Effects of laminated structure on hydraulic fracture propagation in shale

Concrete samples with properties similar to the continental shale core of Well Yanke 1 in the Ordos Basin in the Shaanxi province, China, were made. The control effects of horizontal principal stress difference, the angle between perforated casing and lamina, the lamina thickness and space on hydraulic fracture propagation were modeled on a real tri-axial hydraulic fracturing experiment system, based on the design principle of four factors-three levels of orthogonal experiment. When the horizontal principal stress difference is small, the main fracture surface is parallel to the laminae strike, and the primary fractures will propagate as follows: propagate through the laminae, then deflect significantly at the surface of the laminae, continue along the laminae surface and finally change direction and propagate again through the laminae. When the horizontal principal stress difference is big, the main fracture surface is perpendicular to the laminae strike and the primary fractures will propagate as follows: run through the laminae, change their propagation direction dramatically on the laminar surface, and then run through the laminae. The smaller the difference of the horizontal principal stress, the bigger the angle of fracture deflection, and the better the fracturing result will be; the smaller the angle between perforated casing and laminae, the better the fracturing result will be; when the thickness of shale laminae is moderate, there will be more branching fractures and the fracturing result is better; tuff layers with smaller lamina thickness have better hydraulic fracturing effect. The hydraulic fracturing effect is most sensitive to horizontal principal stress difference, followed by the angle between perforated well casing and laminae.