Logging While Drilling Data Acquisition Challenges in Gas Hydrate Wells - India's Gas Hydrate Expedition - 02
Authors: Singh J. , Kumar P. , Shukla K. M. , Majumdar C. 
Published: March 2018

Natural gas hydrate (methane hydrate) is of growing importance for the oil and gas exploration and production industry. It is a potential boon but can turn into a costly menace. The massive supply of natural gas has a potential to exceed the combined volume of worldwide conventional reservoirs of oil, gas and coal. However, drilling through, analyzing and producing from hydrate formations is very expensive and highly challenging.

Petrophysics - Integrated Logging While Drilling (LWD) tools provided triple combo measurements along with advanced measurements such as sigma and spectroscopy. Nuclear magnetic resonance (NMR) was used as gas hydrates are invisible to NMR and it is fundamental for gas hydrate identification to compare the difference in porosity from NMR when compared with density porosity. Density porosity computation using grain matrix density from the spectroscopy output correlated with NMR porosity to estimate this difference as an indication of gas hydrate presence.

Geology - Since the target formation is highly unconsolidated, high resolution resistivity imaging tools were placed next to the bit to record images just after drilling the formation and before any possible damage. Fracture characterization and bedding plane information were helpful in interpreting the presence and form of gas hydrate, including whether it was disseminated or layered.

Acoustics - The identification of bottom simulating reflector (BSR) clearly demarcates the bottom of any possible gas hydrate deposit. BSR was inferred from the P-wave velocity (Vp). Sharp decrease in Vp below BSR is characteristic feature in presence of gas hydrate. Drilled wells were just below the sea bed and in very unconsolidated formations, thus acquiring P-wave and S-wave velocities were very challenging. With the help of an advanced multipole tool, P-wave and S-wave slownesses were recorded and used for BSR identification and type of gas hydrate dissociation. Plots of saturation computation along

with sonic P-wave velocity were an indication of the type of gas hydrate dissociation, which was correlated with the depositional setting interpreted from the images.

Gas hydrates are predominantly found in deep water, at shallower horizons and at very low temperatures. Drilling through gas hydrates is always challenging as it disseminates under pressure temperature imbalances. The release of such huge volumes of trapped gas is difficult to control and can lead to a disaster. Reservoir characterization is also challenging due to the presence of massive, fractured deposits of gas hydrate in Krishna Godavari offshore, unlike disseminated deposits in sands. State-of-the-art LWD technology was used in National Gas Hydrate Program (NGHP) Expedition-02 in 25 wells to acquire all the required measurements for complete gas-hydrate evaluation. The penta-combo logging run consisted of high resolution resistivity images for fracture evaluation and depositional system identification. Borehole acoustics measurements were used for BSR detection and pore-pressure monitoring. Nuclear- Magnetic-Resonance (NMR) along with density measurements were also used to identify the presence of gas hydrates.

This paper will explain the NGHP-02 in India with associated challenges and findings. It will focus on Logging-While-Drilling (LWD) data acquisition challenges and its utility along with other available core and wireline measurements in gas hydrate deposit characterization.