Frictional coefficient of fracture is an important factor that is needed to analyze a variety of geomechanical problems for faulting and microseismicity associated with CO2 sequestration, and hydraulic stimulation for geothermal and shale gas development. A typical way of determining frictional coefficient is to run laboratory shear tests on recovered cores, but rock cores are rarely available. The purpose of this study is to attempt to correlate the frictional coefficients of fractures to physical properties that are relatively readily measureable, and to estimate the former property indirectly. Two series of frictional sliding experiments were conducted on sandstones with various physical and mechanical properties. In one series of tests, triaxial compression tests were conducted in artificial saw-cut fractures in Berea sandstone (little clay contents). In another series of tests, direct shear tests were conducted in quartz-clay mixed sandstones with various clay contents(5 to 31%). Both tests were performed at room temperature. Our experimental results show that the coefficient of friction tends to decrease with increasing clay contents. Berea sandstone has a frictional coefficient of 0.67. The quartz-clay mixed sandstones have frictional coefficients ranging between 0.36and 0.57clearly depending on clay contents. These results indicate that there is a strong dependency of fracture frictional coefficient on rock clay contents, suggesting that frictional property of fracture can possibly be estimated from rock clay contents. Our results are also quite consistent with previous experimental studies on friction of clay gouges. Since rock clay contents is measurable using geophysical logging (e.g. gamma ray log), our study suggests that frictional coefficients of fractures developed in sandstone can be estimated indirectly.

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