Estimation of Residual Pore Water Content in Hydrate-Bearing Sediments at Temperatures below and above 0 degrees C by NMR

Natural gas hydrates are considered as a high-potential source of unconventional hydrocarbons. Economically viable and environmentally safe production of gas sequestered in hydrates requires estimating the amount of liquid pore water that resides in hydrate-bearing sediments, including one in permafrost. In this work, we present results of estimating the content of residual pore water in gas hydrate reservoirs using nuclear magnetic resonance relaxometry on a low-field NMR rock analyzer with a designed high-pressure core holder at pressures up to 8.0 MPa. The measurements are conducted on artificial samples of fine sand with inclusions of kaolinite and montmorillonite clay. The NMR results reveal the presence of liquid pore water in all hydrate-bearing samples, with its content decreasing under increasing gas pressure and cooling. The content of residual pore water approaches the calculated amount of nonclathrated water, the minimum for hydrate-bearing media, in samples exposed to repeated heating-cooling cycles. Results confirm that samples with higher percentages of clay (especially montmorillonite) contain more residual pore water, which varies from <0.2 wt % in pure sand to 5.0 or 6.8 wt % (at -5.5 and +2.5 degrees C, respectively) in a mixture of sand with 25 wt % of montmorillonite clay. It is also noted that the content of residual water is sensitive to initial moisture content and residual ice content in permafrost hydrate-bearing reservoirs.