Role of natural fractures characteristics on the performance of hydraulic fracturing for deep energy extraction using discrete fracture network (DFN)
Introduction
As one of the significant energy sources, coal plays a critical role in industry and brings enormous economic benefits to society [41]. However, underground mining commonly takes place some hundreds or up to a thousand meters below the surface and the stresses in the rock cannot be released due to the low permeability, leading to many hazards in underground mines. Among these disasters, the rock bursts and coal and gas outbursts are the most dangerous and can be disastrous [12], [25]. These rock brusts and similar events not only damage the working panels, but also endanger the life safety of underground workers. Recently, hydraulic fracturing has been widely used in underground mining to ensure mine safety [14]. By creating new fractures, hydraulic fracturing could destroy the coal and rock mass, weaken the strength, and release stress, thereby reducing the risk of underground disasters [30]. A different although tangentially related issue is coal combustion. Buring coal releases many gases and causes both air pollution and environmental damage. Consequently, the clean energies, such as geothermal source, and CBM, have become the focus of public discussion [2], [18], [21], [35]. However, due to the rock mass permeability, energy extraction faces many challenges and extraction efficiency is relatively low. Therefore, some advanced technologies, such as laser, acid fracturing, microwave fracturing, and in-situ leaching [5], [11], [16], [22], [24], [26], [27], have been applied to improve the extraction efficiency. Among these techniques, hydraulic fracturing is widely used to enhance coal and rock permeability [13], [23], [29], [36]. Although hydraulic fracturing has become a conventional technique to stimulate production, its effectiveness is lower than initially thought. Influenced by many factors, primarily the pre-existing natural fractures, the newly created fractures are very complex, and their growth directions during fracturing are uncontrolled. This results in low extraction efficiency.
Generally, in a dual-porosity model, the coal and rock mass are modeled as being composed of natural fractures and matrix. However, the pre-existing fractures are the main fluid pathways and play a critical role in the efficiency of hydraulic fracturing and the matrix permeability is deemed to be negligible in most cases [15], [28], [34]. Instead of creating new fissures, the fluid has a prior to flow through the pre-existing natural fractures, which can affect the fracture growth and therefore affect the model’s predictions concerning the hydraulic fracturing effectiveness. Therefore, a complex model combining the DFNs is critical for analyzing the hydraulic fracturing effectiveness. Natural fractures have numerous characteristics, such as aperture, persistence, density, and orientation, which could have different effects on hydraulic fracturing. Therefore, how to represent the distribution of natural fractures in coal and rock mass is the core. At present, the finite element method (FEM) and DEM are the common methods used to model the spatial distribution of natural fractures, as shown in Fig. 1. [1], [9], [37]. Because natural fractures have discrete characteristics of natural fractures, DEM offers an ideal tool to reconstruct the spatial distribution of pre-existing fractures, and handle the fluid–solid coupling [40]. However, previous research on hydraulic fracturing has mainly focued on mechanism, permeability enhancement, and stimulated production; the natural fractures have commonly been ignored. Worse still, reports about the influence of natural fractures characteristics on hydraulic fracturing effectiveness are rare.
In this paper, the DFN model was reconstructed to study the hydraulic fracturing efficiency on energy extraction performance using a distinct element method. Moreover, the numerical model was calibratedwith observed data from Cooper basin, South Australia. The model was used to evaluate how changes in the natural fracture’s properties affect hydraulic fracturing efficiency. This research will enhance the hydraulic fracturing community’s awareness of how much hydraulic fracturing’s effectiveness is influenced by original fractures.
Section snippets
The generation of natural fracture using DFN model
In general, the permeability of coal and rock mass mainly depends on the natural fractures, because, compared with the fracture permeability, the permeability of matrix can be ignored. Therefore, the distribution and characteristics of natural fractures are the focus of this study. Fig. 2 shows the procedure for hydraulic fracturing simulation in strata with pre-existing natural fractures. To reconstruct the distribution of fractures, basic natural fracture parameters, such as aperture,
The fluid flow through the natural fractures
Generally, the equations governing fluid flow through the fractured coal and rock mass are the Navier-Stokes equations, which can be described as follows [3], [17].where is the fluid density, is the velocity of fluid flow, is the pressure gradient, F is the body force, is dynamic viscosity, is the laplace operator.
In a double-medium model, the rock mass consists of natural fractures and matrix. The cracks are the main channels of fluid flow, which are discretely
Baseline test of hydraulic fracturing using DFN
To study water flow through fractured coal and rock mass, the fluid characteristics obtained from a hydraulic fracturing experiment in Cooper basin were used in the DFNs models, which are listed in Table 2. The model’s outer boundary is fixed by setting the velocity to zero. These fluid boundaries allow the hydraulic fracturing in the naturally fractured model to be analyzed. For that analysis, an injection well with a radius of 0.1 m and length of 0.3 m is assigned to the centre of model,
The sensitivity analysis of pre-existing fracture network characteristics on hydraulic fracturing efficiency
Based on the above analysis, it is apparent that natural fractures have numerious attributes relevant to hydraulic fracturing, such as aperture, persistence, orientation, and density. More importantly, these characteristics have different contributions to hydraulic fracturing. Therefore, to analyze how much the dependences of hydraulic fracturing effectiveness on the different natural fracture characteristics, a series of sensitive studies were carried out. By changing the characteristics of
Effect of fracture aperture both in pre-existing fractures and hydraulic fracture
Fig. 7 shows the variation of hydraulic fracturing influence zone with the increase of natural fracutres aperture. Analyzing the data indicates that hydraulic fracturing efficiency is decreased with the increase of the pre-existing fractures aperture, which is characterized by the height and length. The main reason for this phenomenon is that the flow channels are narrow when the natural fracture aperture is small. As a consequence, few water is transferred to the production well, and the
Conclusions
Due to the low permeability of coal and rock mass at depth, clean energy cannot be extracted efficiently from underground mines. In order to stimulate production, hydraulic fracturing is widely used. However, the effectiveness of hydraulic fracturing is not as good as is originally expected. The main reason lies in that the natural fractures are existed in the fractured rock mass, and the fluid flows preferentially through the existing channels rather than creating new ones. Meanwhile, natural
Declaration of Competing Interest
The authors declare that the manuscript has no conflict of interest.
Acknowledgments
This work was financially supported by the Fundamental Research Funds for the Central Universities (2017CXNL01). One of the authors wants to express his deepest appreciation to China Scholarship Council (CSC) for the award of Chinese Government Scholarship. The authors would also like to acknowledge the Mining Education Australia (MEA), Collaborative Research Grant Scheme (RES-SE-MEE-PM-59042).
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