Acquisition of Logging-While-Drilling (LWD) Multipole Acoustic log Data during the India National Gas Hydrate Program (NGHP) Expedition 02

Abstract

Abstract Gas hydrates are ice-like crystalline minerals with high energy concentrations, which generally occur in deepwater environments, at low temperature and high pressure, usually close to the seafloor. Gas hydrate research wells are normally drilled riser-less, with multiple wells being drilled in rapid succession. Logging-while-drilling (LWD) is the preferred logging method as the borehole condition in such shallow formations deteriorates very rapidly after drilling. LWD helps minimize the time between drilling and logging. The LWD sonic measurement is key in such projects and is used typically for geomechanics and formation evaluation purposes. The water-bearing sedimentary section within the shallow gas hydrate stability zone is often shale-rich, unconsolidated and the measured acoustic wave slowness is “extremely slow”, with shear slowness above 1400 μs/ft and bulk density of around 1.7 gm/cc. Quadrupole measurements are commonly used in the industry to log shear slowness with LWD tools in slow formations, where the shear slowness is slower than the borehole fluid slowness. However, it is very challenging to log shear slownesses with LWD sonic tool in these extremely slow formations, even with a quadrupole measurement. This paper details the challenges related to borehole acoustics logging in gas hydrate research wells, outlines the key technological enablers and gives an overview of the performance of LWD sonic tools during the National Gas Hydrate Program Expedition 02 (NGHP-02) in the offshore of India. It will spotlight the innovative approach followed to modify the LWD sonic acquisition and processing to adapt to the extremely slow acoustic wave speeds, which facilitated recording LWD shear slowness up to approximately 1400 μs/ft.