Capillary pressure and saturation relations for supercritical CO2 and brine in sand: High‐pressure Pc(Sw) controller/meter measurements and capillary scaling predictions

Capillary pressure and saturation relations for supercritical CO 2 and brine in sand: High-pressure P c (S w ) controller/meter measurements and capillary scaling predictions Tetsu K. Tokunaga, 1 Jiamin Wan, 1 Jong-Won Jung, 1,2 Tae Wook Kim, 1,3 Yongman Kim, 1 and Wenming Dong 1 [ 1 ] In geologic carbon sequestration, reliable predictions of CO 2 storage require understanding the capillary behavior of supercritical (sc) CO 2 . Given the limited availability of measurements of the capillary pressure (P c ) dependence on water saturation (S w ) with scCO 2 as the displacing fluid, simulations of CO 2 sequestration commonly rely on modifying more familiar air/H 2 O and oil/H 2 O P c (S w ) relations, adjusted to account for differences in interfacial tensions. In order to test such capillary scaling-based predictions, we developed a high-pressure P c (S w ) controller/meter, allowing accurate P c and S w measurements. Drainage and imbibition processes were measured on quartz sand with scCO 2 -brine at pressures of 8.5 and 12.0 MPa (45 o C), and air-brine at 21 o C and 0.1 MPa. Drainage and rewetting at intermediate S w levels shifted to P c values that were from 30% to 90% lower than predicted based on interfacial tension changes. Augmenting interfacial tension-based predictions with differences in independently measured contact angles from different sources led to more similar scaled P c (S w ) relations but still did not converge onto universal drainage and imbibition curves. Equilibrium capillary trapping of the nonwetting phases was determined for P c ¼ 0 during rewetting. The capillary-trapped volumes for scCO 2 were significantly greater than for air. Given that the experiments were all conducted on a system with well-defined pore geometry (homogeneous sand), and that scCO 2 -brine interfacial tensions are fairly well constrained, we conclude that the observed deviations from scaling predictions resulted from scCO 2 -induced decreased wettability. Wettability alteration by scCO 2 makes predicting hydraulic behavior more challenging than for less reactive fluids. and Bachu, 2008 ; Benson and Cole, 2008]. The depend- ence of capillary pressure (P c ) on water saturation (S w ) under reservoir conditions is a basic constitutive relation needed to predict CO 2 flow and capillary trapping during sequestration. Indeed, with only knowledge of how (rela- tive) permeability depends on S w , neither fluid flow nor equilibrium saturation conditions are predictable. At later stages of geologic carbon sequestration, rewetting of reser- voirs with native brine results in capillary trapping (resid- ual trapping) of CO 2 , a main storage mechanism [Bachu et al., 2007; IPCC, 2005]. The capillary trapping capacity of reservoir materials has complex dependence on porosity, pore-size distribution, nonwetting phase displacement extent, and the rewetting process [Iglauer et al., 2011b ; Tanino and Blunt, 2012]. Although capillary trapping of CO 2 at its residual saturation depends on P c (S w ) relations, few investigations to date have directly measured these ba- sic relations with CO 2 /H 2 O at reservoir pressures and tem- peratures [Krevor et al., 2011 ; Pentland et al., 2011 ; Pini et al., 2012 ; Plug and Bruining, 2007]. The study by Plug and Bruining [2007] appears to be the only one in which capillary-trapped scCO 2 saturation was determined while 1. Introduction [ 2 ] Geologic carbon sequestration may become an im- portant technology for mitigating CO 2 buildup in the atmosphere from fossil fuel combustion and for moderating climate change [Intergovernmental Panel on Climate Change (IPCC), 2005]. However, predicting the perform- ance of geologic CO 2 sequestration requires reliable under- standing of CO 2 mobility, and its saturation and pressure distribution within reservoirs [Bachu et al., 2007 ; Bennion Earth Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California, USA. Now at Civil and Environmental Engineering Department, Louisiana State University, Baton Rouge, Louisiana, USA. Now at Department of Energy Resources Engineering, Stanford Uni- versity, Stanford, California, USA. Corresponding author : T. K. Tokunaga, Earth Sciences Division, Law- rence Berkeley National Laboratory, 1 Cyclotron Rd., Berkeley, CA 94720, USA. (tktokunaga@lbl.gov)