New Development of 3-D Stochastic Model for Design of HDR/HWR Geothermal Reservoirs System

Abstract

The creation of Hot Dry Rock (HDR) / Hot Wet Rock (HWR) [Takahashi and Hashida, 1992] reservoir systems is critical for development of next-generation geothermal energy extraction methodology. HDR/HWR geothermal systems may be engineered in naturally fractured but weakly permeable hard rocks by hydraulic stimulation. The stimulation involves pumping high-pressure fluid injection into the fractured hot rock, which creates a man-made geothermal reservoir with increased permeability. The design of HDR/HWR geothermal systems requires development of simulation models predicting the extension behavior of hydraulically induced fractures. In this paper, an improved three-dimensional simulation model, based on FRACSIM-3D, is presented, which incorporates the mechanical rock-fluid interaction during hydraulic stimulation, in more rigorous way than the existing two-dimensional model FRACSIM-2D, and quantitatively predict the reservoir growth behavior. In this model, pre-existing fractures are generated stochastically, and their radius distribution is assumed fractal. The fluid flow throw the discrete fracture network is solved by converting it to an equivalent continuum mesh under constant injection pressure. The fluid flow is approximated as Darcy type flow, and the pressure distribution within the fracture is used for the calculation of opening and shear displacements. This requires several complete trial flow solutions, realizing the fracture system anew each time and recalculating the shear displacement using the latest inner fracture pressure. The result of this stimulation simulation using data from the European HDR test site Soultzñsous-forest in France are in good agreement with the existing field data. In addition, the effect of variation of the injection pressure during stimulation on the reservoir extension is simulated with the objective of expanding the reservoir volume.