Zhenhua Han

Investigation of the Quasi-Brittle Failure of Alashan Granite Viewed from Laboratory Experiments and Grain-Based Discrete Element Modeling

Granite is a typical crystalline material, often used as a building material, but also a candidate host rock for the repository of high-level radioactive waste. The petrographic texture—including mineral constituents, grain shape, size, and distribution—controls the fracture initiation, propagation, and coalescence within granitic rocks. In this paper, experimental laboratory tests and numerical simulations of a grain-based approach in two-dimensional Particle Flow Code (PFC2D) were conducted on the mechanical strength and failure behavior of Alashan granite, in which the grain-like structure of granitic rock was considered. The microparameters for simulating Alashan granite were calibrated based on real laboratory strength values and strain-stress curves. The unconfined uniaxial compressive test and Brazilian indirect tensile test were performed using a grain-based approach to examine and discuss the influence of mineral grain size and distribution on the strength and patterns of microcracks in granitic rocks. The results show it is possible to reproduce the uniaxial compressive strength (UCS) and uniaxial tensile strength (UTS) of Alashan granite using the grain-based approach in PFC2D, and the average mineral size has a positive relationship with the UCS and UTS. During the modeling, most of the generated microcracks were tensile cracks. Moreover, the ratio of the different types of generated microcracks is related to the average grain size. When the average grain size in numerical models is increased, the ratio of the number of intragrain tensile cracks to the number of intergrain tensile cracks increases, and the UCS of rock samples also increases with this ratio. However, the variation in grain size distribution does not have a significant influence on the likelihood of generated microcracks.

Hydraulic fracturing process by using a modified two-dimensional particle flow code - method and validation

Hydraulic fracturing had been proved as a very useful tool for unconventional oil and gas development, where the fracturing fluid is injected into tight reservoirs under high pressure to enhance the permeability of rock mass. Although hydraulic fracturing theory, numerical modelling, and laboratory experiments develops fast, knowledge is still limited when the geological conditions are complex. This paper presents a numerical method - particle flow code (PFC) - and validates its power for hydraulic fracturing modelling in complex conditions. Firstly, the bonded particle method (BPM) and fluid-mechanical coupling mechanism are introduced; secondly, Darcys flow in circular particles is simulated; thirdly, a series of numerical simulations is carried out to validate its suitability for hydraulic fracturing modelling; finally, the laminated reservoir will be modelled by BPM. The modelling results show good agreement with classical analytical solution and laboratory test results, which demonstrates that the BPM is a useful and strong tool for understanding the fracturing behaviour of reservoir rocks.

Hydraulic fracturing process by using a modified two-dimensional particle flow code - case study

By using a modified PFC2D model, the authors conducted a series of simulations to examine the influences of in-situ stress ratio, fluid injection rate, anisotropy of laminated rocks, and perforation parameters on hydraulic fracturing process. The simulations indicated that: 1) larger in-situ stress ratio will induce smaller breakdown pressures and faster propagation, and hydraulic fractures will extend along the direction of the maximum principal stress or approach to this preferred path; 2) smaller difference of in-situ stresses or faster fluid injection rate is helpful for creation of complex fracture network; 3) weak layers are preferred locations and directions for fracture initiation and propagation in the laminated rocks; 4) hydraulic fractures initiate easily at the bottoms of perforation channels, and propagate generally along or approaching to the direction of maximum principal stress.