The impact of injection rate, number, and location of injection wells on CO2 trapping efficiency in deep saline aquifers

In this talk, we present a model to explore the spreading of a CO2 plume within a deep saline aquifer, injected from a series of wells arranged around the crest of an axisymmetric anticline structure of finite vertical extent. For a constant CO2 injection rate, we calculate both the fraction of available pore space in the anticline into which CO2 spreads prior to the point at which the plume spills from the lower boundary of the anticline and the maximum pressure within the anticline throughout the injection process. We find that the fraction of the anticline filled with CO2 varies considerably between slow, buoyancy-controlled filling and fast, pressure-controlled filling. At low injection rates, the CO2 plume fills the entire aquifer thickness in the vicinity of the crest, and achieves close to the theoretical maximum storage efficiency. At high injection rates, however, the plume spreads quickly in a thin sheet along the upper boundary of the anticline, leading to much smaller fractions of the available pore space accessed by CO2. We also find that there is a correspondence between the maximum pressure within the anticline produced by a series of equally spaced injection wells and the maximum pressure produced by the simple case of a single injection well at the centre. This allows us to predict the contributions to the pressure build-up within the aquifer from both the pressure spike induced by the injection wells, and the buoyancy of the injected CO2. The results of this modelling provide key insights into the tensions between the injection rate, total injected volume, and pressure build-up in a CO2 storage system, and we discuss some of the challenges this presents.