A new parameter to assess hydromechanical effects in single-hole hydraulic testing and grouting

Grouting or filling of the open voids in fractured rock is done by introducing a fluid (grout) through boreholes under pressure. The grout may be either a Newtonian fluid or a Bingham fluid. The penetration of the grout and the resulting pressure profile may give rise to hydromechanical effects, which depend on factors such as the fracture aperture, the pressure at the borehole, and the rheological properties of the grout. In this paper, we postulate that a new parameter, Å, which is the areal integral of the fluid pressure change over the fracture plane, is an appropriate measure to describe the change in fracture aperture volume due to a change in effective stress. In many cases, analytic expressions are available to calculate pressure profiles and the Å parameter. The approach is verified using a fully coupled hydromechanical simulator for the case of a Newtonian fluid. Results of the verification exercise show that the new approach is reasonable and that the Å parameter is a good measure for the fracture volume change: i.e., the larger the Å parameter, the larger the fracture volume change, in an almost linear fashion. To demonstrate the application of the approach, short duration hydraulic tests and constant pressure grouting are studied. It is concluded that using analytic expressions for penetration lengths and pressure profiles to calculate the Å parameter provides a possibility to describe a complex situation and compare, discuss and weigh the impact of hydromechanical couplings for different alternative grouting strategies. Further, the analyses identify an effect of high-pressure grouting, that of uncontrolled grouting of larger fractures and insufficient (or less-than-expected) sealing of finer fractures under certain grouting conditions.