Effects of Dissolution on the Permeability and Porosity of Limestone and Dolomite in High Pressure CO2/water Systems

摘要

During water-alternating gas (WAG) and others processes involving CO2 injection, interactions between the acid brine and reservoir rock take place, promoting changes in reservoir characteristics. In carbonate reservoirs, these interactions may cause rock dissolution due to acid solution-carbonate reaction. The understanding of these interactions impacts strongly on enhanced oil recovery (EOR) projects using CO2, as well as on the capture and sequestration of CO2 in the reservoir rock as an alternative way to storage of greenhouse gases. This study is dedicated to investigate experimentally interactions between CO2, water and carbonate rock. Experiments involved saturating dolomite and limestone samples by carbonated water under pressures up to 10,000psi in a high-pressure vessel. The vessel was then kept closed for 240h at steps of 48h. At each step, porosity, permeability and mass of the rock were determined. X-ray tomography was carried out for every sample. Analysis of the water composition with regard to the salinity was also made. There were measurable changes in mass, porosity and permeability. Porosity and permeability changes produced improvements or damages on the flow properties, which is related to the fine grains produced during the dissolution process leading to the partial blockage of porous media. Mass reduction was up to 0.95 % and 0.35%, respectively for limestone and dolomite. Permeability varied +/-60% (dolomite) and up to 86% (limestone). Porosity varied +/-3% for dolomite and +/-20% for limestone. The results show that dissolution should be considered in the projects and operations of CO2 injection processes, mainly WAG and carbonated water, which produces acid aqueous solutions able to react to the carbonate rock and causes continuous changes on the rock properties and consequently the oil production system.