Evaluation of hydromechanical coupling in a granite rock mass from a high-volume, high-pressure injection experiment: Le Mayet de Montagne, France

Abstract

An injection experiment was conducted to investigate the pressure domain within which hydromechanical coupling significantly influences the hydrologic behavior of a rock mass. The study site is located in the village of Le Mayet de Montagne in central France, where the local granitic terrain has been extensively characterized. The test well is 840 m deep, with an open-hole diameter of about 16 cm. A heat-pulse flowmeter was initially used to delineate the natural fluid circulation in the well and water was then injected from the surface through a tube anchored on an inflatable packer set at a depth of 275 m. This packer location was chosen in order to reach a depth where the minimum principal stress shifts from vertical to horizontal, thereby avoiding extraneous surface effects. The injection rate was progressively increased from 20 to 780 L/min while fluid velocity in the well was simultaneously recorded as a function of depth with flowmeters; corresponding well-head pressures gradually increased from 0.3 to 6.4 MPa. Fluid-intake zones were identified and their transmissivities were monitored as a function of varying effective stress. When pore pressure remained less than about 2/3 of the minimum principal stress, the hydrologic system responded linearly and Darcy's Law applied. However, as pore pressure approached or exceeded the minimum principal stress magnitude, several new fluid-intake zones emerged. Meanwhile, other intervals exhibited either slight enhancements or reductions in transmissivity apparently due to changes in the far-field pressure conditions. The borehole was capable of supporting hydraulic pressures nearly equal to the magnitude of the far-field maximum principal stress. Results show that transmissive fractures form a dynamic and interdependent network; individual fracture zones cannot be adequately modeled as independent equivalent continua once hydromechanical coupling becomes significant.