Laboratory formation of noncementing hydrates in sandy sediments

Abstract

Natural hydrate-bearing sediment (HBS) predominantly exists in noncementing habit, and its limited availability for use in laboratory studies demands a time-effective and repeatable laboratory process for forming representative samples with natural accumulation habit. This study reports on a three-step laboratory process for forming noncementing methane hydrate in sandy sediments: (1) initial HBS formation under excess-gas conditions; (2) slow saline water (5 wt % CaCl2) injection under strictly controlled pressure-temperature (P-T) conditions; and (3) a temperature warming/cooling cycle. Changes in compressional wave velocity (Vp) of sediment, as well as P-T condition, were monitored throughout the tests. The evolution of Vp, in good agreement with rock physics model calculations, suggested that the transition from cementing hydrate into noncementing hydrate occurs during saline injection as well as temperature warming/cooling cycle. The proposed process appeared to be an efficient and consistent substitute for the existing methods, to form noncementing hydrate habit in sandy sediments.