Closed-Loop Geothermal Well Design with Optimization of Intermittent Circulation and Thermal Soak Times
Authors: McSpadden Albert R. , Coker Oliver D. , Bohlander Spencer 
Published: September 2022

A closed-loop geothermal well design is presented which incorporates both wellbore configuration and completions components as well as a strategy for well operations which together 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. Advantages of Closed-loop Geothermal Systems (CLGS) compared to Enhanced Geothermal System (EGS) designs are also considered.

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 detailed conduction, forced and natural convection and radiative heat transfer modes in both the wellbore and the formation as appropriate. Detailed thermophysical characteristics are incorporated into the model for all wellbore completion components which include industry available OCTG 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 used as a demonstration working fluid and the full spectrum of fluid behavior for all potential phase and quality regimes are accounted for throughout the circulation flow path and at the surface wellhead. 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 reference models are also considered.

Well simulations presented herein achieve repeatable and extended return fluid temperatures in the range of 200°F to over 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 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. Impact of various completion components on operational efficiency can be quantified. In particular, the optimal staging of intermittent circulation operations and their associated thermal soak periods is a featured design option which has not received wide consideration in the literature.