Study on fracture propagation behavior in ultra-heavy oil reservoirs based on true triaxial experiments

As the ultra-heavy oil reservoirs developed by steam assisted gravity drainage (SAGD) in the Fengcheng oilfield, Xinjiang have problems such as huge steam usage, long preheating period, low production, and inaccessible reserve in local parts. Based on the rock mechanics and porosity/permeability characteristics of heavy oil reservoir and interlayer, a series of true triaxial experiments and CT tests considering the fracturing fluid injection rate, viscosity, perforation density and location of fracture initiation were conducted to disclose the propagation behavior of micro- and macro-fractures in the reservoirs and mudstone interlayers. These experiments show that fracturing in the heavy oil reservoirs only generates microfractures that cannot break the interlayer. In contrast, when fracturing in the interlayer, the higher the injection rate (greater than 0.6 m(3)/min), the lower the viscosity, the easier it is to form macro-fractures in the interlayers, and the further the fractures will propagate into the reservoirs. Also, increasing perforation density tends to create complex macro-fracture network in the interbedded reservoirs and mudstone interlayers. The findings of this study can provide scientific guidance for the selection of fracturing layer and the optimization of parameters in the interlayer fracturing of heavy oil reservoirs.