Fluid-dependent shear slip behaviors of coal fractures and their implications on fracture frictional strength reduction and permeability evolutions

Long Fan, Shimin Liu

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Abstract

During coalbed methane (CBM) formation stimulation, large volumes of fracking fluids are injected, and these fluids include water, hot steam, CO2, N2, foams, and their mixtures. These injected fluids can potentially generate new fractures and/or reactivate pre-existing faults/cleats. Coal, unlike other subsurface rocks, has systematic cleats that cause deterioration of the coal's mechanical strength. Unique to coal, fluid sorption-induced swelling and softening are commonly observed. With these unique properties, friction-based fracture sliding in stressed coal exhibits differing mechanisms. To better understand the frictional sliding behavior and permeability evolution in well-cleated coal, sliding friction measurements were conducted using artificially generated coal fractures. Various fluids (helium, carbon dioxide, water, and moisturized methane) were injected into coal to investigate the dynamic characteristics of friction coefficients and permeability evolution during faulting. Unlike the traditional view of stress-based fault reactivation, our results demonstrated that the initial sliding reactivation stress on the coal was not only pressure dependent but also fluid-type dependent. The friction strength of the coal fracture significantly dropped after injecting sorbing fluids, which could lead to fault reactivation at a low pore pressure since the friction failure curve can approach Mohr's circle envelope. Fracture permeability during sliding is sensitive to the softening effect of injected fluids; and water-involved injections of fluids may lead to an abrupt increase of friction due to the dilation caused by interactions between surface teeth and gouge particles. Both the velocity increase and decrease steps showed an increase in velocity strengthening when investigating the rate-state-friction parameters, indicating a continuous friction increase with sliding distance.

Original languageEnglish (US)
Article number103235
JournalInternational Journal of Coal Geology
Volume212
DOIs
StatePublished - Aug 1 2019

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All Science Journal Classification (ASJC) codes

  • Fuel Technology
  • Geology
  • Economic Geology
  • Stratigraphy

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