Numerical Study of Hydraulic Stimulation and Heat Production of Enhanced Geothermal Reservoirs using Discrete Element Technique

English Abstract:

This presentation summarizes some of the result of a study which intends to identify the most important characteristics of Enhanced Geothermal Reservoirs (EGS) and their effect on sustainability of heat production using numerical modeling tools. The numerical modeling approach is based on the discrete element technique and simulates the thermo-hydro-mechanically coupled response of reservoirs with pre-existing discrete fracture network. In the adopted technique, the transient laminar flow through the pre-existing fracture network is simulated. The advective heat transfer by fluid flow, the convection at the boundary between moving fluid and rock, and the conduction of heat through surrounding rock are taken into account. The mechanical solution is fully coupled with thermal and hydraulic responses.

The numerical method is verified first by comparing the model result with the analytical solution for the problem of temperature draw-down in the rock due to injection of cold water into a single fracture. Subsequently, using thermo-hydro-mechanically coupled numerical modeling approach, it is studied how field conditions (e.g., fracture size distribution, density and orientation, fracture mechanical properties, in-situ stress conditions), operational parameters (e.g., injection rate) and well positioning affect the response of EGS during stimulation by evaluating the extent of the stimulated region and the improvement in overall connectivity and permeability. Subsequent to the stimulation phase, the production phase is simulated and the correlation between heat production metrics and in-situ characteristics and operation parameters is studied by evaluating injectivity, productivity and generated thermal energy.

For a sensitivity study, the reasonable range of variation of each parameter is evaluated first. Subsequently, the response of the reservoir over the chosen range is studied. The results shed light on the complex mechanisms that control the response of EGS reservoirs to injection during stimulation and production.