Harvest Geothermal Energy from Low-permeable thin Formations using CO₂: A Doublet Horizontal Well Placement

Harvest geothermal energy using CO<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> as the working fluid from its sequestration reservoir emerges as a feasible carbon utilization approach. For a thin and low-permeable reservoir, horizontal wells have to be placed to achieve feasible efficiency. In a closed or semi-closed thin reservoir, pressure propagation, fluid flow and heat transport are significantly dependent on the horizontal well length, well space, and boundary conditions. This study aims to investigate their coupled impacts on CO<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf>-circulated geothermal harvest in such a reservoir using a response surface modeling method. A doublet horizontal well is placed in a 20m thick, 2km×2km semi-closed reservoir block saturated by CO<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> and brine. Horizontal well lengths, well space, lateral boundary volume factors, are sampled by Latin-Hyper cube approach, and 40 samples are input to a hydro-geothermal model to simulate the CO<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> circulation. Four performance indicators, that is, lifetime, recovered geothermal energy, injectivity, and net injected CO<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf>, are calculated from the 40 simulations. Trained by the simulation data, response surface models are developed mapping the relationships between the performance indicators and input parameters. Using the response surface models, global sensitivity indices (SI) are calculated to evaluate the coupled impacts of selected inputs on the reservoir performance. Well space is demonstrated as the most important parameter, with SI= 0.96, 0.95, 0.56, and 0.50 on the four performance indicators. The findings could facilitate an optimal well placement to maximize geothermal harvest efficiency.