Analytical models of degassing and regassing geothermal systems

Abstract

Geothermal systems naturally contain dissolved carbon dioxide, which may be present in the liquid reservoir at concentrations up to saturation. Observations from several geothermal systems have identified a degassing trend over time associated with production. More recently, attention has turned to the reinjection of CO2 into the reservoir or adjacent aquifers.
The dynamics of CO2 in a geothermal reservoir are influenced by several physical processes. Changes in pressure and temperature can reduce solubility in the liquid phase and may lead to exsolution if concentration is already close to saturation. CO2 may escape the reservoir by migrating vertically through pathways in the caprock or via lateral outflow. CO2 can be directly extracted from the reservoir through wells and may in some cases be partially or fully returned through reinjection. Finally, CO2 can participate in geochemical reactions with the host rock.
Given the potential for complex physical interactions, numerical reservoir modelling is often the preferred method for characterising the state and trajectory of a given geothermal system. However, under certain simplifications a set of closed-form analytical solutions can be developed. This study describes a lumped parameter ODE model of a liquid geothermal reservoir subject to pressure decline, degassing, production-reinjection and mineralisation.
Solutions are developed for a high-gas system where CO2 concentration is solubility limited, and a low-gas system that hosts negative emissions (carbon removal) and mineralisation. The high-gas system undergoes an early period of degassing as pressure declines and then reaches a new equilibrium where net total emissions exceed the pre-production baseline due to an induced upflow effect. The low gas system with additional CO2 reinjection is only able to achieve negative emissions over the long-term when mineralisation is present.