Logging-while-drilling and wireline velocities: Site NGHP-01-10, Krishna-Godavari Basin, India

Abstract

At Site NGHP-01-10, Krishna-Godavari Basin, India, the downhole wireline logging tool measured higher porosity and sonic velocities in gas hydrate-bearing sediments compared to the logging-while-drilling (LWD) acquired data in two closely spaced wells (~10 m). Using a rock physics model that assumes random fractures in unconsolidated sediments using Hashine Shtrikman bounds, we show that difference in physical properties could be due to intra-site difference in the distribution and pattern of hydrate-filled fractures rather than differences in the volume of gas hydrate. Our fracture-inclusive model suggests that between the two holes the porosity and hydrate saturation of the background sediments is similar while porosity and hydrate saturation of the fracture systems change considerably,resulting in the change of the sonic log responses. Relative changes in hydrate saturation between sediments and fractures at 90 m below the sea floor (mbsf) depth (at a prominent seismic horizon), suggests that fracture-filling hydrate could have partly originated as pore-filling material in the sediments and grew as fracture-filling material as a result of fluid flow. The volume fracture profile of hydrate (fracture porosity fracture saturation) between the two holes further suggests that the higher concentration of fractures in the wireline hole may not be a natural in-situ condition but rather a manifestation of the drilling compounded with time lapse in data recording between the LWD and wireline log data. Frequently reported, worldwide, intra-site variability in hydrate saturation from LWD and wireline logs in close proximity could be more related to interpretative methods than actual geologic variability.