[1] Methane gas hydrates, crystalline inclusion compounds formed from methane and water, are found in marine continental margin and permafrost sediments worldwide. This article reviews the current understanding of phenomena involved in gas hydrate formation and the physical properties of hydrate-bearing sediments. Formation phenomena include pore-scale habit, solubility, spatial variability, and host sediment aggregate properties. Physical properties include thermal properties, permeability, electrical conductivity and permittivity, small-strain elastic P and S wave velocities, shear strength, and volume changes resulting from hydrate dissociation. The magnitudes and interdependencies of these properties are critically important for predicting and quantifying macroscale responses of hydrate-bearing sediments to changes in mechanical, thermal, or chemical boundary conditions. These predictions are vital for mitigating borehole, local, and regional slope stability hazards; optimizing recovery techniques for extracting methane from hydrate-bearing sediments or sequestering carbon dioxide in gas hydrate; and evaluating the role of gas hydrate in the global carbon cycle.

Wettability of reservoir mineral surfaces is a critical factor controlling CO2 mobility, trapping, and safe-storage in geological carbon sequestration. Although recent studies have begun to show that wettability of some minerals can change in the presence of supercritical CO2 (scCO2), different laboratories have reported significantly different wetting behavior. We studied wettability alteration of silica in CO2–brine systems through measuring equilibrium water contact angles under wide ranges of pressures (0.1 to 25 MPa) and ionic strengths (0 to 5.0 M NaCl), at 45 °C. Using two independent approaches for each of the experiments, we found the following: (1) Equilibrium water contact angles on silica increased up to 17.6° ± 2.0° as a result of reactions with scCO2. This increase occurred primarily within the pressure range 7–10 MPa, and the contact angles remain nearly constant at pressure greater than 10 MPa. (2) The contact angle increased with ionic strength nearly linearly, with a net increase of 19.6° ± 2.1° at 5.0 M NaCl. These changes in contact angle induced by changes in scCO2 pressure and aqueous solution ionic strength are approximately additive over the range of tested conditions. These findings can be used to estimate the wetting behavior of silica surfaces in reservoirs containing supercritical CO2.

[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 C), and air-brine at 21 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.Citation: Tokunaga, T. K., J. Wan, J.-W. Jung, T. W. Kim, Y. Kim, and W. Dong (2013), 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, Water Resour. Res., 49,[4566][4567][4568][4569][4570][4571][4572][4573][4574][4575][4576][4577][4578][4579]

[1] The injection of carbon dioxide, CO 2 , into methane hydrate-bearing sediments causes the release of methane, CH 4 , and the formation of carbon dioxide hydrate, even if global pressure-temperature conditions remain within the CH 4 hydrate stability field. This phenomenon, known as CH 4 -CO 2 exchange or CH 4 -CO 2 replacement, creates a unique opportunity to recover an energy resource, methane, while entrapping a greenhouse gas, carbon dioxide. Multiple coexisting processes are involved during CH 4 -CO 2 replacement, including heat liberation, mass transport, volume change, and gas production among others. Therefore, the comprehensive analysis of CH 4 -CO 2 related phenomena involves physico-chemical parameters such as diffusivities, mutual solubilities, thermal properties, and pressure-and temperature-dependent phase conditions. We combine new experimental results with published studies to generate a data set we use to evaluate reaction rates, to analyze underlying phenomena, to explore the pressure-temperature region for optimal exchange, and to anticipate potential geomechanical implications for CH 4 -CO 2 replacement in hydrate-bearing sediments.