The Effect of Fluid Content and Effective Stresses on Stick-Slip Friction in Rocks: Implications for Induced Seismicity

Abstract Induced seismicity has become a serious issue in many unconventional plays in the US. These events are primarily associated with the injection of large quantities of produced water at very high rates. It is well known that the frictional properties of rock joints have a large influence on such induced seismic events during wastewater disposal in sub-surface formations. The primary objective of this study is to understand the role of pore fluids, in-situ stress, fracture roughness and pore pressure on the shear failure of rock fractures. A new triaxial apparatus is developed that allows us to measure the frictional properties of fractures under different loading and pore pressure conditions. This apparatus can independently control the pore pressure, shear stress, and normal stress acting on the fracture surface. The frictional properties of rocks and the fracture permeability after a slip event were estimated by keeping the net shear stress and normal force applied on the rock constant. The experiments conducted allow us to directly measure stick-slip and dynamic friction coefficients as well as the creep that occurs after frictional sliding. Tests are conducted with both air and brine as pore fluids. The shear stress is gradually increased, and the displacement of the fracture surfaces is measured. The effective normal stress and pore fluid are varied to measure the impact of these parameters on both stick-slip and creep frictional sliding. The results from the experiments clearly show that the rock surfaces slide in a combination of sudden jerky motions (stick-slip events) and of smooth, gradual movements (creep deformation). The frequency and magnitude of the stick-slip events are important since it is these events that are primarily responsible for induced seismicity. Changes in the effective normal force acting on the fracture mimics the change in pore pressure with water injection. When the shear stress acting on the fracture surfaces overcomes the strength of asperities preventing the sliding, stick-slip sliding begins followed by smooth sliding. Creep deformation is measured as the shear stress and the pore pressure are held constant and the fracture surfaces continue to slide. The coefficient of friction gradually increases as the shear stress increases and the surfaces repeatedly slide over each other. The friction coefficient reaches a plateau value of about 0.6. The effective normal stress or the type of pore fluid has very little influence on this stabilized value of friction coefficient. The friction coefficient appears to be primarily controlled by the roughness of the fracture surfaces. The number and magnitude of stick-slip events is significantly larger when air is the pore fluid (rather than water). This clearly implies that the composition of the pore fluid impacts the likelihood and magnitude of induced seismicity. The magnitude of the stick-slip events also increases as the effective confining stress increases.