Coupled Well-Reservoir Heat Modelling for Closed-Loop Geothermal Wells - A Feasibility Study
Authors: Livescu Silviu , Dindoruk Birol 
Published: April 2022

Naturally occurring hydrocarbon resources have been powering the world since the second half of the nineteenth century with increasing proportionality in terms of the energy mix, allowing tremendous economic growth globally. However, another subsurface resource is estimated to hold several orders of magnitude more energy than all hydrocarbon resources. Geothermal energy is renewable, abundant, and has a small carbon footprint, but its current use is geographically sparse and represents only 1% of the global energy production. To scale it up economically to other regions and applications, several critical problems need to be solved. In a recent paper, the effects of several well parameters were studied on the thermal output, assuming steady-state temperature (or successions of steady-state temperature for a given period) in the near-well region. In this paper, the effects of the transient near-well temperature and heat inflow from the formation are studied.

The hydrocarbon industry is evaluating the opportunity of producing geothermal energy from existing oil and gas wells, as electricity and/or low-temperature waste heat. This can potentially yield significant advantages over traditional geothermal wells, especially in terms of reduced capital expenditure. For instance, the performance of geothermal wells, both injectors and producers, is limited by formation damage issues, such as drilling fluid invasion, fines migration, plugging, and mineral scaling. The scale composition is dependent on the formation mineralogy, for producing wells, and on the injected water quality, for injecting wells. Addressing these issues over the entire well life may be expensive and difficult to predict. Depending on the assumed boundary conditions and other simplifying assumptions, numerical simulations of coupled well and reservoir heat and mass transport may help predict more accurately the thermal output and longer-term economics.

In a previous study, a mathematical model was proposed for closed-loop wells (i.e., U-shaped wells, single pipes in wells, and concentric pipes in wells) to study the effect of several well parameters on the thermal output. The focus on that study was on repurposing existing hydrocarbon wells to geothermal wells. The time- and space-dependent temperature solutions for all well configurations were obtained for time- and space-dependent fluid and flow properties. The near-well temperature was considered as steady-state, at least for a given time frame. A sensitivity study was also performed, showing the effects of several well parameters on the temperature of the fluid flowing to surface. Sensitivity results were included for such parameters as fluid flow rate, well length, inner tubing and annulus diameters, geothermal temperature, and overall heat transfer coefficients. The learnings and outcome from that study can also be incorporated in terms of adding wellbore lift models in various reservoir models. In this study, the effect of the transient temperature in the near-well region is considered and a sensitivity study is performed.

Coupled well and reservoir heat and flow modelling for geothermal systems is important for accurately evaluating their thermal output and economics. A previously developed thermal well model assuming steady-state temperature in the near-well region is extended to account for transient drawdown. Based on the recent interest in the scientific literature in this topic, this study evaluates the effects of the heat conduction in the near-well region and the heat transfer from the near-well region to the well for different well configurations and geometries.