Evaluating proppant performance and fracture conductivity dynamics in the Shenhu marine hydrate reservoir, South China Sea

Hydraulic fracturing, which creates propped fractures to establish high-conductivity pathways near production wells, has emerged as a promising technology for boosting gas production efficiency in low-permeability marine natural gas hydrate (NGH) reservoirs. However, the effect of depressurization and hydrate decomposition on fracture conductivity remains unclear. This study examines the performance of proppants in propping artificial fractures and monitors changes in fracture conductivity and proppant embedment depth during depressurization and hydrate decomposition. Firstly, the changes in fracture conductivity during closure pressure loading and hydrate decomposition were measured by quantitatively injecting pre-cooled methane gas using an API-standard fracture conductivity chamber specifically designed for hydrates. It was found increasing closure pressure to 7.5 MPa led to a decrease in fracture conductivity that was more than 6.9 times greater than that caused by slow hydrate decomposition. The main causes of this conductivity damage were proppant embedment and rearrangement, with rearrangement being the more significant factor. Notably, proppant embedment was more pronounced in hydrate reservoirs compared to other unconventional reservoirs. After the complete decomposition of hydrates, the embedment depth was measured using White Light Optical Profilometry, showing that at a low closure pressure of 7.5 MPa, the embedment depth reached up to 28 % of the proppant diameter. Fractures using 40/70 mesh proppants exhibited higher conductivity compared to those using 30/50 mesh proppants. Additionally, increasing proppant concentration increased fracture conductivity, primarily due to reduced embedment and increased fracture width. Different proppant concentrations displayed distinct mechanisms of conductivity damage: single-layer proppants saw a rapid conductivity decline due to proppant embedment, while multilayer proppants experienced significant reductions from proppant compaction. Seawater flow on the fracture surface further decreased conductivity by 20.68 %, mainly due to the softening and expansion of clay, which exacerbated proppant embedment. This study emphasizes the importance of closure pressure and proppant properties in maintaining fracture network permeability for long-term gas extraction in clayey-silt NGH reservoirs, providing key insights for optimizing marine NGH production.