Study of the Stress State Around Dynamically Pressurized Underground Cavities in Porous Saturated Rock Formations and its Implication for the Fracture Growth Analysis

Abstract

ABSTRACT: A nonlinear constitutive plasticity model for porous saturated rock has been used in numerical modeling of dynamic pressurization of a cavity excavated in the subsurface under various in-situ stress states. Numerical solutions have been first validated with known analytical solutions in the cases of simple strength models and quasi-static cavity pressurization. Then, numerical simulations have been conducted for more sophisticated constitutive models for porous rocks of various porosities and saturations. Sensitivity studies have been conducted for variable cavity pressurization rates, in-situ stresses and material strength properties to established various regimes of gas-driven fracture growth. The model can be useful for a variety of applications such as dynamic well stimulations for unconventional oil/gas reservoirs and enhanced geothermal systems, subsurface compressed hydrogen and air storage, and nuclear waste disposals, and among others. 1 INTRODUCTION Experiments with underground chemical explosions in the subsurface are used to study seismic wave generation and propagation to improve monitoring technique for various subsurface activities (Blandford, 1977). In such experiments, explosions are usually initiated either in cylindrical canisters placed in vertical boreholes (Snelson et al., 2013) or in cavities excavated at the end of a drift tunnel underground (Smith, 1980). The goal of these studies is to understand the character of the ground motion caused by explosions and compare it with the seismic motion due to natural causes or other human activities (e.g. earthquakes, mine collapse etc). Underground explosions may lead to growth of long radial fractures, which may cause generation of shear waves propagating to seismic distances (Johnson and Sammis, 2001; Vorobiev, 2023). It has been shown, that the gas flow into preexisting or shock-wave-generated cracks is a dominant mechanism for dynamic growth of radial fractures which takes place on a longer time-scale relative to the shock wave propagation time (McHugh, 1983; Kutter and Fairhurst, 1971; Fourney, 2016; Rossmanith et al., 1996; Torres et al., 2023). The stress field established around the borehole or cavity where the explosive has been initiated plays an important role in gas-driven fracture propagation. Therefore, modeling the stress field evolution in dynamically loaded cavities improves the understanding of the fracture dynamics to either minimize or maximize the fracture generation process. For example, when the goal is to compare the ground motion caused by earthquakes and explosions one would seek to better contain the explosive products and minimize the fracture generation to avoid the shear motion generation. On the other hand, High Explosives (HE) and propellants have been considered to fracture various rocks in the subsurface especially at depth of a few km where hydraulic fracturing is expensive and less efficient due to high fracture initiation pressure. They have been used to enhance permeability of coal formation for in-situ coal gasification projects (Butkovich, 1976) as well as the permeability of oil and gas reservoirs (Warpinski et al., 1979; Krilov et al., 2008). Also, they have been considered to generate fracture networks to improve circulation between the wells for various geothermal projects (Fourney et al., 1981; Chu et al., 1988). Attempts have been made to find the best high energy source regimes to produce the longest fractures using a fixed amount of energy using both numerical modeling and experiments (Nilson et al., 1991; Wu et al., 2018).