CO₂-water displacement characteristics of saturated sandstones with different pore structures

Objective The pore structures of clastics play a crucial role in assessing the potential of saline aquifers for geologic CO₂ sequestration. Current studies on pore structures during CO₂ sequestration focus primarily on artificial cores. However, compared to artificial cores, natural cores feature more complex pore size distributions and stronger heterogeneity.

Methods This study examined a natural core from deep reservoirs in the Ordos Basin and two artificial cores with different porosities through CO₂-water displacement experiments under water-saturated conditions. Using a nuclear magnetic resonance (NMR) analysis and imaging system for multiphase fluid displacement, this study analyzed the impacts of pore structures (pore heterogeneity) on CO₂-water displacement in these cores.

Results and Conclusions The artificial cores with porosities of 5% and 15% exhibited CO₂-water displacement efficiencies of 67.09% and 46.71%, respectively, whereas the natural core with a porosity of 15% displayed a displacement efficiency of 37.67%. The NMR-derived transverse relaxation time (T₂) spectra of the natural core showed a unimodal pattern, suggesting strong heterogeneity. In contrast, the T₂ spectra of artificial cores manifested bimodal patterns, with uniform pore size distributions and low residual water saturation. Notably, a lower porosity corresponded to high pore connectivity, which allowed for sufficient contact between CO₂ and the core. This characteristic effectively reduced interfacial tension and significantly enhanced CO₂-water displacement efficiency. Compared to the artificial core with the same porosity, the natural counterpart showed significantly reduced CO₂-water displacement efficiency due to its strong heterogeneity, with residual water occurring predominantly in small pores. Therefore, in the assessment of the potential of clastic reservoirs for CO₂ sequestration, it is necessary to thoroughly consider the impact of core heterogeneity and preferentially utilize natural cores or heterogeneous artificial cores in CO₂-water displacement experiments. This will help predict CO₂ sequestration behavior more accurately and avoid overestimated CO₂ sequestration efficiency parameters arising from the application of artificial cores made of homogeneous materials. The results of this study will provide a theoretical basis and experimental support for siting, potential assessment, and injection scheme design for geologic CO₂ sequestration in saline aquifers.