Gas production behavior from hydrate-bearing fine natural sediments through optimized step-wise depressurization

There have been several field tests around the world to recover natural gas from gas hydrate reservoirs, yet they mostly suffer from the low productivity, sand problems and poor economic efficiency. The ultralow permeability of the sediments and an insufficient heat supply act as major barriers. Intense investigations have been carried out to study the gas production behavior from lab-synthesized hydrate-bearing coarse sands; yet the porous matrices could differ significantly from the natural sediments. Few studies have been found to focus on the role of fine natural sediments in the gas recovery performance. In this study, natural marine sediments from the South China Sea are used as the porous matrices, and gas is produced through an optimized stepwise depressurization. Significant differences in the gas production behaviors from fine natural sediments and coarse grains are found. The fine marine sediments may induce the blockage of the production well, even with protection measures; this problem will result in an uncontrolled pressure drop and gas output. Moreover, the stepwise depressurization efficiently contributes to remitting the temperature drop of the reservoir, increasing the minimum temperature from −0.5 ℃ upon a straight pressure drop to 0.27 ℃ upon a step-wise depressurization. This method can therefore help prevent the ice generation during hydrate dissociation. Additionally, a finer pressure gradient of 0.5 MPa will enhance the hydrate dissociation rate by 18.92% compared with the 2 MPa step scenario. The average gas production rate is determined to positively correlate with the Stefan number, which indicates a dominant role of the sensible heat of the reservoir in the efficient gas production. The findings could be applied in the field tests of gas hydrates to avoid ice generation and achieve a stable and continuous gas production behavior.