Coproduction of Hydrate and Shallow Gas in the South China Sea: Performance Evaluation and Interference Analysis of Horizontal Multilateral Well

The multilateral well technique represents a promising approach for enhancing gas recovery in heterogeneous hydrate reservoirs, particularly those with underlying free gas accumulations. However, the optimal deployment strategy for horizontal multilateral wells (HMLWs) on coproduction remains poorly understood. This study presents a novel 3D numerical model designed to systematically evaluate the production performance of HMLWs in this type hydrate reservoir. The findings demonstrate that the effective pressure reduction propagation along the main horizontal wellbore is constrained to approximately 300 m. Deploying branches at the heel-end enhances cumulative gas production by accelerating pressure diffusion and expanding dissociation zones, yielding a 6.1% increase compared to toe-end configurations. Orthogonal optimization identifies an optimal HMLW design consisting of 8 branches with 30 m spacing and 40 m branch length, achieving peak annual production of 1.06 × 107 m3 with minimized well interference effects. Significantly, effective completion length and branch number are identified as the dominant control factors for total gas production. Meanwhile, a novel interference coefficient (m) is introduced to quantify the substantial difference in interference between opposite-side (3.7%) and same-side (21.7%) branch layouts. Comprehensive indicator analysis shows that while vertical multilateral wells (VMLWs) provide stable production efficiency, HMLWs deliver superior total gas yield and energy return on investment (EROI = 19.73), making them economically viable for large-scale exploitation. This study offers valuable operational insights for optimizing hydrate and shallow gas production via HMLWs in silty-clay reservoirs of the South China Sea (SCS).