Multifield coupling mechanism in formations around a wellbore during the exploitation of methane hydrate with CO2 replacement

The exploitation of methane (CH4) hydrate will reduce the mechanical properties of the hydrate-bearing reservoirs. When exploiting CH4 hydrate through carbon dioxide (CO2) replacement, CO2 is permanently sequestered underground while natural gas resources are produced. A coupled thermo-fluid-solid chemistry model for the exploitation of hydrate with CO2 replacement was established to analyze the exploitation process with CO2 replacement and its influence on sand production. The results showed that the presence of CO2 hydrate reduces the formation permeability, which is unfavorable for the dissociation of CH4 hydrate; however, the presence of CO2 hydrate improves the strength of the formations, lowering the risk of sand production. Regarding the significance of the influences of various factors on the amount of sand produced, the various factors are listed in descending order of influence as follows: the CO2 hydrate forming range, CO2 hydrate saturation, drawdown pressure and production time. When CO2 hydrate from in a small area around a wellbore, the extension of the forming range of the CO2 hydrate greatly decreases the risk of sand production; however, after the CO2 hydrate extend 5–7 times the radius of the wellbore, the expansion of this range insignificantly influences sand production.