INJECTION OF LIQUID CARBON DIOXIDE IN RESERVOIR SATURATED BY METHANE AND ITS GAS HYDRATE

The relevance of the study is related to development of theoretical foundations of gas production technologies from gas-hydrate deposits by substitution. The method of carbon dioxide injection is considered, which allows reducing significantly energy costs for development of natural gas hydrate deposits. The main aim of the study is to determine the features of methane replacing with carbon dioxide in gas hydrate when injecting liquid carbon dioxide into gas hydrate formation. Object a porous layer of finite extent saturated with methane and its gas hydrate, the initial pressure and temperature of which correspond to the conditions of stable existence of methane gas hydrate. Methods. On the basis of the equations of continuum mechanics, a mathematical model of heat and mass transfer in the natural reservoir is constructed, accompanied by methane substitution for carbon dioxide in gas hydrate. It is accepted that in the considered case in a layer there are two characteristic zones separated by a mobile boundary of phase transitions. The first (near) pore area is saturated with liquid carbon dioxide and its gas hydrate, and the second (far) area contains methane and its gas hydrate. Result. The authors have obtained numerical solutions for pressure and temperature fields at injection of liquid carbon dioxide into a gas hydrate layer of finite extent and temperature dependences on the substitution border on injection pressure and formation permeability. It is established that at sufficiently low values of injection pressure and permeability the temperature on the boundary of substitution can rise above the equilibrium temperature of decomposition of methane gas hydrate into gas and water. This corresponds to the emergence of a second mobile interphase boundary at which decomposition of methane gas hydrate occurs. The authors determined the dependence of the maximum temperature of pumped liquid carbon dioxide, above which it is necessary to take into account the formation of a mixture of methane and water, on the pressure values at the right and left borders of the formation and its permeability. It is established that the mode with gas hydrate decomposition is implemented at high pressure values on the right boundary of the formation and low permeability and pressure values under which carbon dioxide is pumped. The authors obtained the dependences of the replacement boundary rate, as well as the time of complete replacement of methane gas hydrate by carbon dioxide gas hydrate in the entire formation, on the pressure on the right and left boundaries of the formation, as well as on its permeability.