Aquifer Management for CO2 Sequestration

Storage of carbon dioxide is being actively considered for the reduction of green house gases. To make an impact on the environment CO2 should be put away on the scale of gigatonnes per annum. The storage capacity of deep saline aquifers is estimated to be as high as 1,000 gigatonnes of CO2.(IPCC). Published reports on the potential for sequestration fail to address the necessity of storing CO2 in a closed system. This work addresses issues related to sequestration of CO2 in closed aquifers and the risk associated with aquifer pressurization. Through analytical modeling we show that the required volume for storage and the number of injection wells required are more than what has been envisioned, which renders geologic sequestration of CO2 a profoundly nonfeasible option for the management of CO2 emissions unless brine is produced to create voidage and pressure relief. The results from our analytical model match well with a numerical reservoir simulator including the multiphase physics of CO2 sequestration. Rising aquifer pressurization threatens the seal integrity and poses a risk of CO2 leakage. Hence, monitoring the long-term integrity of CO2 storage reservoirs will be a critical aspect for making geologic sequestration a safe, effective and acceptable method for greenhouse gas control. Verification of long-term CO2 residence in receptor formations and quantification of possible CO2 leaks are required for developing a risk assessment framework. Important aspects of pressure falloff tests for CO2 storage reservoirs are discussed with a focus on reservoir pressure monitoring and leakage detection. The importance of taking regular pressure falloffs for a commercial sequestration project and how this can help in diagnosing an aquifer leak will be discussed. The primary driver for leakage in bulk phase injection is the buoyancy of CO2 under typical deep reservoir conditions. Free-phase CO2 below the top seal is prone to leak if a breach happens in the top seal. Consequently, another objective of this research is to propose a way to engineer the CO2 injection system in order to accelerate CO2 dissolution and trapping. The engineered system eliminates the buoyancy-driven accumulation of free gas and avoids aquifer pressurization by producing brine out of the system. Simulations for 30 years of CO2 injection followed by 1,000 years of natural gradient show how CO2 can be securely and safely stored in a relatively smaller closed aquifer volume and with a greater storage potential. The engineered system increases CO2 dissolution and capillary trapping over what occurs under the bulk phase injection of CO2. This thesis revolves around identification, monitoring and mitigation of the risks associated with geological CO2 sequestration.