Abstract:

The Yanchang Formation in the Ordos Basin has deposited a set of mudshales and fine-grained sandy rocks, rich in shale oil resources, with an estimated resource potential exceeding billions of tons. However, shale oil reservoirs exhibit poor mobility, shallow burial depths, the development of bedding, fractures, and faults in horizontal sections, and unknown fracture propagation patterns, making volumetric fracturing challenging. To address this, cement-encased cores of full-diameter tight sandstone-mudstone and shale from the sublayer in the seventh member of the Yanchang Formation (Chang 7) were used in actual triaxial hydraulic fracturing physical model experiments. These experiments revealed hydraulic fracture morphologies and the fracture propagation mechanism under weak stress fields in shale oil reservoirs. The experiments found that shale oil reservoirs had tight layered structures and weak bonding between rock grains, causing fracturing fluid to easily infiltrate along bedding planes. When the difference between vertical stress and minimum horizontal principal stress was less than 2 MPa, hydraulic fractures predominantly formed horizontal fractures, with the fluid primarily infiltrating along bedding planes or horizontal natural fractures. When this stress difference increased to 7 MPa, vertical cross-layer fractures appeared, forming localized steps that eventually became captured by weakly bonded bedding planes, propagating horizontally along the layers. For fracturing operations, regions with a larger difference between vertical stress and minimum horizontal principal stress, such as wellheads at hilltops, are preferred. This facilitates vertical fracture propagation, improves volumetric fracturing effectiveness in reservoirs, enhances shale oil production, and increases economic benefits.

Key words: interbedded shale oil reservoir, hydraulic fracturing experiment, maximum horizontal principal stress, hydraulic fracture morphology, natural fractures, numerical simulation