Simultaneous oil recovery and residual gas storage: A pore-level analysis using in situ X-ray micro-tomography

We imaged sandstone cores at residual gas saturation (S-gr) with synchrotron radiation at a nominal resolution of (9 mu m)(3). We studied two three-phase flooding sequences: (1) gas injection into a core containing oil and initial water followed by a waterflood (gw process); (2) gas injection into a waterflooded core followed by another waterflood (wgw process). In the gw flood we measured a significantly higher S-gr (=20.6%; S-gr in the wgw flood was 5.3%) and a significantly lower residual oil saturation (S-or; S-or in the gw flood was 21.6% and S-or in the wgw flood was 29.3%). We also studied the size distribution of individual trapped clusters in the pore space. We found an approximately power-law distribution N proportional to s (tau) with an exponent tau = 2.02-2.03 for the residual oil clusters and tau = 2.04 for the gas clusters in the gw flood. tau (=2.32) estimated for the gas clusters in the wgw process was significantly different. Furthermore, we calculated the surface area A-volume V relationships for the clusters. Again an approximate power-law relationship was observed, A infinity V-p with p approximate to 0.75. Moreover, in the gw flood sequence we identified oil layers sandwiched between the gas and water phases; we did not identify such oil layers in the wgw flood. These results have several important implications for oil recovery, carbon geo-sequestration and contaminant transport: (a) significantly more oil can be produced and much more gas can be stored using a gw flood; (b) cluster size distributions for residual oil or gas clusters in three-phase flow are similar to those observed in analogue two-phase flow; (c) there is a large cluster surface area available for dissolution of the residual phase into an aqueous phase; however, this surface area is significantly smaller than predicted by percolation theory (p approximate to 1), which implies that CO2 dissolution trapping and contamination of aquifers by hazardous organic solvents is slower than expected because of reduced interfacial contact areas. (C) 2012 Elsevier Ltd. All rights reserved.