(411g) Studies On the Alteration of Reservoir Rock

The National Energy Technology Laboratory (NETL) has been investigating the various impacts of geological sequestration of CO2 within saline aquifers. For the subject study, an evaluation of the alteration of reservoir rock, well bore cement, and the interface between the two, with supercritical CO2 (SCCO2) was conducted at the laboratory-scale using simulated brine solutions at down-hole conditions. These studies were intended to identify potential leakage pathways for injected CO2 due to degradation of the well bore. Two distinct test series were conducted on core samples of the Mt. Simon sandstone from the Illinois Basin, IL, and the Grand Ronde basalt from the Pasco Basin, WA. LaFarge Class H well bore cement was used for both series. Reservoir rock/cement cores were immersed within a CO2-saturated brine for up to 2000 hours at 35¢ªC and 100 atm CO2. Results suggest that the impact of SCCO2 injection is reservoir-specific, being highly dependent on the reservoir brine and rock type. Brine pH can be significantly altered by CO2 injection, which in turn can dramatically impact the dissolution characteristics of the reservoir rock. Reservoir rock mineralogy not only dictates the mineral-trapping potential for the reservoir, but also the susceptibility of the rock to alteration due to the SCCO2 injection. Finally, well bore cement alteration was identified, particularly for fresh cast cement allowed to cure at SCCO2 conditions. However, this alteration was generally limited to an outer rind of carbonate and Ca-depleted cement which appeared to protect the majority of the cement core from further attack. These studies indicate that at the cement-rock interface, the annular space may be filled by carbonate which could act as an effective barrier against further CO2 migration along the well bore.