Abstract
The limitation of conventional hydraulic fracturing with two long wings of coplanar fractures is well recognized in the context of naturally fractured tight gas or Hot Dry Rock (HDR) geothermal reservoirs. This paper presents a 3D model for an alternative stimulation technology for such reservoirs. The model stochastically simulates actual reservoir representative natural fractures processing field data available from cores and logs. These simulated fractures are then analyzed for deformations with the combination of simple elastic structural mechanics and linear elastic fracture mechanics principles coupling with the injected fluid pressure and in-situ stresses. Finally, the hydraulic conductivity and the reservoir growth pattern are formulated as functions of fracture deformations.
The applicability of the model has been verified using the data of actual fracture stimulation programs conducted in the Hijiori HDR site. It has been found that the model is capable of simulating actual natural fracture distribution in the reservoir. The model is finally applied to a series of numerical analysis with central Australian reservoir conditions to investigate the sensitivity of natural fracture parameters (e.g. size, density and orientation) and in-situ stresses to reservoir growth and conductivity. It is observed that the reservoir growth pattern is mainly influenced by fracture parameters and the relative magnitude and direction of in-situ stresses. Reservoirs with predominantly strike-slip and reverse faulting stress regimes and high deviatoric stresses are favorable for horizontally dominant reservoir growth - a pattern which is highly desirable for efficient HDR geothermal energy extraction. The information provided in the paper is directly applicable to HDR geothermal reservoir development with a high potential for new applications in tight gas reservoirs in which the abundance of natural fractures is so far posing significant complexity to conventional hydraulic fracturing, resulting in multiple fractures, high treatment pressure and premature screen-out.
