Hydraulic fracture has become one of the technical means for efficient development of oil and gas reservoirs, and the brittle rock damage characteristics are the key mechanical indicators for fracturing to form complex fracture networks. The current research on brittle rock characteristics is mainly focused on shale reservoirs, and there is a lack of systematic and in-depth understanding of the characteristics of anisotropic, strongly inhomogeneous conglomerates and their fine-scale damage mechanisms. Through drilling cores and indoor rock mechanics experiments, discrete particle simulation techniques are introduced to reproduce the microfracture expansion mechanism of deeply buried conglomerates, and the study shows that conglomerate brittleness is more sensitive to the surrounding pressure, particle size and volumetric block proportion. Based on the simulation results, the brittleness evaluation index of conglomerate formation is established, which has certain reference significance for understanding the brittle characteristics of deep conglomerate and field fracture design.
The geological conditions of MH block in Xinjiang oilfield in western China are extremely complex. The Permian Upper Urho Formation is a typical low-porosity and low-permeability reservoir. The formation of a large-scale complex artificial seam network around the well perimeter by volume fracturing technology is the key to the efficient development of this type of reservoir, and the brittle characteristics of the formation are the key mechanical indicators for the formation of a complex seam network by fracturing. It is generally believed that the stronger the brittleness of the rock, the better the effect of reservoir modification under fracturing. Most of the current research related to rock brittleness characteristics revolves around shale formations, so focusing on the brittleness characteristics of gravel formations is crucial to the evaluation of fracturing engineering desserts and the optimal design of fracturing schemes.
Brittleness is the property of a rock to fail when it undergoes very small deformation under stress. Many scholars have studied the mechanical properties, deformation laws and damage mechanisms of brittle rocks. Martin & Chandler (1994) summarized the failure characteristics within brittle rocks and classified four stages of the stress-strain curve during compressional rupture of rocks (see Figure 1). Hoek E. (2005) improved the study of the mechanical properties of brittle rocks by suggesting that the basic mechanical properties of brittle rocks depend mainly on the state of crack extension within the rock. Qin et al. (2020) suggested that conglomerates exhibit more significant heterogeneous properties than conventional brittle rocks due to poor particle sorting and large particle size differences. In order to gain a deeper understanding of the fine-scale mechanism of brittle rock damage, scholars have proposed and developed a variety of numerical simulation tools in recent years. The Particle Flow Code (PFC) method proposed by P. A. Cundall & O. D. L. Strack (1979) is particularly suitable for the failure of brittle rocks due to micro-cracks, since it takes into account the contact state between multiple types of particles and their changing characteristics. It has been applied to fundamental research fields such as particle dynamics and failure of different media.
