PNNL

Abstract

Class 1 gas hydrate accumulations are characterized by a permeable hydrate-bearing interval overlying a permeable interval with mobile gas, sandwiched between two impermeable intervals. Depressurization-induced dissociation is considered to be a conventional technology for producing gas from Class 1 gas hydrate accumulations. The depressurization production technology requires heat transfer from the surrounding environment to sustain dissociation as the temperature drops toward the hydrate equilibrium point, and leaves the reservoir void of gas hydrate. Production of gas hydrate accumulations by exchanging carbon dioxide with methane in the clathrate structure has been demonstrated in laboratory experiments and proposed as a field-scale technology. The carbon dioxide exchange technology has the potential for yielding higher production rates and mechanically stabilizing the reservoir by maintaining hydrate saturations. We used numerical simulation to investigate the pitfalls and enhancements of using carbon dioxide injection to enhance the production of methane from Class 1 gas hydrate accumulations. This paper begins a series of papers concerned with using carbon dioxide injection to enhance the production methane from Class 1 through 3 gas hydrate accumulations, those most common beneath the arctic permafrost. Processes not considered in the numerical simulation include: 1) kinetic dissociation, 2) kinetic exchange of guest molecules, 3) formation salinity. Numerical simulations in this study were primarily concerned with the mechanisms and approaches for production enhancement with carbon dioxide injection. To avoid excessive simulation execution times, a five-spot well pattern with a 500-m well spacing was approximated using a two dimensional domain, having well boundaries on the vertical sides and impermeable boundaries on the horizontal sides. Impermeable over- and under burden were included to account for heat transfer into the production interval. Simulation results demonstrate both practical and impractical carbon dioxide injection protocols.