rock matrix. Although the induced fractures contributed substantial enhancement of reservoir permeability, they were sensitive to stress and temperature changes due to the larger effective stresses and thermally induced volumetric expansion.