Methane Hydrate Formation in a Saturated, Coarse-Grained Sample through the Induction of a Propagating Gas Front

Abstract:

We generate methane hydrate in the laboratory in a coarse-grained, brine-saturated, vertically-oriented sample through gas injection. The sample (5.125 inch length; 2 inch diameter; 0.383 porosity) was initially pressurized to 12.24 MPa (1775 psi), cooled to 1 degree Celsius, and saturated with a 7 wt% NaBr solution. A syringe pump was filled with methane gas and connected to the top of the sample at a constant pressure. Another pump was used to withdraw brine from the base of the sample at a constant rate (0.003 mL/min), pulling methane into the sample and initiating hydrate formation. Based on mass balance calculations, derived from the mass of water withdrawn and the mass of methane consumed, the bulk saturations of water, hydrate, and gas reached final values of 0.683, 0.278, and 0.038, respectively. The computed-tomography (CT) scans confirm a downward-propagating low density front, which we interpret as the front of the region where hydrate is forming and free gas is replacing withdrawn water. Assuming that hydrate formation and gas presence is limited to the region behind this front increases the hydrate and gas saturations to 0.452 and 0.062, respectively. Additional analysis of the CT scans indicates a heterogeneous distribution of gas, hydrate, and water within the core and provides insight into hydrate formation behavior and the thermodynamic state of hydrate in gas-rich, coarse-grained systems.