Applying Boundary Conditions for Large Aquifer Models in Geological CO₂ Storage Projects; Why and How?

Around the world many projects are being considered for the geological storage of CO₂ in large saline aquifers. The aerial extent of pressure and saturation plume forecasts, known as Area of Review (AOR) in the United States, can have a significant impact on the monitoring costs and liabilities associated with existing legacy wells within the vicinity of the carbon storage projects. Appropriately modeling how aquifer systems react to large scale CO₂ injection has relevance to potential geomechanical integrity, fault reactivation, seismicity, and impact on underground sources of drinking water (USDW). Static and dynamic models are generally built over a smaller portion of large saline aquifer system. This is typically tied to the balance between model size, fidelity, and computational expenses. Assigning the appropriate boundary conditions to replicate the true extent of the aquifer is investigated in this study. Scoping the aquifer's size, geology, and properties is an important step that allows for the quantification of the appropriate pore volume beyond the area of the numerical model. Results from our study indicate that using pore volume (PV) multipliers is a reasonable approach if accompanied by transmissibility reduction at the interface between the PV modifications and the reservoir model. A simple model was developed to create the true solution for comparison and verification purposes.

Based on the results of this study, we find that using a combination of PV and transmissibility multipliers replicates the true solution more accurately. We demonstrate that large PV adjustments without transmissibility reductions overestimates the true aquifer strength, resulting in overall lower pressures due to large-scale CO₂ injection. We demonstrate a systematic methodology for calculating the simultaneous PV increase and transmissibility reduction that scales easily for a wide range of scenarios.

Further investigation shows that as the PV multiplier grows large, the approach of reducing the transmissibility with a single multiplier starts to choke off the large added pore volume. This study proposes an innovative approach which demonstrates that applying a gradually increasing PV multiplier combined with gradually reducing transmissibility is a more accurate representation of the true aquifer system when compared to a single large PV multiplier and a single transmissibility reduction. The proposed approach was applied to large 3-D models and significant impact was observed in pressure diffusion front shape and extent when compared to more simplified approaches.

Assignment of boundary conditions for a variety of different scenarios were also investigated in this study. It was shown that this novel approach will be the most accurate method of applying appropriate boundary conditions in obtaining reliable AOR forecasts when modeling large aquifer systems for CO₂ storage projects.