Abstract
The Park-Paulino-Roesler (PPR) cohesive zone model (CZM) for coal was established for analyzing mixed-mode I/II fractures using semicircular specimens under punch-through shear (PTS) and three-point bending (SCB) tests. In these methods, the main parameters of the fracture were obtained through SCB tests and PTS tests. And according to the experimental results, the coal specimens show obvious characteristics of ductile fracture under mode I and II loading. Moreover, hydraulic and supercritical carbon dioxide (ScCO2) fracture tests were conducted, and accordingly, it was found that the crack initiation pressure of coal specimens for hydraulic fracturing is 17.76 MPa, about 1.59 times that driven by ScCO2. And the crack initiation time of coal with ScCO2 fracturing is 123.73 s, which is 1.58 times that for hydraulic fracturing. A macrocrack eventually formed in the coal specimen due to the hydraulic drive, which penetrated through the entire specimen. Yet, there was no crack penetrating the whole fracture specimen and several widely distributed secondary cracks in the fractured coal specimens by ScCO2. Furthermore, zero-thickness pore pressure cohesive elements were utilized to investigate multicrack propagation in coals undergoing hydraulic and ScCO2 fracturing. The constitutive relationships of the established PPR CZM were introduced into the cohesive elements. The obtained results are consistent with the hydraulic and ScCO2 fracturing experiment results for the coal specimens. This indicates that the established PPR CZMs can accurately represent the crack propagation behavior in coals for hydraulic and ScCO2 fracturing.
Highlights
As an essential type of clean energy, the exploitation of coalbed methane (CBM) is of significant importance to increase the supply of clean energy, thereby decreasing concerns about greenhouse gases and realizing safe coal mining [1, 2]
The constitutive relationships of the established PPR cohesive zone model (CZM) were introduced into the pore pressure cohesive elements to simulate crack growth in coals caused by hydraulic and ScCO2 fracturing
Hydraulic and ScCO2 fracturing experiments on the coal specimens were performed, and the numerical simulation outcomes were compared with the corresponding experimental results
Summary
As an essential type of clean energy, the exploitation of coalbed methane (CBM) is of significant importance to increase the supply of clean energy, thereby decreasing concerns about greenhouse gases and realizing safe coal mining [1, 2]. The CZM inspired by the studies of Barenblatt [26], Dugdale [27], and Hillerborg et al [28] has been used with success to represent the crack propagation behavior in nonlinear FPZ of ductile materials In this theory, the FPZ is simplified hypothetically to a discrete line or plane corresponding to either a two-dimensional or three-dimensional case, respectively, in which the hypothetical cohesive stress causes the virtual crack to close (see Figure 1). An effective solution for this problem is to apply potential-based models to utilize the initial derivative of the fracture potential energy function [39] This scheme is based on the cohesive stress over the fractured surfaces, while the second derivative reflects the constitutive association. The established model is applied to perform the crack propagation simulation in hydraulic fracturing and ScCO2 fracturing in coals. Comparisons between the test results of hydraulic fracturing and ScCO2 fracturing in coals and the obtained results were utilized to evaluate the performance of the proposed model
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