PNNL

Abstract

Conventional methods of gas hydrate production include reservoir depressurization, thermal stimulation, and inhibitor injection. The leading unconventional technology for gas hydrate production involves the clathrate exchange of CO2 with CH4. This unconventional technology has several distinct benefits over the conventional methods: 1) the heat of formation of CO2 hydrate is greater than the heat of dissociation of CH4 hydrate, 2) exchanging CO2 with CH4 will maintain the mechanical stability of the geologic formation, and 3) the processes are environmentally friendly, providing a sequestration mechanism for CO2. An operational mode of the STOMP simulator has been developed by the Pacific Northwest National Laboratory that solves the coupled flow and transport equations for the mixed CH4-CO2 hydrate system under nonisothermal conditions, with the option for NaCl inhibitor. The simulator solves the coupled nonlinear governing equations for conservation of water, CH4, CO2, and NaCl mass and thermal energy on structured orthogonal grid systems. Recognized mobile phases in the order of decreasing wettability include: aqueous, liquid CO2, and gas. Immobile phases include: hydrate, ice, and precipitated salt; where, the hydrate and ice phases are presumed to be occluded by the aqueous phase. Hydrate temperature equilibrium conditions in the simulator are calculated as a function of gas vapor pressure and mole fraction of hydrate formers from tabular data generated using a fugacity-based equilibrium model. Corrections for inhibitors as a function of aqueous concentration are calculated from a generalized empirical formulation. Hydrate-aqueous and ice-aqueous interfacial radii are computed as a function of the system temperature and hydrate or ice equilibrium temperature, respectively. Interfacial radii are converted to interfacial pressures via interfacial tensions, which are then used to compute saturations via scaled capillary pressure-saturation functions. This approach, combined with the assumption that the aqueous never disappears, yields four phase conditions and two primary variable sets. The simulator is demonstrated on a variety of hydrate production scenarios, including liquid-CO2 microemulsion injection and CO2 exchange.