The effect of pulse frequency on the fracture extension during hydraulic fracturing

Coal and gas outburst is one of the main gas hazards in coal mines, and pulse hydraulic fracturing is a new technique for preventing and controlling gas hazards in coal mines and is presented based on conventional hydraulic fracturing. S2107 is a coal seam with high gas content and coal and gas outburst hazard but with low gas concentrations when extracted. To safely drive air tunnels into S2107, the pulse hydraulic fracturing technique is used on cross-measure boreholes placed from a high-level roadway to the driving roadway to improve the effects of gas drainage and ensure safety during production. The mechanism of coal fracturing via pulsed pressure is analyzed. A variable frequency method is used to strengthen the fatigue damage effect, and the initial pressure is estimated based on previous studies. The layouts of fracturing borehole and guide borehole are designed according to the layer relation, and a new hole sealing method is developed. The results indicate the pressure changes during pulse hydraulic fracturing reflect crack extension, and it is conducive to extending and connecting cracks under pulse pressures. The initial pressure during pulse hydraulic fracturing is negatively related to the water volume and fracturing time but cannot directly influence the fracturing radius. Moreover, this pressure is 30–43 % less than the calculated value. After this application, the gas desorption index, K
 1, at the driving workface decreases to below the critical value. The gas drainage volume of fracturing boreholes and their guide boreholes increases relative to normal boreholes by 3.32-fold and 3.07-fold, respectively. The new technique is promising for preventing and controlling gas hazards in the future.

A new technique for preventing and controlling coal and gas outburst hazard with pulse hydraulic fracturing: a case study in Yuwu coal mine, China

Experimental study of pulsating water pressure propagation in CBM reservoirs during pulse hydraulic fracturing

The reservoir permeability dominates the transport of gas and water in coal seam. However, coal seams rich in gas usually contain various pores and fractures blocked by a large amount of minerals, which leads to an ultra-low permeability and gas extraction rate, and thus an increase of drilling workload. We first propose a thermo-hydro-mechanical-chemical coupled model (THMC) for the acid fracturing enhanced coalbed methane recovery (AF-ECBM). Then, this model is applied to simulate the variation of key parameters during AF-ECBM using a 2D geometry. The effect of different extraction schedules are comparatively analyzed to give an insight into these complex coupling responses in coal seam. Result confirms that the AF-ECBM is an effective way to increase the reservoir permeability and improve the gas production using the proposed model. The range of permeability increment zone increases most dramatically in the way of acid fracturing, followed by none-acid fracturing and acidizing over time. The gas production in order is: acid fracturing (AF-ECBM) > fracturing (F-ECBM) > acidification (A-ECBM)> direct extraction (D-CBM).

A Thermo-Hydro-Mechanical-Chemical Coupling Model and Its Application in Acid Fracturing Enhanced Coalbed Methane Recovery Simulation

Experimental investigation on the characteristics of fractures initiation and propagation for gas fracturing by using air as fracturing fluid under true triaxial stresses

China has a total of 825 high-gas coal mines (state-owned coal mines and state-owned local mines). Most of these coal seams have low permeability, and some even have a high adsorption capacity and poor desorption, such as the Xishan Mine Area. There are 274 pairs of coal and gas outburst mines around the country, representing 45 per cent of the total number of the world’s outburst mines. China has the highest prevalence of coal and gas outburst accidents in the world, with more than 11 500 recorded accidents, leading to the most serious mining death toll in the world. Hydraulic fracturing technology applications in the oil industry are very mature. Much research has been directed toward the flow, stress, and damage (FSD) coupling damage behaviour of hydraulic fracturing1–3, filtration law4, filtration and the impact of pore pressure gradient formed by filtration on the hydraulic crack tip cracking5–6, crack tip stress perturbations7, repeated fracturing8, and geophysical monitoring of hydraulic fracturing9. Currently, hydraulic fracturing technology has been applied to many other industries. The techniques used in natural gas extraction through drilling of surface wells have been applied to the extraction of coal-bed gas by hydraulic fracturing technology10–12, but there are still many problems to study. A coal mine is different from an oil field. The underground tunnels connect directly with the coal seam, so providing a space for underground hydraulic fracturing operations. Based on this, techniques for improving the permeability of gas-bearing coal seams in underground coal mines by hydraulic fracturing have beendeveloped. The underground drills for improving the permeability of the coal seam (thereby weakening it) for hydraulic fracturing are Hydraulic fracturing technology for improving permeability in gas-bearing coal seams in underground coal mines

