Production characteristics of two class water-excess methane hydrate deposits during depressurization

•Two kinds of water-excess hydrates deposits were exploited by depressurization.•Backpressure influenced more on average gas production rate changes of Class 2.•The gas recovery of Class 3 deposits was higher than Class 2 deposits about 20%.•The gas-to-water ratio of Class 3 deposits was higher than that of Class 2 deposits. There are four main classes of methane hydrate deposits in nature. These classes differ in their distributions of the methane hydrate layer, free water layer and free gas layer. The gas production rate, water production rate, gas-to-water ratio, gas and water production recovery during the depressurization dissociation process are different for hydrate samples of different classes. Water-excess hydrate samples of Classes 2 and 3 were prepared in the present work, and they were dissociated at 2.0 MPa, 2.3 MPa, 2.6 MPa and 3.0 MPa. Ice formation was observed when the backpressure was 2.0 MPa. The gas-to-water ratio for Class 3 samples was higher than that of Class 2 samples, regardless of the backpressure. The average gas production rate for the 90% overall gas amount of Class 2 samples was higher than that of Class 3 samples for a backpressure of 2.0 MPa, whereas the opposite result was found for backpressures equal to or greater than 2.3 MPa. Furthermore, the water recovery value of Class 2 samples was higher than that of Class 3 samples by approximately 10%, and the gas recovery value of Class 2 samples was lower than that of Class 3 samples by approximately 20% at the same backpressure. These experiments provided some basic data for the exploitation of Class 2 and Class 3 deposits and find out the influence of backpressure on methane hydrate dissociation of the two class deposits by depressurization.