Combined styles of depressurization and electrical heating for methane hydrate production

Abstract The combined styles of depressurization and electrical heating have an important influence on hydrate recovery and energy use in hydrate exploitation. However, the efficient combined styles of depressurization and electrical heating have not been achieved at present. In this work, six combined styles of depressurization and electrical heating were designed. In order to determine efficient combined styles, a depressurized vertical wellbore and a heated horizontal wellbore were used to model these combined styles and further to dissociate hydrate-bearing samples prepared by the excess-water method. The results showed that electrical heating should be started before depressurization. Specifically, considering hydrate saturation increase of 0.327–2.47% in the hydrate stability region, electrical heating was proposed to start at the onset of fresh hydrate formation. Subsequently, the soaking through electrical heating was performed at a pressure below the equilibrium pressure at the ambient temperature, which increased the averaged hydrate dissociation rate by 7.72%. A lower shut-in pressure for the soaking could enlarge the effective heating radius in samples to improve hydrate dissociation. During depressurization, no electrical heating reduced the averaged water production rate by 80.99% and increased energy efficiency by 18.06%. So electrical heating was proposed to stop in the temperature recovering stage, but whether it was used or not in the temperature reducing stage should depend on exploiting conditions, due to secondary hydrate formation and ice formation at a lower back pressure. This work may offer some reference on the arrangement of depressurization and electrical heating in future field tests for hydrate exploitation.