Kurzbeschreibung
(Englisch)
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In view of Switzerland’s goals to be climate neutral by 2050, the Principal is interested to gain insights into the feasibility of harnessing geothermal energy together with carbon capture, transport, utilization and storage technologies via the ETH-Zurich developed concept of “CO2-plume geothermal” (CPG). There are open questions related to field-piloting CPG, however, that need to be addressed in a timely fashion for this technology to be potentially deployed, upscaled and to have a material impact in the decades after 2030. A first screening of potential pilot sites suggests that the Partner’s well-monitored CO2-storage project (Aquistore) in the Canadian province of Saskatchewan has high potential for a CPG pilot. The Principal now wishes to engage the Partner to undertake a first-stage assessment of the Aquistore site centering on conceptual dynamic reservoir simulations of operative subsurface processes (the “project”).
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Schlussbericht
(Englisch)
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Responding to an interest shown in the Aquistore storage site for a pilot test, the University of Alberta was tasked to study the feasibility of a CO2 circulation test as a first step towards a CO2 Plume Geothermal (CPG) field experiment. The CPG uses CO2 as the working fluid in geothermal power systems while sequestering CO2 in deep saline aquifers. The concept of CPG, as an emerging power generation technology and an add-on to carbon capture and storage (CCS), has the potential to reduce the cost of CCS operations, and might even generate revenue in geothermally favorable regions. More importantly, all the CO2 will eventually be stored during CPG into the targeted subsurface formations.
This feasibility study addresses some of the issues when geologically sequestered CO2 is recirculated to the surface using a well doublet system. This study aims to assist PTRC/SaskPower in identifying the upside/downside risks of running a CO2 circulation test (a key element of CPG), to assist the SFOE with the possibilities and limitations of the CPG technology, and to provide an assessment of key reservoir variables that potentially affect the performance of a CO2 circulation test at the Aquistore injection site. For this feasibility study, we reviewed enhanced geothermal systems and thermosiphon literature to determine minimum requirements prior to conducting CO2 circulation tests, with a focus on reservoir conditions at the Aquistore injection site. A base sector model was extracted from the available full geological model of Aquistore, and it was re-built into a very fine-scale geocellular model for further simulations of the CO2 circulation test. We developed static uncertainty workflows and appraised the most probable realizations of CO2 plume extent and shape from the history-matched numerical simulations and the latest available seismic surveys. Many realizations (layer cake model, stochastic petrophysical properties, inclusion of flexure, and constraining the CO2 plume to the seismic surveys) were conducted to match both CO2 plume and injection history. The dynamic uncertainty analysis of the CO2 circulation test helped to address some of the issues related to the impacts of the extent and shape of CO2 plume, initial water and CO2 saturations within the CO2 plume, heterogeneity in petrophysical properties, and operational variables of both injection and production wells, among others. Simulations suggest that CO2 circulation seems feasible at Aquistore; it does not result in huge volumes of brine production from the aquifer, but it needs significant work on front end engineering design prior to executing the pilot test. In all realizations, there was an initial brine kick prior to continuous CO2 production with a stable water/gas ratio; the initial brine production should not be thought of as a failure of the test. Instead, proper surface facility should be designed to handle this initial brine production. A mass balance of CO2 mass injection and production during the CO2 circulation test indicates that a portion of injected CO2 will permanently be stored in the saline aquifer. This CO2 loss serves the ultimate goal of Aquistore, i.e. permanent CO2 storage. However, CO2 make-up fluid should be added to the injection cycle during any CO2 circulation test. Equally important, the operating conditions of both the injector and the producer can be optimized for a successful CO2 circulation test; these include, but are not limited to, well stimulation, injection/production rates, pressure constraints, and completion designs at both injector and producer, among others. We also estimated the potential complex flow regimes and their impacts on the producer (co-production of CO2 and brine) using simplified two-phase vertical flow models, and commented on the formation of CO2 clathrate during CO2 circulation operation that could result in tubing/pipes blockage, reduced flow rates, or salt precipitation. Future work and knowledge gaps include the impact of potential thermally induced fractures and their propagation (short circuiting the wells), geochemistry of supercritical CO2 (brine/CO2/rock), possible long-term changes in reservoir porosity and permeability, heat extraction behavior during CO2 circulation, CO2 loss, and the need for make-up fluid. The developed model in this study can be used to determine the location of a new producer or an additional brine well for brine disposal, voidage replacement, and reservoir pressure management in future studies
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