Expertise in Complex-Well Construction Leveraged for Geothermal Wells

Abstract

_ This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 204097, “Constructing Deep Closed-Loop Geothermal Wells for Globally Scalable Energy Production by Leveraging Oil and Gas Extended-Reach Drilling and High-Pressure/High-Temperature Well-Construction Expertise,” by Eric van Oort, SPE, Dongmei Chen, SPE, and Pradeepkumar Ashok, SPE, The University of Texas at Austin, et al. The paper has not been peer reviewed. _ In the complete paper, deep closed-loop geothermal systems (DCLGS) are introduced as an alternative to traditional enhanced geothermal systems (EGS) for green energy production that is globally scalable and dispatchable. The authors demonstrate that DCLGS wells can generate power on a scale comparable to that of EGS. They also highlight technology gaps and needs that still exist for economically drilling DCLGS wells, writing that it is possible to extend oil and gas technology, expertise, and experience in extended-reach drilling (ERD) and high-pressure/high-temperature (HP/HT) drilling to construct complex DCLGS wells. Introduction CLGS is considered a subset of EGS, but the authors write that it is a distinct entity. EGS mostly involves well designs that rely on fractures for heat extraction. Such systems are different from CLGS wells in that the latter use closed conduits for thermal fluid circulation and heating. CLGS relies on fluids pumped through a closed loop. The authors treat CLGS systems as being different from EGS systems, with the understanding that drilling technologies discussed in the paper as enablers for CLGS wells apply equally to EGS wells. In the geothermal (GT) domain, the majority of attention and funding currently is assigned to EGS projects. A case is made in the complete paper to continue to develop DCLGS technology because of its favorable risk profile compared with EGS. Part I of the complete paper introduces a hydraulic model coupled with a thermal model suitable for calculating the power generation of DCLGS wells. This synopsis concentrates instead on Part II of the complete paper, in which technology gaps and needs of DCLGS drilling and well construction are highlighted and opportunities identified where oil and gas experience and technology can be directly applied and leveraged. Similarities and Differences of Deep GT and Oil and Gas HP/HT Wells - GT wells generally use larger production hole sizes than typical land wells. - Casing-cement annuli typically are cemented back to surface. - GT wells can be drilled in more-forgiving pore-pressure fracture gradient (PPFG) environments with wider drilling margins than geopressured HP/HT wells in hydrocarbon systems. - Severe lost circulation appears to be a universal problem in deep GT wells. - Drilling costs can account for 50% or more of the total capital costs for a GT energy project. - Data sets on GT wells are much smaller than those for oil and gas wells.