During hydraulic fracturing process, the formation undergoes significant changes in the net stress as a result of the fluid injection and retrieval. These stress changes impact the formation petrophysical properties including permeability, porosity, and the compressibility. In this experimental study, the impact of geomechanical changes on shales petrophysical properties is investigated using a laboratory set-up that was designed and assembled for fast and robust shale core plug porosity and permeability measurements under steady-state conditions. This laboratory set-up is capable of accurately measuring shale core plug ultra-low permeability and porosity because it has a resolution of one millionth standard cubic centimeters per second for gas flow rate and one hundredth cubic centimeters for pore volume measurement. The laboratory set-up allows measurements to be performed under isothermal conditions by automated temperature control. The application of the confining pressure on the shale core plug allows permeability and porosity to be measured under a wide range of net stress.
A sequential set of experiments were performed in this study. During the first set of experiments, the permeability of the core plug was measured by increasing the net stress from 500 psia to 7,900 psia followed by decreasing net stress from 7,900 psia to the initial net stress condition. The next three sets of experiments repeated the same steps as the first set on the same core plug. However, during the third set only the porosity, and during the fourth set only permeability was measured.
The measured porosity and permeability values from the experiments clearly exhibited hysteresis with the net stress changes. The hysteresis was more pronounced for permeability than it was for porosity. Since the permeability changes were non-linear with respect to net stress changes, Walsh method was used to estimate the fracture closure pressure. The results indicate that sequential net stress changes can cause a reduction in fracture closure pressure. This reduction however tends to diminish with increasing number of sequential stress changes. Finally, comparing the porosity and permeability measurements revealed two distinctive behavioral changes for both porosity and permeability that can be contributed to the natural fracture and matrix properties.