Evaluation of thermal stimulation in hydrate reservoirs under hot-water cyclic injection

The techniques of natural gas production from hydrate reservoir are significantly different from those used for conventional oil and gas resources, because the virgin hydrate will dissociate from solid into gas-water mixture under these artificial operations like depressurization and heating. To realistically evaluate the efficiency of hot-water cyclic injection, modeling of the temperature propagation considering complex reservoir thermodynamic properties under different field operations is required. This study proposed a novel numerical model to predict the temperature distributions (in space and time) incorporating three zones (dissociated, transition, and hydrate zones). The double dissociation interfaces in the transition zone, gas production, and evaluation of water cyclic injection efficiency are discussed for a finite 2D reservoir. The modeling was conducted for a real project in a natural gas field located in the Shenhu region, South China Sea. This mathematical model can numerically predict the temperature distributions, phase behaviors of hydrate, gas output, and injection efficiency in any reservoir position and any time under actual field operations. The model draws major conclusions that about every 2.4 ° C increase of the injection temperature or nearly every 25.4 m 3 /day growth of the injection rate results in a 1 ° C increase at the reservoir position r = 1.2 m. Increasing the injection temperature and rate will effectively increase the moving speed of double-phase change interfaces and gas production, but will decrease the energy efficiency ratio and thermal efficiency to a certain extent. Interestingly, when the transition zone exists, the temperature at a given reservoir position will increase periodically with time, and in each cycle, the rate of temperature change decreases from a high value to approximate zero. For the convenience of field engineers, the injection temperature and rate after optimization are recommended as 120 ° C and 32 m 3 /day, respectively, for the studied case. These findings can be employed to analyze these heat transfer problems involving complex phase change behaviors in oil/gas reservoirs where the phase state of the pore fluid changes or in these special rocks or soils in polar and permafrost regions.