Experimental investigation on nonlinear flow properties of fluids through rough-walled shale fractures after shear

Abstract

Abstract For a single fracture, the magnitude of Reynolds number (Re) is proportional to that of hydraulic gradient (J), and J is also a dimensionless parameter representing how fast a pressure drops over a given region. Therefore, J is also a practical parameter for establishing a criterion that quantifies the onset of nonlinear flow in single fractures. This study experimentally analyzed the influences of surface roughness, shear displacement and confining pressure on nonlinear flow behaviors of fluids through 3D rough-walled rock fractures. A total of 10 shale samples with different joint roughness coefficients (JRCs) were prepared after Brazil splitting tests, and the distribution of asperity height distribution for each sample was analyzed using a Gaussian function. The shear displacement (us) that ranges from 1 mm to 10 mm was assigned to generate aperture spaces. Flow tests with flow rate (Q) ranging from 1 ~ 180 ml/min were conducted under confining pressures (σ3) of 1 MPa, 3 MPa, and 5MPa, respectively. The Forchheimer equation fits the Q-J correlations by zero intercept regression well and the fitting correlation coefficients of all cases are greater than 0.99. Normalized transmissivity T/T0 first maintains a value of 1 and then gradually decreases as J increases, indicating that the fluid flow state changes from the linear region to the nonlinear region. Finally, a nonlinear curve fitting formula based on the tests database was proposed to investigate the evolutions of critical hydraulic gradient Jc with varying us and JRC. As us increases, Jc drops sharply. Then, the downward trend gradually slows down, and finally approaches a constant value of approximately 1. Jc increases slightly as JRC increases and shows an exponential growth trend with the increase of σ3.