Determining the geothermal potential of natural gas fields in the Southern North Sea

Abstract

Renewable energy uptake must increase to reduce carbon emissions and curb today’s unprecedented rate of global warming. CO2-based geothermal energy extraction could be used to produce near zero-carbon power from the heat stored in depleted natural gas reservoirs. CO2-plume geothermal (CPG) is a conceptual technology designed to produce power by circulating CO2 through permeable and porous formations sealed by low- permeability cap rocks, ultimately storing industrial quantities of CO2 in the subsurface. Few feasibility studies have considered the application of this technology to wide-scale geographical areas and no offshore CPG prospects have been evaluated. Several factors make natural gas fields in the Southern North Sea (SNS) promising CPG prospects; geothermal gradients are higher on average than those found onshore in the UK, infrastructure is already in place, extensive geological data are available, and seawater can be used as a heat sink to improve CPG performance. In this research, CPG power production is estimated for 64 SNS gas fields using a few identified and simplifying assumptions.

The South Sean field was selected as the field best suited for CPG in terms of power production potential and practicality. Power production was re-estimated for South Sean with increased accuracy. The gas fields are classified as geothermal resources following the United Nations Framework Classification for Resources (UNFC). Findings indicate that moderate amounts of power (in the order of kilowatts electric) could be produced from 50 of the 64 fields. An injection-production well doublet configured to South Sean could produce from 59 kWe to 552 kWe depending on well diameter sizing. This research demonstrates that low parasitic cooling loads can be achieved using seawater as a cooling medium, giving offshore CPG systems an edge over their onshore counterparts. All fields are classified under the UNFC system as having additional quantities in place, highlighting the need for a follow- up economic analysis.