Numerical simulation of calcite vein formation and its impact on caprock sealing efficiency – Case study of a natural CO2 reservoir

Abstract

For the long-term CO2 geological storage, the evolution of the caprock sealing efficiency has received increasing attention. In this paper, the Huangqiao CO2 gas field in Jiangsu Province, China, where the calcite veins have been found in the lower part of mudstone caprock, is considered as a natural analogue site for CO2 geological sequestration (CCS). To ascertain the dynamic formation process of calcite vein and its impact on the evolution of caprock sealing efficiency, a one-dimensional model was designed to represent the fractured reservoir-caprock system. Numerical simulations were performed using the multiphase reactive transport program TOUGHREACT. Sensitivity analyses of fracture permeability and calcite reaction rate were made. The simulation results illustrate that calcium bicarbonate decomposes to form calcium carbonate and release CO2 from solution due to the pressure decay as the CO2-rich fluids migrate upwards along the fracture. The formation of calcite vein decreases the porosity and permeability significantly, which can enhance the integrity and sealing efficiency of caprock. Sensitivity analyses indicate that the formation of calcite vein is facilitated by the fast fluid flow, and calcite vein tends to form in the vicinity of reservoir-caprock interface under the slow-flow condition as the decrease of fracture permeability. Information currently available in Huangqiao CO2 gas field and some other area describing the formation of calcite vein shows good agreement with our simulation results. The mudstone caprock with fractures and faults is able to keep CO2 from leaking for a long time as the case of Huangqiao area. The methods and analyses presented here may be useful for CO2 storage sites with similar conditions.