Fast Marching Method: A New Paradigm for Rapid Simulation of Enhanced Geothermal Systems

Abstract

The Enhanced Geothermal System (EGS) allows access to heat energy stored in hot and dry rock, inaccessible to production via traditional hydrothermal systems. The EGS is expected to significantly contribute to a sustainable energy future. However, a key challenge in geothermal resource development is predicting the flow rate, temperature and thermal output of the produced subsurface fluid. One of the preferred methods for such forecasting is the reservoir simulation which is widely used in the oil and gas industry for predicting subsurface fluid production. EGS reservoir simulations involve complex physics, including non-isothermal, compositional flow within hard rocks containing hydraulic and natural fractures. These simulations are computationally intensive, often requiring several hours to days to complete a single simulation run. In this work, we propose a new paradigm for rapid multi-domain, multi-resolution simulation of EGS that accelerates reservoir simulation by orders of magnitude. Our reservoir simulation method uses a finite-volume-based Fast Marching Method (FMM) to efficiently compute Diffusive Time of Flight (DTOF) and transform 3D simulations into equivalent 1D simulations using DTOF as spatial coordinate. The DTOF, which tracks pressure front propagation, guides the discretization of the 1D mesh. To maintain accuracy, the proposed method preserves 3D resolution near the wellbore and hydraulic fractures while converting the rest of the reservoir to 1D grid, resulting in significant computational speed up. The 3D and 1D domains communicate through non-neighbor connections that incorporate both fluid and heat transmissibility. Our proposed method was applied to a triplet horizontal well model consisting of one horizontal injector located between two horizontal producers for efficient heat extraction. Cool water is injected into the reservoir through the injector, and heated fluids are extracted from the producers. We performed reservoir simulations for 20 years to evaluate the life-scale performance of geothermal projects. The proposed FMM based multi-resolution simulation model provides flow rates, temperature and thermal power output with orders of magnitude speed up compared to full 3D fine-scale simulation. With the accelerated workflow, the proposed approach now allows for performance assessment and optimization of geothermal projects in hours compared to the typical timeframe of several days using commercial simulators, making such modeling practically feasible for routine applications.