Mitigation planning for large-scale storage projects: Multiple injection zones and reservoir pressure reduction engineering design

Effective mitigation plans are an absolutely critical component of mitigation plans for commercial-scale geologic carbon sequestration. One fundamental component of mitigation engineering design is immediate reduction of reservoir pressure. The Southwest Regional Partnership on Carbon Sequestration (SWP) is employing immediate reservoir pressure reduction as a primary mitigation tool in our geologic sequestration field projects. We are also employing multiple injection zones at the SWP deep saline injection site, both to maximize capacity and optimize mitigation plans. We developed models for each of our test sites to forecast optimum density and placement of injection and observation wells. Likewise, we designate certain observation wells as "observation-pressure- reduction," or "OPR" wells. These are wells that serve as observation wells, but are engineered for quick conversion to production (pumping) wells to facilitate immediate pressure reduction, if needed. Results of our reservoir models suggest that immediate pressure reduction may stem geomechanical deformation, stem and/or close crack/fracture growths, shut down "piston-flow" displacement of brines into unintended reservoirs, slow leakage through wellbores, slow leakage of CO₂ through faults, and even induce closure of faults. Much like the injection wells, the distribution of such OPR wells is critical. For example, in ongoing Partnership field-testing, observation wells are being drilled that will serve as OPR wells, and we are using reservoir models to identify well locations that optimize both monitoring and mitigation potential. Reservoir model results also suggest that OPR wells can be converted to injection wells to maximize capacity and control reservoir pressure. For example, as one portion of the reservoir "fills" or if pressure control becomes problematic, the injection well can be converted to OPR mode, and the next well in the series (whether linear or in a grid design) can become an injection well. Simulation results suggest that if pressure reduction wells are used to "make space" for CO₂ by removing brine ahead of the CO₂ front, this pumping will also increase residual gas trapping by promoting horizontal migration. Additional results of our reservoir models suggest several caveats and potential problematic processes: (1) rapid reduction of reservoir pressure decreases CO₂ density, potentially leading to accelerated buoyancy effects, (2) premature CO₂ breakthrough may occur in pressure reduction wells, (3) pressure reduction decreases solubility of CO₂ in the formation water, potentially leading to exsolution and undesired phase changes, and (4) finally, a detailed cost- analysis must accompany such an engineering approach, because reservoir pressures directly affect compression injection costs, e.g., it is possible that pressure reduction wells may reduce or increase net costs of injection, depending on costs associated with water production and handling at the pressure reduction wells. We will show results of this sequestration field engineering approach for specific field tests, including ongoing geologic sequestration field-testing in several U.S. sites, including projects in Utah, New Mexico and Texas. The authors gratefully acknowledge the U.S. Department of Energy and NETL for sponsoring this Southwest Partnership project.