Gas hydrate dissociation by depressurization along with ice occurrence and sand migration

The extraction efficiency and safety of hydrate reservoirs are important for the industrialization of hydrate energy production. In this connection, various depressurization schemes with different amplitudes, rates and modes have been tried in field production tests. In this work, production profiles of laboratory-scale hydrate -bearing sediments were simulated through different depressurization schemes at a constant ambient temperature of 275.45 K. The results show that there are the freezing -melting behavior (FMB) and isothermal stage in the system with the depressurization amplitude (Delta P) range of 1.27-2.67 MPa, but not in the system with a Delta P of 0.83 MPa. In addition, the lower final outlet pressure and the higher depressurization rate increase the ice content and the ice duration of the system. Based on this, the linear equation of beginning freezing time of system (each observation point) can be obtained by fitting. According to the maximum ice content of system (0.06-3.56 E-4m2) under deep depressurization mode, the function of ice content of the system with the Delta P and the production time can be obtained. Moreover, the effects of FMB and sand detaching behavior were analyzed on pore fluid migration. FMB can temporarily block 0-66.56% of the migration space of pore fluid (MSPF). It can be revealed that the pore water velocity of observation points in the absence of FMB increases first and then steadily declines over time. But the former stage due to FMB can be divided into a slight decrease/increase and a sharp increase. Meanwhile, it can be found from the sediment components that the behaviors of skeleton sand detaching and hydrate dissociation can expand the MSPF, with contribution percentages of 0-13.49% and 86.51-100%, respectively. Finally, the high depressurization rate can advance the peak of gas production and water production by 14-22 min and 13-17 min, respectively. The high Delta P can improve the production rate peaks of gas and water about 34-59% and 17.4-41%, respectively, and advance the peak of sand production by 19-60 min. The findings of this work clearly illustrate the importance of the depressurization scheme for the extraction of hydrate resources and provide a basis for the best production scheme.

Automated summary

The production characteristics of laboratory-scale hydrate-bearing sediments were simulated through different depressurization schemes. The results show that different depressurization amplitudes and rates can affect the ice content and duration of the system, and the formation of ice and sand detaching behavior can impact pore fluid migration. Therefore, the depressurization scheme is crucial for the extraction of hydrate resources and provides a basis for the best production scheme.