Thermal shock in reservoir rock enhances the hydraulic fracturing of gas shales

Thermal shock occurs when a material's temperature is changed over a short period of time such that constituent parts of the material deform by different amounts. The deformation of material due to thermal load can be manifested through strain and stress. As the temperature diffuses from hydraulic fracture into reservoir, the temperature changes with x coordinate and the stress/strain can be obtained from the Equation (6). Once the stress at any point exceeds the strength of material, the body fails in one of the three modes of tension, compression or shear. A thermal load on rock, results in the creation and extension of cracks, crushing the grains, or sliding the grain interfaces. In this paper we look into the possibility of stimulating the rock matrix beyond hydraulic fracturing stimulation by cooling down the rock. The physics of temperature reduction in a solid dictates that when a solid is laterally fixed and undergoes temperature reduction, a thermal stress gradient is induced in the solid body. In rock, this thermal stress gradient leads to a differential contraction of the rock, which in turn creates openings, referred to as thermal cracks. We numerically solve the nonlinear gas diffusivity equation, using finite element method and show that the thermal cracks in rock have the potential to improve the productivity of wells placed in tight formations by 20%.