Numerical Study on the Formation of Shear Fracture Network

2017

Energies

Self Cite

In this paper, configurations of pre-existing fractures in cubic rock blocks were investigated and reconstructed for the modeling of experimental hydraulic fracturing. The fluid-rock coupling process of hydraulic fracturing was simulated based on the displacement discontinuities method. The numerical model was validated against the related laboratory experiments. The stimulated fracture configurations under different conditions can be clearly shown using the validated numerical model. First, a dominated fracture along the maximum principle stress direction is always formed when the stress difference is large enough. Second, there are less reopened pre-existing fractures, more newly formed fractures and less shear fractures with the increase of the cohesion value of pre-existing fractures. Third, the length of the stimulated shear fracture decreases rapidly with the increase of the friction coefficient, while the length of the tensile fracture has no correlation to the fiction coefficient. Finally, the increase of the fluid injection rate is favorable to the formation of a fracture network. The unfavorable effects of the large stress difference and the large cohesion of pre-existing fractures can be partly suppressed by an increase of the injection rate in the hydraulic fracturing treatment. The results of this paper are useful for understanding fracture propagation behaviors during the hydraulic fracturing of shale reservoirs with pre-existing fractures.

Section: Methodsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Numerical Analysis on the Formation of Fracture Network during the Hydraulic Fracturing of Shale with Pre-Existing Fractures

2017

Energies

Self Cite

“…Methods that have been employed to increase the permeability of coal seams include dense boreholes, deep-hole blasting, hydraulic flushing, hydraulic slotting, and hydraulic fracturing [7][8][9][10]; however, these technologies have drawbacks. In particular, the range of coal-seam permeability enhancement is small and uneven, and the construction cost is high.…”

Section: Introductionmentioning

confidence: 99%

Structural Optimization of Slotting Nozzle Used to Improve Coal-Seam Permeability

Zhong

et al. 2020

Applied Sciences

Slotting directional hydraulic fracturing is a new method for improving permeability of a coal seam in underground coal mines that can solve problems related to non-uniform permeability enhancement in the seam. A slotting nozzle is the key to this technology: its performance determines the length and stability of the slotted hole. In this study, computational fluid dynamics was used to study the effects of stable segment length L, convergent angle θ, and straight segment length l on the performance of slotting nozzles. The results showed that the order of parameters affecting nozzle performance is: θ > L > l. When the total length of the slotting nozzle was fixed, the dynamic pressure gradually decreased with an increase of L and the rate of decrease slowed down. With an increase of θ, dynamic pressure increased quadratically and the rate of change gradually decreased. With an increase of l, the dynamic pressure decreased quadratically and the rate of change gradually increased. A slotting nozzle (L = 0 mm, θ = 30°, l = 9 mm) was manufactured and measurements of flow coefficient, water jet morphology, and a slotting experiment were carried out. The experimental results showed that the flow coefficient, water jet convergence, and slotting depth of the optimized slotting nozzle were obviously higher than those of the original design, which proved the validity of using this numerical method to optimize the slotting nozzle structure.

“…One of the advantages of DDM is the precise solving of the fracture aperture, which is required by the investigation of permeability, as fracture conductance is sensitively related to the fracture aperture. The details of the model have been introduced in our previous published papers [24][25][26]. For the sake of completeness, the model is briefly introduced below.…”

Section: Hydraulic Fracturingmentioning

confidence: 99%

Numerical Study on the Permeability of the Hydraulic-Stimulated Fracture Network in Naturally-Fractured Shale Gas Reservoirs

2016

Water

Self Cite

Abstract:As hydraulic fracturing is a fluid-rock coupling process, the permeability of the hydraulic-stimulated fracture network in the initial stage has great effects on the propagation of the hydraulic fracture network in the following stages. In this work, the permeability of the hydraulic-stimulated fracture network in shale gas reservoirs is investigated by a newly-proposed model based on the displacement discontinuity method. The permeability of the fracture network relies heavily on fracture apertures, which can be calculated with high precision by the displacement discontinuity method. The hydraulic fracturing processes are simulated based on the natural fracture networks reconstructed from the shale samples in the Longmaxi formation of China. The flow fields are simulated and the permeability is calculated based on the fracture configurations and fracture apertures after hydraulic fracturing treatment. It is found that the anisotropy of the permeability is very strong, and the permeability curves have similar shapes. Therefore, a fitting equation of the permeability curve is given for convenient use in the future. The permeability curves under different fluid pressures and crustal stress directions are obtained. The results show that the permeability anisotropy is stronger when the fluid pressure is higher. Moreover, the permeability anisotropy reaches the minimum value when the maximum principle stress direction is perpendicular to the main natural fracture direction. The investigation on the permeability is useful for answering how the reservoirs are hydraulically stimulated and is useful for predicting the propagation behaviors of the hydraulic fracture network in shale gas reservoirs.