Investigations of reservoir response to net negative CO? reinjection using full scale geothermal reservoir models

Abstract

Geothermal power production is a renewable form of baseload electricity generation. However, it is not an emissionsfree process due to non-condensable gases, such as CO? found naturally underground that are mixed into the working fluid. With the global push toward more sustainable operation of infrastructure, good management of the CO? emissions in geothermal fields has financial, environmental and cultural benefits. One strategy is to inject 100% of the CO? produced by the system in what is known as a net zero reinjection scheme. Furthermore, additional CO? can be captured from other processes, such as the burning of biomass fuels in hybrid biomass geothermal power generation plants and added to the existing reinjection streams. This is known as a net negative injection scheme.
However, successful sequestration of CO? using these methods requires knowledge of how the reinjection strategies affect the system, particularly relative to its natural state. Directly monitoring all CO? emissions across the surface of the geothermal system in sufficient detail is impractical. Estimation of these emissions can be done using numerical modelling, where short-term CO? accumulation under the clay cap is used as a proxy for CO? emissions in the long term. This project used a synthetic model to perform numerical simulation of a no CO? reinjection production scenario, a 100% CO? reinjection scenario and a 110% CO? reinjection scenario using Waiwera, an open-source geothermal flow simulator.
In the case with no CO? reinjection we observe a steady decline in production CO?. The two cases where we reinject CO? (100% and 110%), we show production CO? is consistent with the base case for the first 3.5 years, then we observe a steady increase over the duration of the simulation. In the base case, CO? leaves the model either through production or the surface. When injection of CO? occurs, the CO? is replenished in the production zone of the reservoir from reinjection and the up flow resulting in elevated CO? production and more
CO? going through the alteration in time.