In Situ Deformation Analysis of a Fracture in Coal under Cyclic Loading and Unloading

Abstract

The deformation analysis of fractures is vital for advantageous development of oil and gas fields, especially the coalbed methane (CBM) reservoir, since the change of fracture parameters can be directly evaluated through fracture deformation analysis. Then the flow capacity of CBM and the effect of various stimulation methods can be analyzed. In this study, X-ray CT image analysis is used to quantitatively characterize the deformation of a coal fracture in situ, and the evolution of fracture aperture under cyclic loading is presented. Furthermore, dimensionless permeability at different confining pressures by the Lattice Boltzmann method is simulated. The current results indicate that the fracture deformation changes significantly under cyclic loading. A dramatic change is observed in the initial loading stage, in which the coal is strongly compacted, and the fracture aperture and permeability are reduced to 13.9% and 0.1%, respectively, when the confining pressure is loaded to 10 MPa. When unloading to 0 MPa, the fracture aperture and dimensionless permeability are far less than that of the initial 0 MPa. It is worth noting that the deformation of the second cycle fracture is weaker, and the change range of permeability and aperture of coal fracture becomes smaller, but when unloading to 0 MPa in the second cycle, the fracture permeability can be restored to 50.8% compared with 0 MPa of the loading stage. Additionally, a special phenomenon has been observed that under cyclic loading, even when the confining pressure reaches 10 MPa, some areas of the fracture are still not closed. We infer that there are some large pore structures in fracture space, and high confining pressure leads to fracture closure, but the deformation of the pore structure is not prominent compared with the fracture space. These characteristics of fracture deformation are of great significance for the production of CBM and are worthy of further study.

Keywords

• deformation analysis
• X-ray CT image
• cyclic loading
• fracture
• coal