Consolidation and hardening behavior of hydrate-bearing pressure-core sediments recovered from the Krishna–Godavari Basin, offshore India

Abstract

Abstract Evaluating the consolidation properties and hardening behavior of reservoir with high concentrations of gas hydrate are crucial for the safe and development of gas hydrate as an energy resource. In addition, investigating the stiffness properties of the hydrate-bearing sediment promotes the prediction accuracy of hydrate content estimation from seismic survey and logging data. The hydrate-bearing pressure-core sediments recovered from the Krishna–Godavari Basin during India's National Gas Hydrate Program Expedition 02 were tested herein using a newly developed high-pressure oedometer chamber (HOC) and transparent acrylic cell triaxial testing (TACTT) system. The compression and swelling behavior of the sediments were observed by one-dimensional consolidation and isotropic loading and unloading compression tests using HOC and TACTT. Lateral earth pressure (at-rest stress) was measured using HOC, and compressional-wave (P) and shear (S) wave velocities were measured by TACTT under various stress conditions, including estimated in situ effective stress. The results revealed that the consolidation curve of the hydrate-bearing sediment is higher than that of the hydrate-free sediment. In the case of the normally consolidated state, the bulk volume of the hydrate-bearing sediment was compressed to the original consolidation curve of the hydrate-free sediment after hydrate dissociation. However, when the sediment experienced over-consolidation, it did not compress owing to the hydrate dissociation. In addition, the K0 value of the hydrate-bearing sediment was greater than the hydrate-free sediment and exhibited a larger hysteresis during loading and unloading. The isotropic loading and unloading tests confirmed that hydrate existence hardened the compression and swelling indexes. Based on the P and S wave velocities, the morphology of hydrate in the sediment in situ was assumed to be of a load-bearing type. An empirical equation was then proposed to estimate the shear modulus using the relation between S wave velocity, hydrate saturation, and effective confining pressure.