Numerical Analysis of Experiments on Thermally Induced Dissociation of Methane Hydrates in Porous Media

Numerical simulation is essential for the prediction and evaluation of hydrocarbon reservoir performance. Numerical simulators developed for the description of the behavior of hydrates under production and the corresponding flow of fluids and heat accounting for all known processes are powerful, but they need validation through comparison to field or experimental data in order to instill confidence in their predictions. In this study, we analyze by means of numerical simulation the results of an experiment of methane hydrate dissociation by thermal stimulation in unconsolidated porous media heated through the vessel walls. The physics captured by the model include multicomponent heat and mass transfer, multiphase flow through porous media, and the phase behavior of the CH₄ + H₂O system involved in methane hydrate formation and dissociation. The set of governing equations consists of the mass and energy conservation equations coupled with constitutive relationships, i.e., the dissolution of gas in H₂O, relative permeability and capillary pressure models, composite thermal conductivity models, and methane hydrate phase equilibria. The model geometry describes accurately the hydrate reactor used in a recent experimental study investigating methane hydrate dissociation behavior [Chong et. al. Appl. Energy 2016, 177, 409–421]. The cumulative gas production is estimated and validated against three tests of experimental data involving different boundary temperatures, showing a good agreement between observations and numerical predictions. The predicted evolution of the spatial distributions of different phases over time shows that hydrate dissociation progresses inward from the reactor boundary to the center, methane gas accumulates to the top of the reactor because of buoyancy, and water migrates down to the bottom of the reactor because of gravity. A sharp hydrate dissociation front is predicted, and the estimated location of hydrate dissociation front suggests a linear relationship with the square root of time. A sensitivity analysis on the thermal conductivity of sand under fully saturated conditions is conducted to elucidate its effect on the gas production behavior. In addition, the energy efficiency ratio computed from the simulation of this boundary-wall heating technique varies from 14.0 to 16.2. Deviations between observations and predictions of the evolution of the temperature profile are attributed to initial heterogeneous distribution of the hydrate phase in the hydrate reactor

Pore-Scale Investigation of CH₄ Hydrate Kinetics in Clayey-Silty Sediments by Low-Field NMR

Clayey-silty sandy media have been widely discovered in naturally occurring hydrate-bearing sediments in the South China Sea. However, the phase change behavior of CH4 hydrate (MH) and the resulting pore structure change and the migration of fluid in clayey-silty sediments remain less known and warrant investigation. In this study, we examine the pore-scale behavior of MH formation and dissociation in clayey-silty sediments and the associated fluid migration by low-field nuclear magnetic resonance (NMR). Based on T2 spectra measurement, MH starts to grow in small pores (pore size <1 μm) first and in large pores (pore size >10 μm) subsequently. The presence of clay, i.e., Na-MMT, practically retards the overall growth kinetics of MH evidenced by the low H2O conversion (<10%) to MH in clay-associated small pores. During depressurization, MH starts to dissociate in sand-associated large pores first. Free water migrates to clay-associated small pores and partially converts to clay-bound water. MRI visualization depicts the heterogeneous spatial distribution of both MH and the residual water in the process. The experimental results provide possible explanations on the spatial heterogeneity of MH in clay-silty sediments in nature and on the multiphase fluid migration during energy recovery from MH reservoirs