Molecular dynamics simulation of methane in sodium montmorillonite clay hydrates at elevated pressures and temperatures

Computer simulation has been used to study the structure and dynamics of me thane in hydrated sodium montmorillonite clays under conditions encountered in sedimentary basins. Systems containing approximately one, two, three an d four molecular layers of water have followed gradients of 150 bar km(1) a nd 30 K km(1), to a maximum burial depth of 6 km (900 bar and 460 K). Metha ne is coordinated to approximately 19 oxygen atoms, of which typically 6 ar e provided by the clay surface. Only in the three- and four-layer hydrates is methane able to leave the clay surface. Diffusion depends strongly on th e porosity (water content) and burial depth: self-diffusion coefficients ar e in the range 0.12 x 10(9) m(2) s(1) < D < 12.65 x 10(9) m(2) s(1) for wat er and 0.04 x 10(9) m(2) s(1) < D < 8.64 x 10(9) m(2) s(1) for methane. Bea ring in mind that porosity decreases with burial depth, it is estimated tha t maximum diffusion occurs at around 3 km. This is in good agreement with t he known location of methane reservoirs in sedimentary basins.