Methane-hydrate （MH）, has been considered as a new resource of natural gas in the future. However, in the past several ?led production tests, serious sand problems occurred during the depressurization, because of the unconsolidated sand characteristic of MH reservoirs. Unlike conventional oil and gas reservoirs, MH reservoirs locate in very shallow formations without intensive compaction; MH existing in the pore space also play a role of cementing sand grains. Therefore, sand grains would lose their solidarity during the depressurization for MH dissociation, separate from each other, fluidize and migrate into a production well. To solve this problem, we have proposed a new method, which stabilizes a MH reservoir by grout material. To verify its effectiveness, we conducted a series of experiments using quartz sands, cement-grout and TBAB hydrate under normal pressure. First, we con?rmed that the grout material could prevent a sand problem from a synthesized formation without hydrate. Then, we verified that permeability and uniaxial compressive strength （UCS） could be sustained in the grouted hydrated cores. In addition, we examined the factors affecting the physical properties of grouted cores, such as grain size, hydrate saturation, salinity of formation water, temperature, concentration of grout material and additives, and curing time. Finally, in accordance with the insights obtained above, we tried to optimize the strategy to prevent the sand problem using the grout material in hydrate reservoirs, from the viewpoints of maintaining their permeability and strength （UCS）.

Solar thermal, wind power, and geothermal heat have become popular as the new energy with the small impact of greenhouse gas emissions, and Methane hydrate is one of them. IPCC （2007） showed that CH₄ has high global warming potential which is 23 times as large impact as CO₂ in 2007. It means that changing CO₂ from CH₄ plays the part of decreasing greenhouse gas emissions. In addition that methane is combustible material, and would become the best fuel energy. However, methane hydrate dissolves sea water with pressure and/or water temperature changing in the seafloor because of unstable solid. Therefore, it is important to measure volume of methane gas for recognition of methane hydrate in the seafloor. However, the studies of measuring volume of gas in the seafloor are hardly found. Therefore, we investigate possibility of using the Time Domain Reflectometry （TDR） method as measuring volumetric water content in seafloor sediments because a gas phase in a core sample of the seafloor sediment may be obtained as V_a＝V－（V_w－V_s） where V is the total volume of a core sample （m³）, V_a is a gas phase of a core sample （m³）, V_w is a liquid phase of a core sample （m³）, and V_s is a solid phase of a core sample （m³）. We recognize TDR method is useful of measuring volumetric water content in seafloor sediments. However, we found each site needs each calibration under the sediments used for volumetric water content measurements by TDR method.

Low salinity waterflooding （LSWF） has been used as a promising technique for enhanced oil recovery（EOR） in sandstone and carbonate reservoirs. Many mechanisms for the effect of LSWF have been proposed, but the wettability alteration is the most important factor affecting LSWF for EOR. In the reservoir, the crude oil is attached to rock surface via electrostatic forces at high salinity and forms oil-wet surface. The electrostatic equilibrium will be disturbed when the low salinity water flooded to the reservoir. Thus, the rock surface changes from oil-wet to mix-wet or waterwet surface and improves the oil recovery. The characteristics of crude oil/brine and rock/brine interfaces signi?cantly influence the wettability alteration. The surface complexation models for the interfaces have been proposed to understand and estimate charge and potential distribution at the interfaces based on thermodynamic equilibrium. Further, the models were extended to understand the impact of ionic adsorption. This paper provides a comprehensive review of crude oil/brine interface. This review includes the basis of surface complexation models （diffuse double layer or triple layer）, analysis and comparison of the proposed surface complexation for crude oil/brine, the main parameters used to develop the models such as surface site densities and equilibrium constants for dissociation and ionic adsorption, the effect of polar groups on interface properties, and the importance of crude oil/brine interface properties on wettability alteration.

This review paper provides the current information on crude oil/brine interface and can be used as guide to select brine composition in LSWF for wettability alteration in both sandstone and carbonate reservoir.