NUMERICAL STUDIES OF GAS PRODUCTION FROM OCEANIC HYDRATE ACCUMULATIONS

George J. Moridis
Lawrence Berkeley National Laboratory, University of California, U.S.A

Current estimates of the worldwide quantity of hydrocarbon gas hydrates range between 10¹⁵ to 10¹⁸ m³ (Sloan, 1998). Even the most conservative estimates of the total quantity of gas in hydrates may surpass by a factor of two the energy content of the total fuel fossil reserves recoverable by conventional methods. The magnitude of this resource could make hydrate reservoirs a substantial future energy resource. While current economic realities do not favor gas production from the hydrate accumulations, their potential clearly demands evaluation.

Although oceanic accumulations form the bulk of natural hydrate deposits, recent numerical simulation studies and field tests of gas production from hydrates have focused exclusively on permafrost deposits because of their relatively easier accessibility. Such deposits were formed under conditions different from those prevailing in marine systems, and exhibit different characteristics.

This study addresses the issue of gas production from oceanic hydrate deposits over the entire spectrum of geologic scenarios. In addition to the three classes of hydrate deposits encountered in the permafrost (i.e., Class 1 deposits, with a free gas zone underneath the accumulation; Class 2 deposits, underlain by a mobile water zone; and Class 3 deposits, with no underlying zones of mobile fluids), a new class (Class 4) is encountered in oceanic environments. Class 4 deposits are characterized by heterogeneous hydrate saturations widely dispersed in the porous matrix, and by the absence of important permeability boundaries (a distinctly different feature, absent from Classes 1 to 3).

Several production scenarios are analyzed using the EOSHYDR2 model (Moridis, 2002), a member of the TOUGH2 family of codes (Pruess et al., 1999). Production strategies, involving the main hydrate dissociation methods (depressurization, thermal stimulation, use of inhibitors, and combinations thereof) are evaluated in each of the four classes of oceanic hydrate deposits. Additionally, the sensitivity of production to important conditions and properties is investigated. These include the initial deposit pressure and temperature, the initial saturation distributions, the boundary conditions, and the hydraulic and thermal properties of the hydrate-impregnated system, as well as the operational parameters of the dissociation method. The effect of spatial heterogeneity in the hydrate saturation on gas production is also studied.

The results of the study indicate that the hydrate deposit class dictates the dissociation method, and, consequently, the production strategy. Under oceanic conditions, depressurization appears to be the most promising method, but the appeal of thermal stimulation increases from Class 1 to Class 4 hydrates. Combining depressurization, thermal stimulation and inhibitor use (i.e., sea water) dramatically increases the gas production potential. In general, gas production is strongly affected by (a) the initial hydrate temperature and saturation, (b) the thermal conductivity (for dissociation through thermal stimulation), and (c) the pressure drop and the hydraulic properties (permeability, and relative permeabilities) of the hydrate-bearing sediments (for depressurization-induced dissociation).

Of particular interest is the potential of Class 2 hydrates, production from which is not hampered by the water disposal problems encountered in permafrost environments. Additionally, depressurization-based production from Class 2 hydrates is unaffected by dissociation-induced hydrate cooling (which hampers further dissociation) because of the large heat capacity of the moving water in contact with the hydrate. Production from Class 4 hydrates does not appear to be appealing for the low hydrate saturations usually associated with such deposits.