Monte carlo analysis of heat extraction simulation under the decreasing trend of permeability enhancement in the hot dry rock reservoir

Over the past half-century, field tests for enhanced geothermal system (EGS) worldwide have revealed the significant risks associated with the development of low porosity and low permeability hot dry rock (HDR) reservoirs. In view of the transmission and traceability of production uncertainty, the realization of stochastic characterization of permeability distributions and the corresponding output simulation preview can provide a general and objective evaluation of EGS production under the current permeability enhancement technology. Based on the Monte Carlo method, a traversal simulation of geothermal energy extraction is conducted to achieve a predictive analysis of thermal energy production under the same enhanced permeability conditions. Firstly, the overall decreasing trend of permeability is determined based on the magnitude and expansion range of permeability enhancement in artificial reservoirs, and the turning bands method (TBM) is used to generate random distributions, superimposing decreasing trends and random distributions to construct permeability distribution samples with overall determinism and local randomness. Secondly, a multiphysics coupling model is established, and geothermal extraction simulations are conducted on the constructed permeability distribution samples under unified initial and boundary conditions. Finally, the production evaluation indicators for each sample are derived from the numerical simulation results, and the distribution characteristics of the production indicators are analyzed. Based on statistical analysis of the production indicators, this paper highlights that enough flux which can sweep adequate stimulated reservoir volume (SRV) is the key to different production patterns. When setting the critical values of 20 kg/s of production flow and a service life of 15 years, the failure rate of samples is approximately 1/4. This indicates that, even under conditions of maximum permeability enhancement by 5 orders of magnitude and an expansion range of 350 m, the EGS system still faces significant production capacity risks. The two key points for effectively constructing EGS artificial reservoirs are the effective connection of enough high-permeability domains and the avoidance of dominant seepage channels. That is, the development of HDR requires reservoir enhancement technologies with higher controllable technique. Thus, under the current technological conditions, a conservative and cautious attitude should be maintained towards the decision of EGS construction.