Long-term numerical simulation of a joint production of gas hydrate and underlying shallow gas through dual horizontal wells in the South China Sea

Recent field tests to recover natural gas from marine gas hydrate reservoirs in Japan and China have exhibited worldwide attention to this strategic energy form; significant challenges still remain in improving the accumulative gas yield for a better economic efficiency. According to the geological survey in marine hydrate reservoirs, there exists a concomitant free gas layer underlying the gas hydrate reservoir. Consequently, here we propose a new scheme to jointly produce the gases from hydrate layer and its underlying shallow gas layer. It was found that a dual horizontal well deployment respectively in the corresponding two layers could contribute to a 4.1 times cumulative gas yield comparing that of a single vertical well scenario. This implies a remarkable enhancement of gas productivity via making full use of the gases in the shallow gas layer. Notably, a potential risk of interlayer failure could occur upon the great interbedded pressure difference (maximum 10.7 MPa) arising from the varying behavior of pressure propagation in the layers. This can be effectively alleviated by separately controlling the depressurization scheme in different layers; a mild step-wise depressurization was suggested in the more permeable shallow gas layer while the hydrate layer with a lower permeability should experience a sharper pressure drop. This was found beneficial reducing the interbedded pressure difference by about 70% without intervening the gas production. An energy return on investment (EROI) analysis showed a promising positive energy harvest of our optimized layer-dependent pressure scheme. It could be therefore a potential method to be applied in the field tests in the South China Sea to improve the economic efficiency without disturbing the reservoir stability.