Laboratory Investigation of CO₂ Trapping in Carbonates Aquifers - The role of Micropores

Carbonate rocks have complex pore systems and their suitability for CO₂ storage is contingent on favorable pore structure. Unfortunately, there is insufficient data about the screening and selection process for gas storage in carbonate rocks. To test the suitability of some carbonate core samples for storage, we carried out pore analysis of the core samples, which included helium porosity/permeability, capillary pressure, and NMR relaxation test. A porous plate drainage experiment at a capillary pressure of 200 psi followed by NMR measurements were conducted to determine the T₂ cut-off of the pores. CO₂ coreflood experiments were also carried out at high pressure and temperature to determine the storage efficiency of the rock. The overall results show that the presence of intragranular pores can significantly hamper storage capacity, especially if they form a significant portion of the pore systems. High T₂ cut off values (≈ 200 ms) were found in the cores, where the intraparticle pores have high capillary entry pressure (> 200 psi). Analysis of initial-residual saturation from coreflooding data showed that the core samples have a trapping efficiency ranging between 1.8 to 2.2. However, further observation revealed that only the intergranular pores were available for CO₂ storage, and only a fraction of these pores was drained by the injected CO₂. In some carbonate rocks, the intragranular pores constitute about half of the total pore system which further reduces the storage capacity of the rock. These observations highlight the importance of a well-defined screening criteria for gas storage in carbonate rocks. The fraction of intragranular to intergranular pores and the T₂ cut-off should be considered as one of the most important screening criteria in carbonate rocks.