Numerical investigation of the formation displacement and caprock integrity in the Ordos Basin (China) during CO2 injection operation

Abstract

Theoretical and numerical studies of coupled hydro-mechanical processes during geothermal and hydrocarbon production, nuclear waste disposal, and CO2 sequestration in geological media have become a research focus worldwide. However, their application in China, especially CO2 sequestration, is still in infancy and may need several years to develop. In this paper coupled hydro-mechanical processes in CO2 migration, formation uplift and the state of caprock integrity as a result of CO2 sequestration have been investigated by simulation at field scale for an integrated CCS (CO2 Capture and Sequestration) project launched in the Ordos Basin, China. The paper aims at attaining long-term safety and integrity of caprock to ensure success for subsequent project operations at field scale. The linked TOUGH2MP-FLAC3D simulator has been used to study the coupled hydraulic-mechanical responses to primary stresses, injection rate and reservoir permeability during CO2 injection and post-injection periods by means of 4 case studies. In the short-term operation of a CO2 sequestration project launched in the Ordos Basin 0.1Mt CO2 has been injected in four saline aquifers within a period of 1 year and 9 years of relaxation time allotted for simulation runs. Contrary to using hydraulic field conditions alone, a combination of hydraulic-mechanical effects accounts for lateral migration of CO2 in aquifer layers that leads to partial volumetric expansion of the reservoir. Amongst the main factors affecting hydro-mechanical processes, injection rate has the most significant impact. This is followed by reservoir permeability that is responsible for gauging the uplift trend rate in the post-injection period through hindering the dissipation of pore pressure. Results indicate that the scope of pore pressure attained will be larger in the isotropic stress state than in the compressional stress condition, pointing to the significance of primary stress. Calculation of the maximum storage pressure within safety limits using the integrity and shear criteria reveals that failure of caprock integrity increases with the amount of CO2 injected. During injection however, the risk of caprock failure, especially in thin sandstone-caprock interlayered systems, increases with time. This paper estimates the pressure buildup in the four injection aquifers in the Ordos Basin, as a basis to ensure safety in the long-term operation of the CO2 sequestration project, hence providing important reference for future commercial operations at field scale.