Laboratory study of the factors affecting hydraulic fracturing effect for inter-salt oil shale layers, Qianjiang Depression, China

Abstract

This study aims to investigate the potential factors affecting hydraulic fracturing of inter-salt oil shale reservoirs in the Qianjiang Depression, China. Using the inter-salt shale samples, the re-crystallization seepage tests, rock mechanical tests under high temperature and pressure, salt rock creep tests, and direct shear tests were conducted. The testing results suggest several major factors that affect hydraulic fracturing effects in the end. First, the seepage of reservoir and fracturing fluid through hydraulic fractures leads to salt dissolution and crystallization, reducing the effective seepage area of fractures. Second, the salt crystal may block the pore throats or micro fractures after brine invades the shale, decreasing the overall permeability. Third, the low strength and obvious plasticity of inter-salt shale and the strong creep characteristics of salt rock raise difficulties for proppant to effectively support fracture walls, thereby sharply narrowing the hydraulic fracture width. Lastly, the weak interfaces (bedding planes and lithology interfaces) in inter-salt oil shale reservoirs restrict the height of hydraulic fractures, resulting in the disconnection of seepage channels between multiple inter-salt shale reservoirs. Thus, several factors together reduce reservoir permeability, weaken the fluid flow capacity in the fracture, narrow the fracture width, and limit the effective stimulation volume, resulting in weaken the effect hydraulic fracturing.