gas hydrate - CCOP

STOMP-HYD: A New Numerical Simulator for Analysis of
Methane Hydrate Production from Geologic Formations
Mark White 1) and Pete McGrail 2)
1) Hydrology Group, Pacific Northwest National Laboratory, U.S.A.
2) Applied Geology and Geochemistry, Pacific Northwest National Laboratory, U.S.A.
ABSTRACT: Conventional methods of gas hydrate production include reservoir
depressurization, thermal stimulation, and inhibitor injection. A leading
unconventional approach for gas hydrate production involves the exchange of CO 2
for CH 4 . This unconventional concept has several distinct benefits over the
conventional methods: 1) the heat of formation of CO 2 hydrate is greater than the
heat of dissociation of CH 4 hydrate, providing a low-grade heat source to support
additional methane hydrate dissociation, 2) exchanging CO 2 with CH 4 will
maintain the mechanical stability of the geologic formation, and 3) the process is
environmentally friendly, providing a sequestration mechanism for the injected CO 2 .
An operational mode of the STOMP simulator has been developed at the Pacific
Northwest National Laboratory that solves the coupled flow and transport
equations for the mixed CH 4 -CO 2 hydrate system under nonisothermal conditions,
with the option for considering NaCl as an inhibitor in the pore water. The
simulator solves the coupled nonlinear governing equations for conservation of
water, CH 4 , CO 2 , and NaCl mass and thermal energy on structured orthogonal grid
systems. Recognized mobile phases in the order of decreasing wettability include:
aqueous, liquid CO 2 , 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. Gas hydrate equilibrium temperature is 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 phase
never disappears, yields four phase conditions and two primary variable sets. The
simulator is demonstrated on a variety of hydrate production scenarios, including
liquid-CO 2 microemulsion injection and CO 2 exchange.
Keywords: numerical simulation, STOMP, methane hydrate, CO 2 exchange,
depressurization, thermal stimulation, inhibitor.
New Energy Resources in the CCOP Region - Gas Hydrates and Coalbed Methane 77