Non-Isothermal Injectivity Considerations for Effective Geological Storage of CO2 at the Aquistore Site, Saskatchewan, Canada
Authors: Rangriz Shokri Alireza , Chalaturnyk Rick J. , Nickel Erik 
Published: September 2019

Monitoring data of episodic transient heat and flow conditions, caused by intermittent cold CO2 injection in Aquistore, has shown a linkage between injectivity index and downhole injection temperature. Taking leverage access to invaluable field performance data collected from this highly instrumented Canadian CCS demonstration project, the focus of this paper is to understand and quantify the potential non-isothermal mechanisms involved in cold CO2 injection. Understanding this phenomenon is important as it has serious implications on containment, conformance, and injectivity technologies for effective geological CO2 storage.

To account for transient heat and fluid transport during cold CO2 injection in Aquistore, a non-isothermal EOS-based fluid flow simulation, of a high-resolution detailed geological model built based on an extensive characterization program, was calibrated with periodic monitoring data of downhole pressure, temperature, and injected mass rate. Due to the possibility of non-isothermal effects on near-wellbore stress fields, local induced fractures, and permeability alterations, in addition to dynamics of CO2-brine interactions, coupled reservoir geomechanical modeling techniques were then employed for further calibration. The uncertainties associated with the subsurface geological modeling, leaking aquifer boundaries, reservoir heterogeneity, rock thermal, petrophysical, and geomechanical properties were considered for both isothermal and non-isothermal conditions.

Processing of DTS (Distributed Temperature Sensing) data from both injection and observation wells indicated dynamic perturbations in subsurface temperature due to injection operations. Geological characterization, performed through high-resolution 3D seismic images, core, and log data, and the existence of a leaking aquifer, were found to have significant impacts on CO2 plume evolution. Through history matching process of non-isothermal flow simulation, for both injector and observation wells, the extent of the cold region was estimated, and found to be mainly controlled by rock thermal properties, permeability, and injection rate. Our analysis suggested that cold temperature front was limited to near-wellbore region due to substantial heat loss by conduction, besides radial decay of convective flow.

Further non-isothermal coupled simulations indicated a large, but near-wellbore-limited reduction in effective horizontal stresses, induced by cold CO2 injection. Employing different values of thermal expansion coefficients, local potential open-mode fractures were observed; however, fracturing of entire formation was not experienced. This phenomenon was associated with local permeability enhancement, and potential improvement in CO2 injectivity. A comparison of isothermal and non-isothermal analyses on reservoir performance during CO2 injection was lastly provided.

Our analysis of subsurface injection and coupled processes in relation to geologic CO2 sequestration delivers critical insights on how and under what conditions these non-isothermal effects are generated. This ultimately provides a predictive tool to better characterize the reservoir behaviour, injectivity issues, and spatial location of a subsurface CO2 plume.