Study on the model of staged and cluster fracturing to control the gas in driving roadway by horizontal well in roof strata

To address the issue of low gas extraction efficiency in driving roadway of broken soft and low permeability coal seams in Huainan mining area, a model of segmented cluster fracturing gas management in horizontal wells at the roof of coal seams was proposed. The crack expansion process of hydraulic fracturing in horizontal wells of coal seam roof was studied using numerical simulation and physical similarity simulation methods. The gas production effect of segmented and clustered fracturing was investigated using productivity simulation, and a comparative analysis of residual gas content between fracturing in clusters and without clustering was conducted. The numerical simulation results of crack extension shown that, the cracks in the horizontal wells at the roof of coal seam could expand to the coal seam and open up the coal seam completely. Due to the plasticity of the coal seam was greater than that of the sandy mudstone at the roof, a wider fracturing crack was formed in the coal seam than at the roof. The physical similarity simulation results of crack extension shown that, considering the development of the mudstone pseudo-roof, when the horizontal drilling was placed in the sandstone at the roof of the broken soft coal seam, and under reasonable vertical distances and high-displacement fracturing fluid construction, if coal seam developed a relatively thin mudstone pseudo-roof, the fractures could extend through the interface of direct roof and pseudo roof (siltstone- mudstone interface) and the interface of pseudo-roof and coal seam (mudstone-coal seam interface) along the shot hole aperture and into the underlying coal seam. The fractures extended to form a curved and irregular step-like pattern, which can achieve the goal of fracturing and reforming the broken soft coal seam. However, when the coal seam developed a thick mudstone pseudo-roof, it had a blocking effect on hydraulic fracturing, making it difficult for the cracks to open up the underlying coal seam. The productivity simulation results shown that, under the same stratigraphic environment and construction conditions, after 3 years of extraction, single-stage non-cluster fracturing could produce a larger gas extraction impact range. However, the gas content of driving roadway could not be uniformly reduced, and there was a gas extraction gap between the fractured segments. Fracturing in clusters produced a smaller gas extraction impact, but it could more evenly reduce the gas content in driving roadway. After the engineering verification in the Panxie coal mine in Huainan, with a construction discharge of 10 m³/min, the longest fracture length could reach up to 193.8 m, the maximum height of fracture was 27.0 m, and the daily gas production of a single well reached 1 490 m³/d. The gas extraction volume of 2 years was 31×10⁴ m³, indicating that the segmented and cluster fracturing technology of horizontal wells at the coal seam roof was an effective model for efficient gas extraction in the driving roadway of broken soft and low-permeability coal seams in Huainan area.