Flow processes and pressure evolution in aquifers during the injection of supercritical CO2 as a greenhouse gas mitigation measure

摘要

Regional saline aquifers offer the greatest potential for very large-scale underground CO₂ storage as a means of mitigating greenhouse gas emissions. Their dynamic storage capacity, in terms of induced increases in formation pressure, will limit the rate at which CO₂ can be injected and may ultimately limit the amount of CO₂ that can be stored. Generic flow models were generated to examine the effects on pressure evolution of various reservoir parameters (dimensions, permeability, porosity, presence and nature of flow barriers). CO₂ injection involves dominantly hydrogeological (single-phase flow) processes in much of the reservoir and surrounding adjacent strata, with additional two-phase flow effects around the CO₂ plume itself. Large, thick aquifers with no significant flow barriers can accept high injection rates (c. 10 million tonnes of CO₂ per year) without undue pressure effects. However, flow barriers, such as faults, increase induced pressures considerably; for reservoirs with such features, careful site characterization and operational planning will be required for large storage projects. The principles established from the generic modelling were applied to a real aquifer storage operation at Sleipner in the North Sea. Here, CO₂ is being injected into the Utsira Sand, a large relatively homogeneous reservoir. Modelling indicates that pressure increase should be negligible. In fact, observed wellhead pressures do show a small rise, but this can be attributed to temperature changes in the fluid column in the wellbore. Pressure changes in the reservoir are likely to be very small.