Low permeability of coalbed has always been a main bottleneck impeding the safety production of coal mines and the high-efficient gas recovery. A static fracturing technology, relied on hydraulic-driving modules to expand outwards for producing artificial fissures in coalbed, is proposed here for coalbed reservoir stimulation. The mechanical model of borehole fracturing is established to clarify the associated mechanisms for fracturing stimulation based on the elastic-plastic mechanics. The numerical results indicate when the fracturing load is over twice as much as in situ stress, the concentration of hoop stress around borehole would be released and the range of fracturing-induced fissures gradually extends with the rising load. While the lateral stress coefficient of strata rising from 1 to 3, the stress distribution around borehole evolves from a ring to a saddle shape, resulting in the horizontal extension of fissures in the early stage. According to underground monitoring, a significant improvement of coalbed permeability up to 5 times has been achieved in 16031 tailgate, in Guhanshan coalmine, China.
On account of a specific energy structure and a gigantic consumption, China has already been the world’s largest coal producer with the output of 3.41 billion tons in 2016, meanwhile suffering serious gas disasters [
However, most of the coalfields in China were formed in Permo-Carboniferous system, experiencing several large-scale tectogenesises, which brought about fragmented coal and mylonitic coal [
On the other hand, conventional methods of gas drainage face difficulties: long preextraction time, quick reduction in gas flowrate, and huge drilling project. The reservoir stimulation techniques are needed to enhance coalbed permeability, mainly including hydraulic fracturing [
A static fracturing technology, relied on hydraulic-driving modules to expand outwards for producing a large amount of artificial fissures in coalbed, was proposed here. Firstly, the fracturing model was established based on the elastic-plastic mechanics. Then, the effect of fracturing load on stress distribution and the initial fissure position were discussed by using FLAC3D software. Eventually, three groups of fracturing tests were conducted in 16031 tailgate tunnel, Guhanshan coalmine, to verify the permeability improvement of No. 2-1 coalbed.
The field trials of the static fracturing technology have been conducted in Guhanshan coalmine, located in the northeast of Jiaozuo coalfield. Nowadays, the coalmine is mainly recovering No. 2-1 coalbed, and the design production capacity is about 1.2 million tons of anthracite annually. The average thickness of No. 2-1 coalbed is approximately 2.8 m, with the dip angle of 12°∼17°. According to identification results over the years, the maximum of relative gas emission quantity reaches 33.32 m3/t and the absolute gas emission quantity is 75.59 m3/min. Undoubtedly, Guhanshan coalmine faces a high risk of coal and gas outburst both in driving and mining periods.
In order to extract gas gushed from No. 16031 working face, a single row of drainage boreholes was arranged in the range of 10∼680 m of the lower rib of No. 16031 tailgate. The aperture of borehole is perpendicular to the middle line of the tunnel. The depth of borehole is 70 m, and the initial drilling height is 1.2 m away from the floor within the deviation of ±10 cm. The angles of odd and even borehole are −2° and −3°, respectively. The borehole spaces are, respectively, 0.8 m and 1.6 m in the range of 10–74 m and of 74–680 m away from the open-off cut. Three groups of the static fracturing tests were implemented as shown in Figure
Location of 16031 tailgate and drilling layout.
A static fracturing equipment (Figure
Operation chart of static fracturing technology.
The static fracturing technology aims to relieve or even eliminate the hoop stress concentration around borehole to produce more gas flow channels by exerting a high-pressure load on the hole wall. Providing the surrounding coal around borehole is submitted to the homogenous elastic-plastic material, the borehole model should be regarded as a microcircular tunnel complied with the Mohr–Coulomb criterion. Since the axial length and the buried depth of borehole are much more than 20 times its diameter, the model is simplified as the plane strain model as shown in Figure
Mechanical model of borehole fracturing.
