Analyzing the process of depressurization-induced gas production from natural marine sediments

Abstract

Though understanding the gas production behavior from methane hydrate in sediments by depressurization are critical for the utilization of gas hydrate resource and has been widely studied, very few researches are reported using natural marine sediments as porous matrices. Significant differences have been found between the natural sediments and widely used coarse sands; difficulties could be commonly encountered in the formation and production process arising from the fine grain sizes and thereby low permeability. In this work, gas production behavior in hydrate-bearing natural marine sediments from the South China Sea was investigated by depressurization with different gas production pressures (2 MPa, 3 MPa and 4 MPa)and pressure gradients (1 MPa, 2 MPa and 4 MPa). Results show that the gas production process consists of two main stages: the rapid free gas liberation stage when the pressure and temperature drop quickly; the hydrate decomposition stage when the gas production slows down and temperature remains relatively stable and then rises to the backpressure driven by heat transfer from the ambient. A high pressure gradient and low production pressure were found to efficiently facilitate the gas production process. The minimum temperature during production significantly varies, under the combined effects of fast gas release and hydrate decomposition, which could play a crucial role in the production behavior. Furthermore, the difference in the thermal conductivity of the natural marine sediments with common coarse grains seriously affects the heat transfer and gas production behavior, with heat transfer from the ambient to the production well with a radial temperature gradient. The inhomogeneity of hydrate saturation vertically resulting from the low permeability of natural sediments also results in a non-uniform temperature distribution during production, which could locally trigger the secondary hydrate formation and freezing thereby hindering the gas production. Therefore, significant differences could be present in the natural sediments, making this work helpful in understanding the production behavior from natural hydrate deposits.