Calculation of minimum stress in mini hydraulic fracturing experiments and consideration of the effect of back stress

Abstract:

In this study, we present an analysis of laboratory experiments on mini hydraulic fracturing, that included the determination of the fracture closure pressure using standard methods and the comparison this pressure with the minimum loads applied to the sample. The comparison showed the inequality of these values. Experiments were conducted for two fracture orientations: vertical and horizontal. In both cases, the calculated minimum stress was found to be inconsistent with the known one from the characteristics of the experimental setup in the exact laboratory experiment. To eliminate the identified discrepancy, it was proposed to take into account the effect of back stress. This stress occurs due to an increase in pore pressure when fluid is filtrated into the reservoir from a hydraulic fracture. It affects the walls of the fracture, contributing to its closure. In this work, we evaluated the influence of this effect on the determination of the minimum stress in the medium in laboratory experiments on mini hydraulic fracturing. Some assumptions about how to account for back stress in the calculation of minimum stresses were considered. It was found that taking into account the back stress when calculating the minimum stress from a found closure pressure significantly reduces the difference between the calculated values and experimental data.