Sensitivity Analysis of Low-Temperature Geothermal Reservoirs: Effect of Reservoir Parameters on the Direct Use of Geothermal Energy

Abstract

Geothermal energy is a vast source of renewable energy. The latest improvements in the field of Engineered Geothermal Systems (EGS) have opened a new chapter for the use of geothermal energy. The areas lacking conventional hydrothermal resources can harness geothermal power with the help of EGS. Low-temperature geothermal reservoirs using EGS technology to increase the permeability to geothermal fluids can be used for residential as well as commercial space heating, thus, reducing the carbon footprint of space heating compared to using natural gas or other fossil resources. The eastern United States generally has lower temperature gradients than the western United States; However, West Virginia, in particular, has higher temperature gradients compared to other eastern states. A recent study at Southern Methodist University by Blackwell et al. has shown the presence of a hot spot in the eastern part of West Virginia with temperatures reaching 150°C at a depth of between 4.5 and 5 km. This study examines a reservoir at a depth of around 5 km resembling the geology of West Virginia, USA. The temperature gradients used are in accordance with the SMU study. In order to assess the effects of the reservoir conditions on the lifetime of a low-temperature geothermal system, we have performed a sensitivity analysis study on seven natural and humancontrolled parameters within a geothermal reservoir: reservoir temperature, injection fluid temperature, injection flow rate, porosity, rock thermal conductivity, water loss (%) and well spacing. The sensitivity analyses used two different methods of parameter variation, ‘One Factor At a Time (OFAT) method’ and a Plackett-Burman design. For both the OFAT and Plackett-Burman designs, all seven of the parameters mentioned above were used. The OFAT method was performed by changing one parameter at a time, while keeping the rest at constant base case values. A 30-year timeframe of operation was used to run the reservoir simulations using TOUGH2 numerical simulation software developed at the Lawrence Berkeley National Laboratory using the EOS1 equation of state module for pure water. A porous medium approach was taken to design the reservoir. For the full-parameter sensitivity analysis, a two-level (L=2) Plackett-Burman experimental design was used, with the cumulative hot water production discounted to the current year as the measured variable for comparison. The discount rate chosen was 5% (to illustrate direct-use systems incorporated into public utilities), resulting in the contribution to the net present value of a reservoir. The effects of the parameters on the real and discounted production rates were assessed in this analysis. The results of this study provide a preliminary assessment of the effects of various reservoir parameters on the economic viability of low-temperature geothermal utilization. They also provide a comparative approach between the parameters for the optimized exploitation of a reservoir. As expected, the initial reservoir temperature has the most significant effect on the reservoir productivity.