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In order to explore the evolution law and mathematical characterization method of natural fissure structure of coal under uniaxial loading, a 3D numerical model was reconstructed based on computed tomography (CT) scanning experiment to accurately describe natural fissure structure, and the uniaxial compression numerical simulation of coal was carried out. The length, opening, fissure degree and fractal dimension were introduced to characterize the spatial morphology of fissure, and the variation of characterization parameters with increasing load was analyzed. The results reveal the evolution characteristics of fissure structure of coal under uniaxial loading. The study shows that the coal is mostly composed of small-scale fissure, and the fissure evolution can be divided into two stages during the loading process. In the first stage, namely the initial stage of loading, the fissure is compacted and closed. Compared with the non loading, the upper limit of fissure length decreases by -1-1%, the fissure degree decreases by 35%, and the fractal dimension decreases by 22%. In the second stage, fissure in the coal begin to develop with the increase of load. At peak load, the length increases by 10% on average, the distribution frequency of the minimum interval of fissure length increases by 12%, and the distribution frequency of the minimum interval of fissure opening increases by 50%. The research results provide a theoretical basis for revealing the laws of coal fissure propagation under loading and effectively implementing hydraulic fracturing technology. [ABSTRACT FROM AUTHOR]

Summary: The heterogeneity of tight reservoirs significantly influences hydrofracturing behaviours, such as crack morphology, type, initiation, propagation, and distribution. The accurate characterisation of the influencing mechanisms has become a pivotal issue in enhancing the fracturing stimulation of tight reservoirs, as well as in the prediction of tight oil and gas production. In this study, the hydrofracturing behaviours of heterogeneous glutenites and their influencing mechanisms were numerically investigated based on bonded particle models (BPMs). The geometry and mechanical properties of the natural glutenites were obtained using microfocus computed tomography (CT) and triaxial tests and were used to construct heterogeneous BPMs. The hydrofracturing behaviours of the heterogeneous BPMs under various in‐situ stresses were analysed with hydro‐mechanical coupling effects considered and compared with those of homogeneous BPMs under the same conditions. The numerical results show that gravels in heterogeneous glutenites inhibit crack propagation. The shear cracks that appear in the initial stage of crack development subsequently propagate and distribute around the injection hole, and there are fewer hydraulic shear cracks than tensile cracks. The crack morphologies of BPMs are found to be consistent with the experimental forms. The numerical simulation provides a way to understand the mechanisms that govern the hydrofracturing crack types and propagation of heterogeneous reservoirs. [ABSTRACT FROM AUTHOR]

Unconventional natural gas reservoirs usually have composite or heterogeneous microstructures, and heterogeneity has significant influence on the initiation and propagation of hydrofracturing cracks. Numerical simulation has advantages over in situ or experimental studies in the examination of the integrate hydrofracturing process. Many numerical modelling studies have been conducted to investigate the patterns of underground hydraulic fracturing. Unfortunately, few models have adequately simulated the 3D dynamic hydraulic fracturing process while maintaining accurate heterogeneous structures. This study adopted numerical simulation to investigate the processes of the initiation and the propagation of cracks in a heterogeneous material based on the CDEM algorithm, which couples finite and discrete element methods. A numerical model was used to represent the actual heterogeneous structure of a physical specimen. The initiation position and process of propagation of cracks influenced by geostress differences and heterogeneity are discussed. The efficiency of the simulation work was verified by the 3D reconstruction models in terms of the experimental results. The results indicated that material heterogeneity has considerable effect on crack initiation, but that crack propagation is controlled primarily by the geostress ratio. [ABSTRACT FROM AUTHOR]

