Numerical Modeling of Secondary Thermal Cracks in Hot Dry Geothermal Reservoirs

Abstract

Previous studies have shown that during the fluid circulation process in geothermal reservoirs, the thermal stress components which are parallel to the fracture surface are almost one order larger in magnitude than its component normal to the fractures surface. The very large parallel components of thermal stress indicate the great possibility of the generation of secondary thermal creaks in the operation process of geothermal reservoirs. These new thermal fractures could allow fluid to flow to regions of hotter rock, thus significantly enhance the performance of geothermal reservoirs.

A few literature on the secondary thermal fracture can be found in the public domain (Hsu 1978; Barr 1980). However, most of these work are based on impractical assumptions, such as uniform temperature of the fracture surface, closed new thermal fractures, etc. In our paper, we use the extended finite element method (XFEM) to determine the spacing and length of secondary thermal fractures for a two-dimensional problem. The fluid flow in the fracture results in nonuniform temperature of the fracture surface, which would cause the thermal fractures occur in the places adjacent to injection wells first. The former thermal fractures would cause the stress redistribution around and thus affect the generation of the following thermal fractures. Also, the opening of the thermal fractures would allow low-temperature fluid to flow in and then be driven to propagate further. The numerical model is used to take in account the effect of the cooling rate of rock (here it is the fluid injection rate) on the distributions of spacing and length of the thermal fractures, which would be of importance to the practical geothermal operations.