Proxy-model for Flow and Transport in Geothermal Reservoirs

Abstract

The hot water produced from a geothermal doublet possesses energy, which once utilized, the water cools down and is re-introduced back into the same reservoir at a sufficient distance using an injector well. As cold water flows through the reservoir, it acquires thermal energy from surrounding in-situ rocks. This process recurs until a substantial drop in rock temperature occurs and the water is unable to be recharged adequately; as a result, cold water starts to "break-through" into the production well and the doublet soon needs to be abandoned.
This breakthrough time can be predicted using reservoir simulation. Significant work has been done in the past to determine the effect of different parameters on the breakthrough time and accuracy of models have been improved by incorporating real world physics. Despite being capable to predict breakthrough time, accurate high-fidelity 3D models require significant time in uncertainty quantification and data assimilation analysis due to CPU demanding simulations.
In this project, a physics-based proxy model is developed to predict flow and heat transport in low-enthalpy reservoirs. Streamlines that describe flow in a system and mostly controlled by steady-state pressure distribution are traced using Pollock's method (Pollock, 1988) and the reservoir is divided into streamtubes. Rock-heat depletion is modelled by semi-analytic model along streamlines. The objective is to predict geothermal doublet breakthrough time using only a limited number of streamtubes, thus minimizing simulation time and CPU resources.
Comparison with accurate high-fidelity model reveals that results for proxy model are optimistic; the error for pressure and temperature distributions, as well as the breakthrough curves is within the acceptable tolerance. The time required to simulate the proxy-model is less than the high-fidelity model. And as the number of streamtubes (to simulate the proxy model with) decrease, the time required for simulation further decreases but conversely the error between the breakthrough curves increases.