Simulation Study of sc-CO2 Based Silylation for Decreasing Severity of Water Blockage and Salt Precipitation during Geological CO2 Storage in Deep Saline Aquifers
Deep saline aquifers are often favorable for underground CO2 sequestration due to their large capacity and relatively low likelihood for resource conflicts. However, many possible issues can arise during CO2 injection. Often these aquifers have a significant salinity level (as these often present minimal resource conflict issues) and as such salt precipitation near the injection wellbore can be problematic. Furthermore, when water blockage occurs, salt precipitation can be exacerbated since large amounts of water remain near the wellbore. Altering the rock wettability towards less water-wet can alleviate water blockage and in turn reduce the likelihood or severity of salt precipitation. Previous lab experiments have shown that supercritical CO2 (sc-CO2)-based silylation can effectively functionalize rock surfaces with hydrophobic silanes. In this study, numerical models were constructed to evaluate the combined effects of multi-phase fluid flow, water evaporation and salt precipitation assuming a change in wettability (thus impacting the relative permeability characteristics of the reservoir) resulting from the silylation process. The aim of this study is to evaluate the efficacy of this chemical treatment to address near wellbore salt precipitation induced by CO2 injection. According to the simulation results, a decrease in injectivity due to salt precipitation is more significant when water blockage is also present. Injectivity is deteriorated prominently in high salinity reservoirs with water blockage since evaporation into the injected CO2 phase will cause significant salt precipitation. In a representative formation, the injectivity decline is worse (up to 68.6% relative injectivity change (RIC)) when both salt precipitation and water blockage are considered since the latter provides more trapped brine inducing more salt accumulation around the wellbore. With hydrophobic silylation, the combined effects of salt precipitation and water blockage on RIC are decreased on an absolute basis by up to 7%. Depending on techno-economic considerations, this method is encouraged to be implemented as early as possible during a CO2 injection program to minimize salt accumulation from the outset.
According to the simulation result, a decrease in injectivity due to salt precipitation is more significant than that due to water blockage. Salt precipitation decreases pore space by forming additional solid in it, while water blockage affects the mobility of CO2 due to relatively high irreducible water saturation. The negative impact of water blockage gradually diminishes (as the water evaporates) while salt starts growing within the pore space. With a high level of trapped brine, the injectivity is deteriorated more severely (up to 68.6% of relative injectivity change (RIC)) when both salt precipitation and water blockage occur. Different capillary behavior may induce localized precipitation considering the heterogeneity either caused by water blockage or naturally employed in the geological structure of a deep saline aquifer.
Beside the ability of sc-CO2 based silylation in altering wettability, applying this method reduces the severity of salt precipitation indirectly. The alteration turns some trapped brine moveable and thus decreases irreducible water saturation. This phenomenon prevents large accumulation of salt near perforation of the injection well. In case of salt precipitation endorsed by water blockage, this method counteracted the injectivity around 5 to 7% of RIC (absolute basis). In addition, this method is encouraged to be implemented before continuous CO2 injection for the geological sequestration because the expected mechanism will not work when salt precipitation comes up covering the pore surface.