Sampling disturbance in hydrate-bearing sediment pressure cores: NGHP-01 expedition, Krishna–Godavari Basin example

Sampling natural sediments causes unavoidable disturbance as recovered sediments experience changes in stress and strain during drilling, core recovery, transportation, handling, and early stages of testing. In hydrate-bearing sediments, the potential for sampling disturbance may be aggravated, since pressure and temperature changes can lead to hydrate dissociation and gas exsolution. Pressure core technology attempts to recover and characterize hydrate-bearing sediments while preserving them under in situ pressure and temperature conditions, which is an essential requirement to assess the mechanical, physical, chemical, and biological properties of natural hydrate-bearing sediments. Previous studies on near-surface sampling effects are extended in this study to evaluate additional sampling disturbances relevant to hydrate-bearing sediments: (1) hydrate dissociation due to mechanical extension, (2) negative pore pressure generation during unloading (Mandel–Cryer effect), (3) secondary hydrate formation, (4) changes in hydrate mass as a function of changes in pressure and temperature within the stability field, (5) hydrate anomalous preservation and its benefits for pressure core handling and testing, and (6) relaxation/aging following sampling. Results provide valuable insight to sampler design, coring and operation procedures, high pressure chamber design, and pressure core testing techniques.