Frictional properties of simulated anhydrite-dolomite fault gouge and implications for seismogenic potential

The frictional properties of anhydrite-dolomite fault gouges, and the effects of CO2 upon them, are of key importance in assessing the risks associated with CO2 storage in reservoir formations capped by anhydrite-dolomite sequences, and in understanding seismicity occurring in such formations (such as the Italian Apennines). Therefore, we performed velocity-stepping direct-shear experiments on simulated dolomite, anhydrite and 50:50 anhydrite/dolomite gouges, at representative in-situ conditions (120 degrees C and sigma(e)(n) = 25 MPa). They were conducted under vacuum, or else using water or CO2-saturated water as pore fluid (P-f = 15 MPa). Friction coefficients varied between 0.55 and 0.7. All dry samples exhibited velocity-weakening behavior, whereas all wet samples exhibited velocity-strengthening behavior, without or with CO2. This is consistent with trends previously reported for such gouges. A compilation of literature data shows that the transition from velocity-strengthening to velocity-weakening occurs in these materials between 80 and 120 degrees C when dry, and between 100 and 150 degrees C when wet. This implies little seismogenic potential for wet dolomite, anhydrite and mixed gouges under CO2 storage conditions at 2-4 km depth. Seismic slip in the Italian Apennines at depths of similar to 6 km and beyond may be explained by the velocity-weakening behavior expected in anhydrite and especially dolomite at temperatures above 150 degrees C. (C) 2016 Elsevier Ltd. All rights reserved.