Closed-Loop Geothermal Well Design: Simulation Study of Intermittent Circulation and Thermal Soak Times

Summary A closed-loop geothermal well design is presented that incorporates both wellbore configuration and completion components, as well as a strategy for well operations, which together can potentially achieve meaningful production of thermal energy. Planning and optimization of intermittent circulation enable “thermal soak” periods to thermally charge the working fluid while mitigating thermal depletion in the reservoir. Technical challenges of a viable closed-loop downhole heat exchanger scheme are discussed given its exclusive dependence on thermal conduction. The advantages of closed-loop geothermal systems (CLGS) compared with enhanced geothermal system (EGS) designs are also discussed as a motivation for the consideration of CLGS. Fully transient and closely coupled thermal-hydraulic simulations using an industry-standard software model were performed on a representative well design and schedule of well circulation operations. The simulation model accounts for conduction, forced and natural convection, and radiative heat transfer modes in both the wellbore and formation as appropriate. Detailed thermophysical characteristics are incorporated for all wellbore completion components, which include industry-available oil country tubular goods grades and sizes, specialized variations such as vacuum-insulated-tubing (VIT), insulating fluids including nitrogen, conventional and foamed cements, and syntactic foam as well as the variation in the Earth formation. Water is the demonstration working fluid, and the full spectrum of fluid behavior for all potential phase and quality regimes is accounted for throughout the circulation flow path. Resultant transient temperatures over an extended sequence of flow and shut-in periods are reported inclusive of near-by Earth formation temperatures out to the far-field boundary. Comparisons with analytical models are also considered. Well simulations presented herein achieve repeatable and extended return fluid temperatures in the range of 200°F to greater than 400°F. In combination with a pad well concept, this allows for long-term steady energy generation. Clearly, the generation of useful temperatures and ultimately justifiable enthalpy delivery with closed-loop configurations is a challenge. Further work on innovative design concepts, refinements such as integration with surface plant processes to optimize surface pressures and pump requirements as well as the recycling of heated water, and the identification of optimal locations for deployment will progress this work. Advantages of fully closed-loop well systems include avoidance of potential problems associated with traditional geothermal and EGS wells such as induced seismicity and bedding plane slippage, formation interface skin quality, reservoir degradation over time, and introduction of corrosive formation species into the wellbore, and disposal thereof. Combined optimization of both wellbore configuration components and staged circulation and thermal soak periods is shown here to provide a realistic option for significant steady heat generation. The impact of various completion components on operational efficiency can be quantified. In particular, the staging of intermittent circulation operations and their associated thermal soak periods is a featured design option.