Hydro-mechanical Analysis of Pressure Recovery after a Perturbation of a Shut-in Phase

We performed stimulation experiments in ten isolated intervals at different depths in the injection borehole (BH10) in the research mine 'Reiche Zeche' in Freiberg, Saxony, Germany. The pressure fall-off after the termination of pumping at pressure levels exceeding the jacking pressure of the hydraulically induced fractures is controlled by fracture growth, leak-off from the fracture into the surrounding rock, and the closing of the fracture. Closing of the fracture is actually the only of the three process that counteracts the pressure decrease. Furthermore, closing has two stages, the reduction in fracture aperture before and after the two fracture surfaces touch each other. We address these two stages as mechanically open and closed in distinction to hydraulically open and closed. Once the two fracture surfaces are in contact the further decrease in effective hydraulic aperture is controlled by the normal stiffness of the fracture. A recovery of interval pressure, here addressed as flowback, was observed when the interval pressure was briefly released during shut-in sequences. The pressure recovery bears information on the normal stiffness of the created fracture, a mechanical property difficult to determine in in-situ experiments. However, the pressure signal is a convolution of (i) leak-off, (ii) flow in the fracture, and (iii) the closing. The objective of our study is to characterize observed backflow data and to establish criteria for the dominance of the three processes relying on a generic analytical model. The field data reveal that the interval pressure can recover in less than 5 seconds to 1/3 of the pressure when the brief release was initiated. Furthermore, we observed that the storage capacity of the interval affects the field observations.