Characteristics of fracture propagation in compact limestone formation by hydraulic fracturing in central Sichuan, China

Abstract

Compact limestone formations are significant and serve as enrichment petroleum sources in the central Sichuan basin. However, they are difficult to stimulate by hydraulic fracturing owing to their low porosity, low permeability, and high density. The fracture geometry is neither a bi-wing fracture found in a homogeneous sandstone formation nor a complex fracture network occurring in a shale formation with well-developed discontinuities and high brittleness. In this study, a series of laboratory stimulated experiments were conducted to investigate the fracture initiation and propagation behavior in compact limestone formation. Concurrently, the effects of the horizontal stress difference, pump rate, fracturing fluid viscosity, and completion method were also studied. The results revealed that a complex fracture network was formed with difficultly in a compact limestone formation. The fractures initiated from open-hole section or perforations to propagate in four forms, resulting in three categories of the main fracture geometry: transverse fracture, longitudinal fracture, and complex fracture. Complex fracture formations occurred under a small horizontal stress difference, whereas a simple fracture was induced by a large horizontal stress difference, similar to a transverse or longitudinal fracture. Moreover, the cement strength, development degree, and distribution position of the discontinuities had a significant effect on the fracture geometry. A low fluid viscosity or low pump rate was demonstrated to be an effective method for activating and opening the discontinuities, forming complex fractures under a small horizontal stress difference rather than under a large horizontal stress difference. However, the variable pump rate fracturing with a low-viscosity fluid could promote the interaction of the induced fracture and discontinuities, leading to a large stimulated reservoir volume under the large horizontal stress difference. In addition, perforated fracturing generated a more complex fracture at a higher fracture pressure than an open-hole fracturing. Furthermore, an acid fluid should be injected into limestone formation before the fracturing operation to reduce the rock strength, resulting in a low fracture pressure of limestone formation.