The Effects of Gas Type and Rock Heterogeneity on Residual Trapping in Carbonate Samples: A Pore Scale Experimental Study

Abstract This study aims to develop an improved understanding of the pore-level physics underlying trapping phenomena during the flow of aqueous and gaseous phases in carbonate rocks, by evaluating the influence of key parameters such as gas type, rock heterogeneity, and micro-porosity. Multiple miniature core samples (76-81 mm in length, 8 mm in diameter) were extracted from selected carbonate outcrop and reservoir rocks. These samples were solvent-cleaned using a flow-through technique to establish water-wet conditions. Subsequently, an optimized differential imaging approach was employed to accurately quantify in-situ sub-resolution micro-porosity. This was followed by a series of two-phase gas (nitrogen or methane)/brine drainage and imbibition experiments that were conducted using a state-of-the-art miniature core-flooding apparatus integrated with a micro-CT scanner. A detailed analysis of pore-scale occupancy maps was employed to evaluate the impact of micro-porosity, gas type, and rock heterogeneity on fluid saturations and distributions. At sub-resolution scales, micro-porosity calculations showed strong agreement with laboratory measurements obtained via both Mercury Injection Capillary Pressure and helium porosimetry techniques. By incorporating sub-resolution micro-porosity into saturation calculations – through segmentation-based methods ensuing CT-number-based analysis – valuable insights were gained into its impact on fluid saturation levels. Notably, we observed a substantial influence on the saturation of the wetting phase (brine), which was particularly pronounced in methane/brine systems compared to nitrogen/brine counterparts. Our findings underscore that neglecting micro-porosity in carbonate rocks could lead to significant errors in fluid saturation calculations. Additionally, we observed a preferential invasion of smaller pores by nitrogen at the latter stages of drainage, resulting in a higher final gas saturation during this process, as evidenced by in-situ pore fluid distribution maps. Conversely, methane exhibited limited invasion of smaller pores, leading to lower gas phase trapping and a higher recovery factor (40.53%) due to waterflooding compared to that in the nitrogen/brine system (34.56%). Cluster analysis performed after each displacement process corroborated these findings. The variations observed in fluid saturation and pore occupancy were attributed to differences in the fluid properties of the non-wetting phases employed, e.g., density and viscosity, consequently affecting the threshold capillary pressures required for invasion of the smaller pores. Measurements of residual gas saturation in carbonates have been scarce in the literature, partly due to their inherent heterogeneity caused by the highly irregular pore/throat geometries, non-uniform porosity/permeability distributions, and the presence of both macro- and micro-porosity. Obtaining more accurate estimations of residual saturations necessitates a deeper understanding of the pore-level displacement physics of trapping mechanisms in carbonates, which was the main objective of this study. Such insights are crucial for evaluating reservoir performance effectively.