Well Log Characterization of Sediment Porosities in Gas-Hydrate-Bearing Reservoirs

Abstract

Abstract With growing interest in natural gas hydrates, it is becoming increasingly important to be able to determine the volume of gas hydrate and included gas within natural gas hydrate accumulations. Gas volumes that may be attributed to gas hydrates are dependent on a number of reservoir parameters, one of the most difficult reservoir parameters to determine is porosity. Well logs often serve as a source of porosity data; however, well-log calculations within gas-hydrate-bearing intervals are subject to error. The well-logging devices that show the greatest promise of yielding gas hydrate reservoir porosities are the gamma-gamma density and neutron porosity logs. Well log response modeling has revealed that under most conditions, the bulk-density of a water-bearing sedimentary section is almost identical to the bulk-density of a gas-hydrate-bearing sedimentary section as measured by a gamma-gamma density logging tool. At relatively high porosities (O>40%) and gas-hydrate saturations (Sh>50%), however, the downhole log derived bulk-density porosities need to be corrected for the presence of gas hydrate. A neutron well-log response computer simulator, SNUPAR, has been used to calculate nuclear transport and capture parameters for various gas-hydrate-bearing reservoirs. The calculated thermal neutron capture cross section of various hypothetical gas-hydrate-bearing reservoirs indicates that methane hydrate has little effect on neutron porosity measurements within "normal" reservoir conditions (O<40%) and low gas-hydrate saturations (Sh<50%). Within this study, density porosity and neutron porosity nomographs have been developed with which it is possible to correct for the effect of high gas-hydrate saturations on the log derived porosities. In the field verification phase of this study, downhole density and neutron porosity log data (in some cases corrected for the presence of gas hydrate) have yielded accurate porosities for gas-hydrate-bearing reservoirs on the North Slope of Alaska. P. 765