Characterising the effect of gas hydrate in pore pressure and stress using NGHP-02 downhole logs in Krishna Godavari offshore Basin, India

Abstract

The present study demonstrates the impact of gas hydrate concentration and its morphology on pore pressure and sediment stress. We analyse the well-logging data of the 2nd Indian National Gas Hydrate Program (NGHP-02) in Area B of the Krishna Godavari Basin. Overpressure zones are delineated by observing porosity with depth and log inferred compaction trends. Eaton, Bower, Miller and porosity-based models are used for predicting pore pressure. The predicted pore pressures are ranging between ∼3 and 4.5 MPa, and ∼2.8 and 4.4 MPa respectively at Hole 02–22A and 02–23A, where the estimated fracture pressures and horizontal stresses are nearly equal to vertical stress at both holes. Within the gas hydrate stability zone (GHSZ), overpressure is not found; rather, our findings show that pore pressure falls with increasing gas hydrates saturation at both holes. This could be due to a decrease in water concentration during hydrate formation. Consequently, the effective stress of sediments increases, causing gas hydrate-filled vertical fractures to heave apart in the minimum horizontal stress directions. We observe overpressure below the gas hydrate layers at two holes, which is due to the load of the overlying sediments, low permeability created by the high concentration of gas hydrate at the bottom of GHSZ (about 40% and 80% at Hole 02–22A and 02–23A, respectively), and may be the microbial gas being produced at a faster rate than they are being expelled due to the low permeability. It is noticed that the strike directions of fractures vary with depth at both holes, which shows the anisotropic effects of stress conditions on gas hydrate-filled fractures during the time of formation.