Production characteristics of methane hydrate reservoirs in the near wellbore region: Insight from depressurization rate

In this work, we experimentally simulated the production process of three-phase saturated hydrate reservoirs through single-vertical well depressurization, focusing on the effects of depressurization rate on the phase change, heat and mass transfer behaviors near the wellbore. Based on the reservoir characteristics of Shenhu area in the South China Sea, the samples with a three-phase coexistence of free gas, free water, and hydrate were prepared in laboratory. We analyzed the potential impacts of depressurization rate on the region near wellbore during actual reservoir production. Experimental results showed that in the initial stage of depressurization, a low depressurization rate could cause more hydrates to form, whereas a high depressurization rate tended to concentrate gas and water production at wellhead. During depressurization stage, the cumulative gas production at wellhead increased as the depressurization rate decreased, while in the subsequent constant pressure stage, a low depressurization rate could lead to a reduction in the average gas production rate. Apart from the Joule-Thomson effect caused by gas flow, the reservoir temperatures were significantly controlled by the formation and dissociation of hydrates in various locations. Throughout the production period, a significant negative correlation was observed between the depressurization rate and the hydrate dissociation rate. A low depressurization rate helped to reduce the amounts of water production near wellbore and the consumption of hydrate dissociation on the sensible heat from the surrounding sediments. For actual reservoir production, an excessive depressurization rate may cause fluids with high velocity to impact the wellbore, resulting in its damage or collapse, while an insufficient depressurization rate is not conducive to hydrate dissociation.