Evolution law of physical parameters and hydrate reservoir productivity under multi-stage depressurization

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In the process of gas hydrate depressurization production, the reasonable depressurization rhythm and depressurization amplitude have significant impact on improving production and reducing engineering geological risks. Considering the basic stability of the reservoir, this study constructs mathematical models of gas hydrate decomposition kinetics, multiphase flow in the reservoir, and the disintegration and migration of rock matrix particles containing hydrates. Based on actual data from the first trial production in Japan's Nankai Trough, the validity of the model has been verified. The study analyzed changes in reservoir physical properties and productivity under multi-stage depressurization conditions. The influence of different pressure reduction rhythms on productivity changes and the evolution laws of porosity, permeability and saturation over time and space were discussed. The research disclosed the multi-stage depressurization mode can modulate the decomposition rate and sand production rate of natural gas hydrates through the progressive reduction of reservoir pressure, guaranteeing production capacity while attaining sand production control and minimizing the risk of blockage, thereby striking a balance between production efficiency and sustainability. This study provides a crucial theoretical basis for the design optimization of natural gas hydrate depressurization extraction schemes. The research outcomes not only guide the parameter configuration optimization during depressurization but also offer scientific support for establishing production prediction models.