Experimental and numerical simulation study on hydraulic fracture propagation law of coal seam

Abstract

In this article, the expansion law of hydraulic fractures in coal seams was explored using a combination of hydraulic fracturing experiments and numerical simulations with coal samples. The results show that (1) the minimum horizontal stress difference is high in the coal seam with undeveloped laminae, which will confine the fractures entirely within the coal reservoir, thus affecting the effect of multilayer fracturing. The high elastic modulus of the spacer is favorable for vertical fracture expansion. Critical displacement is essential in determining the vertical fracture breakthrough from the formation. The concentration of fracturing fluid is proportional to the height of the fracture and inversely proportional to the length of the fracture. (2) The fracture pressure decreases significantly during the development of natural laminae in coal. The fracture prefers to expand along the direction of the laminae with a smaller angle between the laminae and the maximum horizontal principal stress. The fracture passes directly through the interface to form a vertical fracture when the adhesive strength of the interface is high. When the interfacial cementation strength is low, the cracks extend along the interface and then turn to produce new vertical cracks and finally form “Z” cracks.

Keywords:

• Coalbed methane (CBM)
• cohesive unit
• crustal stress
• hydraulic fracturing
• natural crack

Additional information

Funding

The study was supported by the General Program of National Natural Science Foundation of China, grant number 51874249, the State Key Program of National Natural Science Foundation of Sichuan Province, grant number 2022NSFSC0019, the Joint Funds of National Natural Science Foundation of China, grant number U21A20105, the Major National Science and Technology Project of China, grant number 2016ZX05042-003, and the Major National Science and Technology Project of China, grant number 2016ZX05065-003.