Influence of permeability and laminations on fracture initiation and propagation in highly deviated wells: A case study in Weizhou Offshore thinly sand-mud interbedded reservoirs

To clarify the effects of significant permeability heterogeneity and laminations on fracture initiation and propagation within the offshore Weizhou sand-mud thinly interbedded reservoirs, a series of laboratory true triaxial physical simulation experiments on hydraulic fracturing in highly deviated wells was conducted. Saturated full-diameter cores from the L-3 Formation of the Weizhou oilfield were used for the experiments. An innovative fracturing simulation wellbore suitable for highly deviated wells and a spiral fiber optic monitoring method were also designed. By comparing and analyzing the breakdown pressures and fracture morphologies under conditions of different permeability streaks (matrix, high-permeability, and low-permeability) and lithologies (muddy siltstone and shale), the aim of this study was to reveal the mechanisms by which permeability differences and sand-shale "lamination" structures affect fracture propagation. It was found that high permeability significantly lowered the fracture initiation pressure and induced shear failure in the near-wellbore zone, which led to more complex fracture morphologies. Compared to the bedding planes of shale, the sand-shale laminations were observed to act as a "dynamic process zone", causing fractures to exhibit a unique "stair-shaped" propagation pattern. This process continuously consumed hydraulic energy and increased the tortuosity of the fracture path. The results of this study provide an important theoretical basis and experimental reference for the fracturing design and fracture control in similarly complex reservoirs.