On the Role of Thermal Stress in Reservoir Stimulation

Abstract

The paper presents a numerical study of the role of thermal stress on fracture propagation in the vicinity of a wellbore and on the fracture permeability during injection operations. For the case of wellbore stimulation and considering the regime of relatively long crack growth, the number of initial cracks and the rock fracture toughness have a small influence on the crack length. At times, particularly when the in-situ stress is isotropic or has a weakly anisotropy, the direction of crack growth becomes unstable and straight radial cracks start to grow in circumferential direction. In the presence of an anisotropic stress field, the cracks grow in the direction of maximal compressive stress, as expected, but the crack length is controlled by the minimal compressive stress. The influence of thermal stress on fractures and their propagation is considered for the problem of sudden cooling of a rock half-space, injection/extraction process in fractures, and cooling of a wellbore. Interaction of multiple secondary thermal fractures and their trajectory is considered. Results indicate that length of the secondary cracks is mainly determined by the temperature distribution in the geothermal reservoir. In the case of instant cooling of the half-space surface, the cracks length is proportional to the cooling depth L. Under suitable conditions of in-situ stress regime and cooling, thermal stimulation can lead to significant fracture propagation. However, a timely increase in reservoir permeability requires pressurization of the thermal cracks.