124 / 2023-06-21 09:20:38

Experimental study of gas hydrate decomposition in sediments with different saturation levels under gradient depressurization mode

natural gas hydrate; gradient pressure drop; saturation; gas production; gas production rate

Natural gas hydrate (NGH) is a kind of unconventional natural gas resource with abundant reserves. At present, hydrate extraction methods mainly focus on depressurization, heating, chemical reagent method and carbon dioxide replacement method, among which depressurization method is considered as one of the most promising methods for natural gas hydrate extraction. However, nowadays, most studies focus on the effects of sediment components, water saturation, porosity, etc. on the decomposition of hydrate depressurization, as well as water production and heat transfer during the depressurization process, and most of the depressurization schemes used in these studies only involve single-stage depressurization (SDP), in which the bottomhole pressure is directly reduced from the initial pressure of the system to the final pressure without any pressure-holding point in between. However, there are few studies on the magnitude and path of pressure reduction by simulating the real subsea sediment composition. Accordingly, in this paper, four types of gradient depressurization modes and four different depressurization intervals of gas hydrate decomposition tests were conducted under two hydrate saturation conditions, 24-35% and 46-64%, by using the gas hydrate decomposition simulation test system to simulate the borehole pressure and salinity in the test mining area. The results show that the decomposition time of high hydrate saturation relative to low hydrate saturation decreases to different degrees when the gradient is depressurized to 3 MPa, and the decomposition time of II-5, III-5 and IV-5 decreases by 37.57%, 21.57% and 43.20% relative to II-1, III-1 and IV-1, respectively. The peak instantaneous decomposition rate of hydrate all appeared in the early stage of mining, with faster rate in the initial stage of decomposition in the larger pressure drop interval and slower rate in the initial stage of decomposition in the lower pressure drop interval, and the peak instantaneous decomposition rate gradually decreased with the increase of pressure drop gradient. The larger the pressure drop, the greater the driving force of hydrate decomposition, and the greater the heat required for its decomposition per unit time, which will easily lead to the shortage of heat supply at the hydrate decomposition interface within a certain period of time, limiting the decomposition of hydrate and the phenomenon of secondary generation, although the gradient pressure drop will still cause further temperature drop, but during the gradient pressure drop, each temperature drop is smaller than the drop at one time, which will reduce the unpredictable hydrate reformation or occurrence. The temperature drop during the gradient pressure drop is smaller than that during the first drop, thus reducing the probability of unpredictable hydrate reformation or cementation and improving the gas production efficiency. The average rate of hydrate decomposition decreases to different degrees with the increase of pressure gradient in the gradient pressure reduction mode, in which the average rate of hydrate decomposition decreases by 61.27%, 64.64% and 69.21% for II-5, III-5 and IV-5, respectively, compared with that for I-5. The peak gas production rate may occur immediately after the start of extraction during natural gas hydrate extraction, and it is important to reduce the peak gas production rate and enhance the stability of hydrate decomposition process to ensure the reservoir and wellbore stability. This study provides a theoretical reference for the development of a pressure reduction scheme for hydrate extraction in the Shenhu sea area of the South China Sea.