Evaluating the Geothermal Potential of Hot Sedimentary Aquifers Using a Hybrid Approach

Abstract

ABSTRACT Flow through geothermal reservoirs is highly complex, and often includes contributions from both fracture networks and the porous rock matrix. Discrete Fracture Network (DFN) models are proven, effective tools for the characterization of rock masses especially where fracture dominated fluid flow is encountered; whereas more conventional tools, such as Finite Volume (FV) methods, are more numerically favorable for simulating problems where detailed multiphysics is required. This paper presents a workflow that combines discrete and continuum descriptions that captures the salient features of the geological materials whilst also remaining numerically tractable. DFN models of fractured rock masses are typically developed using statistical distributions to generate realistic three-dimensional (3D) descriptions of the natural fracture network. Superimposed with this fracture description, is a matrix-orientated description based on an intact rock property model. Integration of these two descriptions into a single continuum rock mass description is achieved through a novel discrete-continuum upscaling process which combines fractures and intact properties into a unified form, providing effective mass permeability and geomechanical descriptions. The composite rock mass description is then carried forward into a numerically efficient multiphysics solver that provides effective simulation of both temperature and flow in a fully coupled manner to evaluate the performance potential of geothermal reservoir units. In addition, it will be demonstrated how the presented work can naturally embed within the stochastic framework of DFN and permit a probabilistic based evaluation. This paper presents application of the hybrid DFN-FV workflow for a hot sedimentary aquifer. The application is presented in terms of the characterization steps and a description of the input used, which is then supplemented with the dual fracture and matrix description. The demonstration will also touch on the efficient gridding of geological domains and provide example simulation results of multi-well injector and producer fluid flow and heat transfer. The work in this paper shows how the DFN-FV approach can be systematically employed to help with the success of geothermal well placement and completion studies in hot sedimentary aquifers.