Analytical study of CO2–CH4 exchange in hydrate at high rates of carbon dioxide injection into a reservoir saturated with methane hydrate and gaseous methane

Abstract

An injection of carbon dioxide into depleted methane deposits has a dual-purpose: to stimulate the production of the residual natural gas and to store carbon dioxide. Of a great interest is the special case when a methane deposit contains methane hydrate. When carbon dioxide is injected under suitable thermodynamic conditions, the methane hydrate is converted to carbon dioxide hydrate. The mechanism of conversion during the injection of gaseous carbon dioxide into a reservoir, saturated with methane hydrate and methane is investigated. This mechanism is based on the difference between the heat required to dissociate methane hydrate and the heat released during the formation of carbon dioxide hydrate. As a result, a self-sustained replacement reaction may develop. In this work a new mathematical model which describes the replacement reaction of CH4 by CO2 in hydrate at a high injection rate is developed. A moving interface on which carbon dioxide replaces methane in hydrate is introduced and full system of boundary conditions is derived. Using an analytical similarity solution, various reaction regimes are explored. The obtained critical diagrams give the range of the main parameters at which the conversion of CH4 hydrate to CO2 hydrate is realized.