An comprehensive analysis of the hydraulic fracturing behavior of coalbed methane wells of Xishan coalfield and its revelation

Abstract

Hydraulic fracturing is a widely used technique for increasing the permeability of coal seams. Its impact on the production capacity of coalbed methane(CBM) wells is significant. This paper makes detailed observations on the exposed coal seams and fracture distribution of 5 CBM wells in Tunlan block of Xishan coalfield, and studies the distribution characteristics of coal structure, microfractures, quartz sand and coal fine by using stereoscopic microscope, scanning electron microscope, and micro CT. The fracturing effect and influencing factors of CBM wells are analysed by combining regional geostress and hydraulic fracturing construction parameters. The research results indicate that the macroscopic fractures morphology produced by hydraulic fracturing are complex and diverse, including horizontal, vertical, X and T-shaped fractures. The degree of coal body fragmentation increases with distance from the CBM well. The coal body closer to the CBM well is mainly composed of fragmented and granular coal, while the coal body further away is mainly composed of undeformed coal with primary fractures. The quartz sands are mainly deposited in horizontal fractures, with only a small amount in T-shaped fractures. Quartz sand rubs, collides, and embeds strongly with the coal fracture surface. The fracturing fluid is then superimposed to fracture the coal body, resulting in a large amount of coal fines. This causes the coal fines to wrap around the quartz sand and block the fractures. The coal fines within the macroscopic fracture are primarily a result of the fracturing of the coal body, the flushing of the coal body by fracturing fluid, and the friction between the quartz sand and the fracture surface during hydraulic fracturing. Coal fine generation in micro-fractures during coal body fracturing. Coal fines and quartz sand accumulate in the fractures, creating enormous resistance to the carrying fluid and preventing the quartz sand from migrating to the far end of the CBM well. The magnitude and direction of the geostress, as well as the strength of the coal seam roof and floor, are important factors that influence the opening and direction of the hydraulic fracturing fractures. The low viscosity of the carrying fluid cannot suspend quartz sand, and the accumulation of quartz sand and coal fine can block the fractures, easy to cause quartz sand and coal fine to aggregate and block fractures.

Keywords

• coalbed methane well
• hydraulic fracture behavior
• fracture observations
• coal texture
• quartz sand
• coal fines