Numerical and Experimental Studies of Coupled THM Processes in Fractured Tight Rocks
Authors: Abdullah A. , Rashid H. U. , Olorode O. M. 
Published: September 2023

ABSTRACT
Several researchers address the question of the validity of coupled thermal, hydraulics, and mechanical (THM) simulation results by comparing them to published experimental data. However, the initial and boundary conditions of these experimental studies are typically not fully understood by the users of these results, leading to misleading assumptions in the simulation models. To address these gaps in the study of THM processes, we present an experimental and high-resolution, fully dimensional numerical study of THM processes in tight rocks with stochastic fractures in 3D. The idea is to provide experimental data and reference solutions for subsequent work on multiscale fracture modeling in tight rocks. The performance of these studies at high pressures, temperatures, and stresses makes the results representative of subsurface conditions in fractured tight rocks such as enhanced geothermal systems (EGS) and unconventional oil and gas (UOG) reservoirs. The quantification of the mechanical deformation of these rocks in response to pressure and temperature changes during production is essential for the safe development of these tight rocks.

INTRODUCTION
Crude oil and natural gas currently contribute approximately 68% of the total energy consumed in the United States. Of this oil and gas consumption, 65% comes from shale oil reservoirs while 79% comes from shale gas reservoirs (Nalley and LaRose, 2022). These rocks are typically tight and naturally fractured, requiring the combination of horizontal drilling and hydraulic fracturing to be produced commercially. The process of hydraulically fracturing rocks involves injecting cold fluids into subsur-face rocks at pressures sightly higher than the minimum (compressive) effective stress in the reservoir. To optimize such coupled THM processes in the subsurface, we need models that are accurate representations of the coupled processes in the subsurface.
Despite the significant contribution of unconventional oil and gas (UOG) reservoirs to meeting the energy demands of the United States, there has been an increased concern about its environmental impact over the last decade. This includes the mechanical deformation and slip across faults because of the increased compressional stresses when oil and gas are produced, and the increased tensional and shear stresses when the wastewater from production and hydraulic fracturing are disposed of in basement complex rocks below the UOG reservoirs. These processes are related to the well-known problem of induced seismicity, which has increased significantly since the onset of UOG reservoir development (Murray, 2015). For instance, in 2015, North-Central Oklahoma saw a 900-fold increase in its rate of M3+ earthquakes (earthquakes with a magnitude of 3 and above) compared to what it was before 2009 (Langenbruch and Zoback, 2016). In 2014, Oklahoma surpassed California (historically the most seismically active region in the United States) in the number of M3+ earthquakes annually. To mitigate these environmental risks, it is essential to study the complex processes by which the earth moves or slips in response to anthropogenic activities such as wastewater injection (Hornbach et al., 2015), conventional oil and gas production (Van Wees et al., 2014), hydraulic fracturing (Meng and Wang, 2018), improved/enhanced oil recovery (Rubinstein and Mahani, 2015), development of geothermal resources (Majer et al., 2007; Rathnaweera et al., 2020), underground storage of carbon dioxide (Rutqvist et al., 2016; Vilarrasa et al., 2019), hydrogen and other gases (Burtonshaw et al., 2022), etc.