Yuwei Li

TL;DR: In this article, the authors proposed an evaluation method of brittleness index based on energy method, established the evaluation model based on three types of damage constitutive relations, and finally analyzed the effects of the damage variable of peak strain of different types of different rocks on brittiness.

Abstract: Brittleness is one of the most important mechanical properties of rock: it plays a significant role in evaluating the risk of rock bursts and in analysis of borehole-wall stability during shale gas development. Brittleness is also a critical parameter in the design of hydraulic fracturing. However, there is still no widely accepted definition of the concept of brittleness in rock mechanics. Although many criteria have been proposed to characterize rock brittleness, their applicability and reliability have yet to be verified. In this paper, the brittleness of rock under compression is defined as the ability of a rock to accumulate elastic energy during the pre-peak stage and to self-sustain fracture propagation in the post-peak stage. This ability is related to three types of energy: fracture energy, post-peak released energy and pre-peak dissipation energy. New brittleness evaluation indices B 1 and B 2 are proposed based on the stress–strain curve from the viewpoint of energy. The new indices can describe the entire transition of rock from absolute plasticity to absolute brittleness. In addition, the brittle characteristics reflected by other brittleness indices can be described, and the calculation results of B 1 and B 2 are continuous and monotonic. Triaxial compression tests on different types of rock were carried out under different confining pressures. Based on B 1 and B 2, the brittleness of different rocks shows different trends with rising confining pressure. The brittleness of red sandstone decreases with increasing confining pressure, whereas for black shale it initially increases and then decreases in a certain range of confining pressure. Granite displays a constant increasing trend. The brittleness anisotropy of black shale is discussed. The smaller the angle between the loading direction and the bedding plane, the greater the brittleness. The calculation B 1 and B 2 requires experimental data, and the values of these two indices represent only relative brittleness under certain conditions. In field operations, both the relative brittleness and the brittleness obtained from seismic data or mineral composition should be considered to gain a more comprehensive knowledge of the brittleness of rock material.

TL;DR: In this article, a theoretical evaluation model of coal brittleness has been established based on the statistical damage theory and the energy evolution law of rock failure process, the calculated results were in good agreement with the experimental results, which proved that the theoretical analysis method of coal BRittleness proposed in this paper can be used to analyze the applicability of coal seam fracturing.

TL;DR: In this article, a new index for evaluating coal brittleness was established from the perspective of energy evolution during coal failure, and the applicability of the new index and the influence of the confining pressure and cleat orientation on the coal brittleness were analyzed.

TL;DR: In this article, a comprehensive and quantitative analysis of Nigerian oil and gas project performance from technical and non-technical perspectives with data from 65 projects was carried out, and a set of recommended practices was developed based on the risk level to assist project planners, international companies, investors, and policymakers to mitigate risks of developing oil-and gas projects.