The mechanical parameters in complex lithological reservoirs, such as the development degree of natural fractures and fracability, exhibit considerable differences, which result in an unclear fracture propagation mechanism that poses difficulties to fracturing treatment design. A fracability evaluation model that is suitable for different lithological reservoirs was established through a case study of Tamulangou Formation in Hailar Basin, China, and on the basis of rock mechanics test results, brittleness, fracture toughness, horizontal stress difference, and natural fracture development degree. Then, the effect of pumping rate and fluid viscosity on the propagation of multiple fractures was studied using the true tri-axial fracturing simulation experimental system, and the geometry of fracture initiation was observed through the distribution of tracers. Results showed that (1) Tuff had moderate fracability (0.35≤FT≤0.65) and was sensitive to pumping rate and fluid viscosity. (2) Siltstone had moderate fracability (0.40≤FS≤0.68) and was sensitive to changes in pumping rate and fluid viscosity. (3) Breccia had poor fracability (FB<0.45) and was not sensitive to changes in pumping rate and fluid viscosity. The findings of this study can provide guidance for the fracturing treatment design of complex lithological reservoirs.

1. Introduction

Complex lithological reservoirs have the characteristics of large lithology differences, complex structures, significant anisotropy of reservoir physical properties, and uneven development of natural fractures. The physical and mechanical heterogeneities of complex lithological reservoirs mean that different lithological rocks exhibit dissimilar fracability [1-4]. As the reservoir fracability vary with lithological layers or regions, hydraulic fracture propagation behaviors may be different from conventional reservoirs, tight shale and sandstone reservoirs [5-9]. At present, the existing fracability evaluation models are mainly applied to conventional sandstone and shale reservoirs, which may not suitable for complex lithological reservoirs [10-11]. Therefore, it is necessary to establish a fracability evaluation model for complex lithological reservoirs, and study the hydraulic fracture propagation mechanism of different lithological reservoirs, so as to provide guidance for the fracturing treatment design of lithological reservoirs [12].

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