A New Apparatus to Measure the Coefficient of Friction and Changes in Fracture Permeability
Authors: Sathyanath Arun , Sharma M. Mukul 
Published: August 2024

ABSTRACT:
The injection of large volumes of fluid into the subsurface can potentially result in seismic events or earthquakes. In this context, it is crucial to understand the conditions such as the frictional properties that control subsurface faults slip and cause the seismic events. A new triaxial apparatus has been developed that can characterize the frictional properties of rock samples. The new apparatus is different from the previous test system designs in its ability to measure additional parameters accurately such as fracture permeability in addition to frictional properties. The apparatus measures both the dynamic friction coefficient, the magnitude of stickslip events, and changes in the fracture permeability during and after frictional sliding. It also enables us to estimate the effect of pore pressure and fluid/brine flow through the fracture. This ability to measure changes in fracture permeability after frictional sliding is unique and provides additional insight into changes that can occur in the subsurface due to fault slip. Another advantage is its ability to independently control shear stress on the rock sample, allowing for the direct measurement of shear stress rather than calculating it from total stress values. By independently controlling the confining stress, the axial shear stress, and the pore pressure using three hydraulically driven pumps the entire range of frictional properties can be measured. Each load can be applied in different modes: constant strain rate loading, or constant stress.

1. INTRODUCTION
The significant increase in seismic events reported in West Texas and Oklahoma (Ellsworth, 2013) are attributed to anthropogenic activities such as wastewater disposal (Moein et al., 2023). The disposal of wastewater into the reservoir causes an increase in pore pressure which reduces the normal stress acting on naturally occurring faults (Hubbert & Rubey, 1959) and allows the fault to slip (Healy et al., 1968; Kisslinger, 1976; McClain, 1970). When the frictional resistance (proportional to the normal stress acting on the fault) drops below a threshold, tectonic shear stresses can lead to fault slip. This is the primary reason behind induced seismicity. Hence understanding these events and the frictional properties of rock controlling it are crucial to mitigating the adverse effects of induced seismicity.
Characterizing the frictional properties of rock is a vital step in understanding the occurrence of induced seismic events. Friction controls the shear-type dislocation movement that occurs during an earthquake event (Ohnaka, 1975). The hypothesis is that the type of fault, mineralogy of the rock, the orientation of the fault, fluid injection rate, pore pressure, chemical or physical interaction of rock with pore fluids (Moein et al., 2023), and composition of the brine could also affect or alter the frictional characteristics of the rock. Characterization of frictional properties includes estimation of the dynamic frictional coefficient of rock, type of slip (whether it is an aseismic or seismic), and the rate at which the rock slips. Understanding the factors that control the frictional characteristics helps us better understand and potentially mitigate these events.