/pdf/hydraulic-fracturing-technology-for-improving-permeability-p1vbjnd8au.pdf

Hydraulic fracturing technology for improving permeability in gas-bearing coal seams in underground coal mines

Hanging roofs can occur on a very large scale (generally can reach 15,000 m2) during the initial mining stages in longwall coal mining operations and are often a result of hard roof at the active mining face. Besides, when it suddenly fractures and caves, some risks and disasters would occur such as shock loading, wind blasts, and sudden gas extrusion from gob to working face. To control the size and competency of the hanging roof, Φ32-mm fracturing drills can create holes in the roof near the front and back coal walls, and Φ32-mm linear fracturing drills can be used to create holes in the roof at the face ends. The length of these drill holes is 3 to 5 times the height of the mining face and is based upon the principle that the gob should be fully filled with a caved roof to support vertical stresses, as well as the assumption that the broken-expansion coefficient of the rock is given as an empirical value. Considering the fracture propagation range and the support form of the cut, the drill-hole spacing is set to be between 1 and 2 times the single-hole crack propagation range. Hydraulic fracturing can generate main fractures and wing-shaped branching cracks inside the roof rock and can transform and weaken the roof structure, forming weak fracture plane favorable to roof caving. These fractures cause the roof to cave in a timely and adequate manner under in situ ground pressures. Successful field test in the no. 2 mine of Cuncaota, Shendong group indicated that the propagation range of hydraulic fractures is approximately 4–8 m. When the active face advanced 22.5 m, the roof beyond the hydraulic supports caves entirely, and gradually, the gob is fully filled with the caved roof and is absent of large voids. Thus, hazards such as shock loading, wind blasts, and sudden gas extrusion can be reduced or eliminated.

Using hydraulic fracturing to control caving of the hanging roof during the initial mining stages in a longwall coal mine: a case study

The invention discloses a hydrofracturing-based method and hydrofracturing-based equipment for diminishing dust and eliminating outburst of a coal bed. The method comprises the following steps of: firstly, drilling a long hole towards a working surface in a return airway of the coal-excavating working surface, sealing the hole with a hole sealer for hydrofracturing and forming a hydrofractured seam in the coal rock body; secondly, adding sand into a sand mixer and conveying the mixed fluid of water and sand into the hydrofractured seam by using a mud sand pump, wherein thin sand is added first and then thick sand is added; thirdly, connecting a mash gas extracting pipeline to the hydrofractured hole, draining water through negative pressure and extracting mash gas; and finally, removing the mash gas extracting pipeline from the hole sealer, and connecting a static-pressure water-injecting pipeline for performing static-pressure water injection into the coal bed after the hydrofracturing and the mash gas extraction and stopping water injection till the water flows to the position 3 to 5m ahead of the coal wall of the working surface. By employing the expansion of the main hydrofractured seam of the coal bed, the expansion of wing-shaped branched seams and the soaking effect of the absorbed water, the structure of the coal bed is changed, the strength of the coal bed is weakened and the permeability of the coal bed is improved, so the aims of weakening the strength of the coal bed, increasing the permeability of the coal bed, eliminating the outburst and diminishing the dust are achieved.

Hydrofracturing-based method and equipment for diminishing dust and eliminating outburst of coal bed

Experimental investigation on the impact of initial pore pressure on breakdown pressure of borehole radial fracture for unsaturated mortar hydraulic fracturing under true triaxial stress

Weichao Xue

Papers

Hydraulic fracturing technology for improving permeability in gas-bearing coal seams in underground coal mines

Using hydraulic fracturing to control caving of the hanging roof during the initial mining stages in a longwall coal mine: a case study

Hydrofracturing-based method and equipment for diminishing dust and eliminating outburst of coal bed

Experimental investigation on the impact of initial pore pressure on breakdown pressure of borehole radial fracture for unsaturated mortar hydraulic fracturing under true triaxial stress