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

•The long-term joint production from hydrate and shallow gas layer provides a 4.1 times enhancement of the cumulative gas yield.•The regulated layer-dependent scheme effectively alleviated the interbedded pressure difference by ∼70%, avoiding the interlayer instability.•An energy return on investment (EROI) analysis was carried out to evaluated the optimized joint production method.