Abstract

This paper investigates the problem of hydraulic fracturing in the impermeable brittle medium. We prepared some rectangular granite specimens with single pre-exiting flaw (inclination angle of 30°). Through applying hydraulic pressure into the pre-exiting flaw, hydraulic fracturing test was performed on the specimen. The fracturing process was monitored and analysed using the digital image correlation method. The quantified global strain field proves that most part of the crack evolution during the fracturing are dominated by tensile effect. Before a visible crack appears at certain location, the strain at this point will increase sharply, which represents the essence of brittle failure. Combining with failure morphology, the quantified global strain field will contribute to improving the basic understanding of brittle failure. This study will provide a method for investigating the hydro-failure mechanism of granite from a microscopic point of view.

1 Introduction

The hydraulic fracturing process has been extensively used to enhance the production of oil and gas from underground reservoirs for many decades. Experimental studies both in the laboratory and in the field have been conducted to obtain a better understanding of the hydraulic fracturing characteristics (Yew and Weng 2015). Among these studies, how hydraulic fractures initiate, propagate and coalescence with each other is still not clear, because they mainly rely on post-testing observations and on indirect (usually using acoustic emissions) observation of the hydraulic fractures (Silva and Einstein 2018). To change this situation and meet the requirements in the laboratory tests, some researchers have improved the test equipment to observe directly the hydraulic fracturing process using high-speed camera and electron microscope (Silva and Einstein 2018; Silva et al. 2015). This test device makes it possible to investigate the real-time process of the hydraulic fracturing.

The digital image correlation (DIC) can accomplish noncontact deformation measurement which compares the undeformed image and the deformed image to track the relative displacement (Roux et al. 2009). It has been widely used to observe crack propagation and process zone development in laboratory rock tests. R.G. Jeffrey et al. (Jeffrey et al. 2015) measured the fracture path along with fracture and interface displacement in the two-dimensional fracturing of Eidsvold siltstone. C. Zhao et al. (Zhao et al. 2018; Zhao et al. 2018) observed and analysed the full-field strain and failure features of the rock-like materials under uniaxial compression using the DIC method. B.Q. Li et al. (Li and Einstein 2017) evaluated the photographic data of granite subjected four point bending with the DIC method and compared it with the acoustic emission results. H.Z. Xing et al. (Xing et al. 2018) studied the Full-field strain and strain-rate fields of rock materials under dynamic compression using the highspeed three-dimensional digital image correlation (3D-DIC) method. These studies have proved that the DIC method is suitable for measuring strain field of rock material. However, it has not yet been applied to the hydraulic fracturing of rock, in view of the fact that the equipment in traditional hydraulic fracturing experiments hinders the observation of deformation.

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