Assessment of Geothermal Energy Extraction from the Mt. Simon Sandstone at University of Illinois at Urbana-Champaign Using a Doublet Well System

The presence of low-temperature sedimentary basins in the midcontinent of theUnited States hasspurred interest in utilizing geothermal energyfrom deep saline aquifers to reduce the useof fossil fuels for direct heating and cooling. However, developinggeothermal resources is hinderedby high capital costand risks associated withthe feasibility of extracting theresources. To reduce risk anduncertainty in estimating the extractable energy from the Mt. Simon Sandstone (MSS)in the Illinois Basin (ILB), amodeling workflow was developedto assess feasibility of deliveringgeothermal energyusing a two-well (doublet)system.The proposed Deep Direct-Use (DDU) Geothermal Energy System (GES) would directly supply geothermal energy toheat agricultural research facilities(ARF)at the University of Illinois at Urbana-Champaign (U of IL). The total amount of geothermal energythat will be transported to the ground surface was determined by modelingtemperature changes from the MSS reservoir to the surface.A geocellularmodel informed the reservoir modeling, which was developed withhydraulic and thermal propertiesmeasured in boreholes drilled within a 36-square mile (93 km2) areaof aroundthe U of IL.Geothermal reservoir simulations were performed to estimate maximumrates for extracting and injecting the geothermal fluid andevaluate the sensitivity of reservoir temperature distribution with changingthewell spacing, extractionand injection rates, and seasonal (ambient ground surface)temperatures.Reservoir modeling results predict maximumextractionandinjection rates that greatly exceedtherequired flow rateof954m3[6,000 bbl/d]to meet the ARF heating ~2 MMBtu/hour–-and maintaina temperature difference of 11°C(20°F)between the extracted and injected fluid.A 2-D,axisymmetric wellbore model extending from the ground surface to the bottom of the MSS(~1,751 m [5,745 ft] depth) was used to simulate temperaturechanges during extraction and injection. Thismodel was calibrated to distributed temperature sensing (DTS) logfrom a CO2storagewellat the IBDP.The calibrated wellbore model was used to evaluate how variations in the extraction rate, wellbore insulation, and thermal propertiesof the wellbore materials(i.e., tubing, casing, cement)impact the temperature during extraction. Additionally, the effects of rate, tubing radius, and fluid temperature during injection were investigated. Wellbore modeling results indicated that installing a vacuum-insulated tubing or placing silicate foam around the extraction well tubing wouldpreserve the heat stored in extracted geothermal fluidand limitsthe temperature change(loss)to <0.6°C(<1°F). Overall, the temperature changedecreased as the extractionand injection rateswere increased. Changes in formation heat capacity and tubing radiushad negligible effectson the temperature change, whereas thetemperaturechangewas reduced as the injection temperaturesincreased.Resultsfrom reservoir and wellbore modeling indicatethat enormous geothermal resourcefrom the MSScan be extracted tomeetARF energydemand. The findings from geothermal reservoir simulations and wellbore modelinginformed the design of the surface facilities and doublet well system