PNNL

Abstract

Geologic accumulations of natural gas hydrates hold vast organic carbon reserves, which have the potential of meeting global energy needs for decades. Estimates of vast amounts of global natural gas hydrate deposits make them an attractive unconventional energy resource. As with other unconventional energy resources, the challenge is to economically produce the natural gas fuel. The gas hydrate challenge is principally technical. Meeting that challenge will require innovation, but more importantly, scientific research to understand the resource and its characteristics in porous media. Producing natural gas from gas hydrate deposits requires releasing methane from its clathrated form. The conventional way to release methane is to dissociate the hydrate by changing the pressure and temperature conditions to those outside of the hydrate stability region. The guest-molecule exchange technology releases methane by replacing it with a more thermodynamic molecule (e.g., CO2, N2). This technology has three advantageous: 1) it sequesters greenhouse gas, 2) it releases energy via an exothermic reaction, and 3) it retains the hydraulic and mechanical stability of the hydrate reservoir. Field testing of the guest-molecule exchange technology is currently in the planning stages for site within the Prudhoe Bay area, Alaska North Slope, where the average hydrate-bearing layer conditions are 6.9 MPa and 4.4°C. Previously a numerical simulator was developed with capabilities for modeling gas hydrate production from geologic reservoirs using four technologies: 1) depressurization, 2) thermal stimulation, 3) inhibitor injection and 4) CO2-CH4 guest molecule exchange. The original form of the simulator assumed equilibrium conditions between the mobile and hydrate concentrations of CO2 and CH4; where, the mobile components are those in the aqueous, gas, and liquid-CO2 phases. Additionally the simulator ignored dissolution of CH4 into the liquid-CO2 phase. Simulation results from this simulator predicted rapid pore plugging by the injected CO2, regardless of the form of injected CO2 (i.e., subcritical gas, liquid, supercritical gas, aqueous dissolved). In support of the technical planning for the arctic field demonstration, this paper takes a second look at the injectivity of liquid CO2 into hydrate bearing formations, using a new kinetic exchange model between the mobile and hydrate concentrations of CO2 and CH4. Simulation results using the kinetic implementation of the simulator demonstrate the importance of guest molecule exchange kinetics in maintaining formation injectivity.