Effects of external temperature and dead volume on laboratory measurements of pore pressure and injected volume in a rock fracture

Abstract

The accurate evaluation of pore pressure and injected volume is crucial for the laboratory characterization of hydromechanical responses of rock fractures. This study reports a series of laboratory experiments to systematically demonstrate the effects of external temperature and dead volume on laboratory measurements of pore pressure and injected volume in a rock fracture. We characterize the hydraulic aperture of the fracture as a function of effective normal stress using the exponential aperture model. This model is then employed to predict the pore pressure change and injected volume in the fracture without the influences of external temperature and dead volume. The external temperature changes in the cyclic loading test due to the Joule-Thompson effect for fluids. The effect of external temperature on pore pressure change in the fracture can be well explained by thermal pressurization of fluids. Our results also show that the external dead volume can significantly lower the pore pressure change in the fracture during the cyclic loading test under undrained conditions. The injected volume can also be substantially enlarged due to the external dead volume in a typical pore pressure system. Internal measurement of the pore pressure in the fracture using a fiber optic sensor cannot exclude the influences of external temperature and dead volume, primarily because of the good hydraulic communication between the fracture and pore pressure system. This study suggests that the effects of external temperature and dead volume on pore pressure response and injected volume should be evaluated for accurate laboratory characterization and inter-laboratory comparison.

Keywords

• Fracture aperture
• Thermal pressurization
• External dead volume
• Pressure attenuation
• Injected volume
• Fiber optic sensor