The most promising near term solution to reduce CO2 emissions is to capture the CO2 produced from coal and gas fired power plants and inject it into deeply buried saline aquifers. However a large amount of these plants are far away from deep sedimentary basins and therefore alternative solutions have to be sought. Detailed computer simulation, using compositional numerical model software CMG-GEMGHG, was used to study the effects of aquifer anisotropy (heterogeneity), well completion technique (perforation and well orientation), brine production and injection rates on CO2 injectivity, and capability to retain and store reasonably large amount of CO2 for an extended period of time. The objective of this study is to find the optimal combination of operational parameters (injection rates, well completion techniques and brine withdrawal strategy), and how the interplay of these parameters with aquifer anisotropy (heterogeneity) affects, and can be manipulated to optimally harness CO2 storage potential and leakage risk mitigation in shallow saline aquifers.
Injected CO2 exists in three main forms in the aquifer, supercritical, dissolved and aqueous forms with amount depending largely on prevailing formation pressure and the depth of well placement. Without brine withdrawal, horizontal injector wells with the vertical sections closed have higher CO2 injectivity compared to vertical and deviated wells with bottom half perforated. With brine withdrawal more CO2 was injected and anisotropy (heterogeneity) (Kv/Kh ratio) has significant effect on CO2 injectivity, but well perforation and orientation has negligible effect, except for an extended (85 years) injection period when aquifer pressure has reached the maximum allowable. The impact of anisotropy (heterogeneity) on CO2 injectivity was observed to decrease with increase on injection rate for injection well placed in uppermost layers. This study shows CO2 sequestration in shallow saline aquifer has significant potential. Results from this study can potentially provide guideline for well completion technique, injection rate and brine withdrawal strategy, depending on aquifer properties, that can be applied in CO2 sequestration in shallow saline aquifer.