A geothermal energy techno-economic analysis for downhole wellbore hydrogen production from biogas with subsurface carbon retention

Abstract

Improving overall resource efficiency enhances energy security. Biogas is an important asset within waste management, transforming a range of organic waste into a higher-value product. By creating integrated partnerships, sector coupling highlights the synergies of Geothermal Energy, District Heating, Industry-CO₂, Biowaste and Agriculture. This paper offers a perspective on a novel geothermal methodology for the wellbore reformation of biogas to generate hydrogen production with in situ carbon capture and storage (CCS) and proposes a new disruptive approach with a more immediate, direct and effective route to net zero. The methodology is referred to here as Carbon Injection and Gasification Geothermal (CIGG). The CIGG process combines several processes (i.e., hydrogen generation, carbon capture and biogas upgrading) with low-grade heat geothermal to eliminate process steps, saving process energy, costs, and materials, to create one, combined, sustainable solution. To capture these synergies, a wellbore methane reformation tool is proposed that exploits the natural geo-pressure from geothermal reservoirs and their associated formation fluid (hereafter power fluid). The hot injected CO2 waste stream eliminates the temperature depletion of the formation that is normally associated with geothermal power fluids. The immediate, in situ, downhole capture of CO₂ will also enable improved geothermal power efficiencies from any CO₂ partially recirculated within the power fluid. With geothermal wells having an expected life span of 15–25 years these synergies will enhance energy security for the long term. The CIGG process is proposed as a true win–win for both the energy economy and environmental stewardship, future-proofing biogas assets against emerging climate laws that restrict carbon production. It is climate-beneficial while creating a more holistic, sustainable CCS system that is a free byproduct of a net-energy production system, which simultaneously reduces carbon footprint to accelerate net zero goals. A techno-economic analysis was performed to estimate the cost of hydrogen generation, together with analysis supported by chemical reactions simulation covering energy and mass balance. These estimates show that with a biogas delivery of 4 MMSCFD (with 50% CO₂ content), from 4 to 5 medium–high volume biomass Anaerobic Digestion plants (each generating 0.8–1.0 MMSCFD of biogas), it is possible to generate hydrogen at around 3 to 4 USD per kg from feeding 2 geothermal wells. Using a CIGG methodology, geothermal wells do not need to be drilled deep (e.g., 5000–7000 m) to reach hot reservoirs at >200 °C with normal geothermal temperature gradients. These high temperatures can now be realized using power fluids from shallower (e.g., 1500–2000 m), better quality, sedimentary reservoirs through heat recovery from the wellbore methane reformation tool. Importantly, geothermal power is now not limited by the geothermal depth of hot reservoirs. With a corresponding reduction in geothermal well costs by >50%, well depths will no longer dictate geothermal project economics. CIGG will create unrealized global scaling into geographical zones with high agricultural (or urban) biowaste and shallow sedimentary reservoirs of low geothermal gradient, enabling development of marginal projects, and expanding each sector in tandem.