Providing the cohesion and the friction angle of coal are always constant and the lateral coefficient
If
The integration constant
Thus, the elastic-plastic solutions of stress can be expressed as follows:
When
Putting Mohr–Coulomb criterion into the equation of static equilibrium, we can also get the differential equation:
The solution of differential equation is expressed as follows:
The integration constant
Thus, the radial stress on the plastic zone can be written as
The hoop stress is expressed as follows:
The hoop stress on the plastic boundary can be determined with inputting condition:
Therefore, the radius of plastic zone is represented as follows:
When
When the stresses distribute nonuniformly (
Two cases (the azimuth
Firstly, providing
To satisfy the generation condition of tensile fissures around borehole, the hoop stress
While providing
The fracturing load
Accordingly, providing
The numerical model was established based on the geological and mining conditions of Guhanshan coalmine. According to the borehole histogram, rock strata were regarded as isotropic material without considering the influences of structural plane, interlayer, and groundwater. In the model, a borehole (
Numerical model.
To investigate the effects of fracturing load on stress distribution around borehole, the loads of 15∼30 MPa were, respectively, exerted on hole wall. The stress and plastic zone distribution under different loads are shown in Figures
Stress contour under fracturing loads of 15∼30 MPa.
Plastic zone distribution under the load of 15∼30 MPa.
Figure
Figure
In the field, the horizontal stress in strata is always larger than the vertical stress under the action of tectonic movement. Thus, the characteristics of stress distribution around borehole were discussed under different lateral coefficients as shown in Figure
Stress contour around borehole under different lateral coefficients.
Figure
In order to determine the difference of permeability coefficient of coalbed before and after fracturing, a cross-borehole was firstly drilled and sealed to measure the residual gas pressure of No. 2-1 coalbed. Then, an orifice flowrate was installed one day after the gas pressure gauge removed to measure the daily gas flowrate, and the measurement duration was recorded. The permeability coefficient can be evaluated based on the theory of unstable radial gas flow in coalbed as follows [
Figure
Changes of permeability coefficient of coalbed.
Drainage parameters (gas concentration, drainage pressure of the system, the diameter of drainage pipe, and the flow velocity of gas) of the three groups of measurement; boreholes were totally monitored for 84 days so as to determine the variation of gas flowrate before and after fracturing. Figure
Curves of gas flowrate and gas concentration.
For the traditional drainage method, the gas flow started at the formation of borehole and the results indicate clearly that the gas flowrate gradually reduced in a short period of 26 days after the beginning of gas drainage. However, by fracturing operation, a large amount of absorbed gas would be desorbed into free state with the improvement of the permeability of coalbed, resulting in a sharp increase in gas drainage concentration. The flowrate of mixed gas also increased owing to the fissure formation caused by fracturing, which demonstrates the significant improvement in efficiency of gas drainage by using the fracturing technology. According to the statistical analysis of each measurement borehole, after fracturing, the gas drainage concentration increased by 2.16 times on average, and the pure flowrate of gas recovery flow increased by 2.05 times.
A static fracturing technology to enhance the permeability of coalbed is proposed here for the high efficiency of gas recovery. The mechanism of borehole fracturing is clarified based on the mechanical analysis and numerical modeling. The results demonstrate the following (1) When the fracturing load is over twice as much as in situ stress, the concentration of hoop stress around borehole is released and there are more tensile fissures generated with a rising load, which results in the promotion in coal permeability. (2) When the lateral coefficient >1, tensile fissures are generated in the horizontal direction of borehole and vice versa. The stress distribution around borehole gradually evolves from a ring to the saddle shape with the lateral coefficient rising from 1∼3. (3) After the fracturing operation, the gas drainage concentration increased by 2.16 times on average, and the pure flowrate of gas recovery flow increased by 2.05 times.
In the future, more experimental effects and field applications are needed in order to understand the evolvement of fracturing-induced fissures and the characteristics of gas flow in coalbed, associated with different drainage periods by the static fracturing stimulation.
The research data used to support the findings of this study are included within the article. Request for more details should be made to the corresponding author.
The authors declare that they have no conflicts of interest.
We appreciate Guhanshan coalmine for providing experiment support for our study and are grateful to the China National Natural Science Foundation (Grant no. 51109076).
The graphical abstract: static fracturing system for ECBM and the effect on gas drainage.