Gas breakthrough pressure for hydrocarbon reservoir seal rocks:: implications for the security of long-term CO2 storage in the Weyburn field

This paper reports a laboratory study of the gas breakthrough pressure for different gas/liquid systems in the Mississippian-age Midale Evaporite. This low-permeability rock formation is the seal rock for the Weyburn Field in southeastern Saskatchewan, Canada, where CO2 is being injected into an oil reservoir for enhanced recovery and CO2 storage. A technique for experimentally determining CO2 breakthrough pressure at reservoir conditions is presented. Breakthrough pressures for N-2, CO2 and CH4 were measured with the selected seal-rock samples. The maximum breakthrough pressure is over 30 MPa for N-2 and approximately 21 MPa for CO2. The experimental results demonstrate that the Weyburn Midale Evaporite seal rock is of high sealing quality. Therefore, the Weyburn reservoir and Midale Beds can be used as a CO2 storage site after abandonment. The measured results also show that the breakthrough pressure of a seal rock for a gas is nearly proportional to the interfacial tension of the gas/brine system. The breakthrough pressure of a CO2/brine system is significantly reduced compared with that of a CH4/brine system because of the much lower interfacial tension of the former. This implies that a seal rock that seals the original gas in a gas reservoir or an oil reservoir with a gas cap may not be tight enough to seal the injected CO2 if the pressure during or after CO2 injection is the same or higher than the original reservoir pressure. Therefore, reevaluation of the breakthrough pressure of seal rocks for a given reservoir is necessary and of highest priority once it is chosen as a CO2 storage site.