In this study, the hydraulic fracture propagation mechanism in low-permeability layered rocks was investigated using the true triaxial hydraulic fracturing test system and the Continuum-based Discrete Element Method. Firstly, the mechanical properties of natural cores in reservoirs and interlayers were determined using uniaxial and triaxial testing machines. Secondly, the Kaiser stress effect method was employed to calculate the in-situ stress. To characterize the thickness distribution characteristics of reservoirs/interlayers in layered rocks, the concept of reservoir thickness ratio k δ was proposed. The influence law of k δ on the fracture pressure P f of layered rocks was explored. The fracture elongation r was proposed to describe the extension state of fractures in reservoirs and interlayers, and the effect of k δ on fracture elongation r were analyzed. The spatial morphology and geometrical characteristics of fractures were characterized based on the 3D fractal dimension and fracture opening. The 3D damage variable D V was defined to reveal the damage evolution law of layered rocks during fracture propagation. The research results reveal the fracture propagation mechanism in hydraulic fracturing of layered rocks and provide theoretical guidance for the effective implementation of hydraulic fracturing technology. This study is important for the efficient reconstruction of low-permeability reservoirs with layered media and improving oil and gas recovery. • Reservoir thickness ratio k δ charactering reservoir/interlayer thickness proposed. • 3D reconstruction models of fractures spatial morphology established. • Influence of k δ on the fracture propagation behaviors of layered rock explored. • Spatial morphology and geometrical characteristics of fractures quantified. • 3D damage variable and damage evolution law during fracture propagation obtained. [ABSTRACT FROM AUTHOR]

A quantification method for evaluation of the fracability of shale is essential for optimizing hydraulic fracturing of shale gas reservoirs and enhancing shale gas recovery. To quantitatively evaluate the fracability, seven sets of shale cores are drilled from the reservoirs at different depths of an oilfield in the east of China. The influences of six fracability-related mechanical and physical characteristics of shale, i.e. brittleness, brittle mineral content, clay mineral content, cohesion, angle of internal friction, and unconfined compressive strength, are analyzed. A mathematical model taking account of significance of the influencing factors is proposed based on analytic hierarchy process (AHP) to evaluate the fracability according to their different effects on shale fracability. The analysis indicates that the fracability decreases with the increase of reservoir depths. The hydraulic fracturing tests of the shale cores are conducted to verify the accuracy of the quantification method. The fractal geometry is used to characterize the fracture degree of the shale. It is shown that a larger fractal dimension of the fracture network corresponds to a better fracability of shale. The more complex the fractures are, the larger the fracability of shale is. The experimental data coincide with the results of the proposed evaluation model. [ABSTRACT FROM AUTHOR]

Tight multilayered hydrocarbon reservoirs are typical in unconventional oil and gas extraction, in which widespread bedded interfaces have crucial effects on hydraulic fracturing propagation behavior. However, because of the complex natural properties of bedded interfaces in natural layers, such as geometric structure, distribution, strength, and contact conditions, their effects on fracture propagation and governing mechanisms are not well understood. In particular, traditional theoretical models are not suitable for characterizing the propagation behavior of hydraulic fractures in multilayered reservoirs and conventional numerical methods cannot effectively simulate the interactions between hydraulic fractures and bedded interfaces. In this study, the adaptive finite element-discrete element method was introduced to probe the effects of bedded interfaces on hydraulic fracture propagation. Different numerical models of five deviation angles of representative bedded interfaces were established based on the geometric and physical parameters of natural multilayered reservoir rocks. Some novel techniques (local remeshing near the fracture tips to guarantee accurate fracture propagation path, mesh coarsening to improve the computation efficiency, characterization technology for bedded interfaces and contacts between layers, and hydro-mechanical coupling between fluid flow in fracture network and porous rock matrix in the fracturing process) were comprehensively utilized. Furthermore, the fracturing-induced microseismic damaged and contact slip events were identified based on the computed moment tensors to detect the interactions between the hydraulic fractures and bedded interfaces. The results of the cases with single and multiple perforations in horizontal wells show that the bedded interfaces disturb the stress continuity, reselect propagation direction of hydraulic fractures in multiple perforations, induce hydraulic fracture growth, and promote the occurrences of fracturing-induced damaged and contact slip events. The offset and deflection of hydraulic fractures increase with increasing deviation angles between hydraulic fractures and bedded interfaces and the offset vanishes when the deviation angle is 0°. • Adaptive finite element-discrete element method was proposed. • The effects of bedded interfaces on hydraulic fracture propagation were studied. • Models of bedded interfaces of multilayered reservoir rocks were developed. • Bedded interfaces disturb the stress continuity and provoke fracture growth. • Bedded interfaces promote fracturing-induced microseismic events. [ABSTRACT FROM AUTHOR]