The primary mechanisms to initiate hydraulic fractures in rock include overcoming the tensile resistance of the rock, overcoming its shear resistance, or a combination of both. The focus of this study is the solid-liquid interaction in hydraulic fractures using basic measurements and models to aid in investigating these mechanisms. The materials of study were Opalinus Shale and hydraulic oil. The basic measurements included liquid density, viscosity, surface tension, and contact angle with the rock. These measurements were used to determine the characteristic capillary length, capillary number, spreading, and imbibition parameters. Two fracture scenarios were considered for a range of apertures:
A- liquid inside a pre-existing fracture was modelled as parallel plates to determine the Laplace pressure and adhesion force.
B- liquid penetrating a newly created fracture was modelled as a capillary to determine the dynamic and equilibrated rise.
This study provides a theoretical framework to assess solid-liquid interaction effects in fractures and introduces the capillary draw amplification coefficient. The results of this study provide insight into the lag or lack thereof between the liquid front and the fracture tip during hydraulic fracturing and how the liquid interacts with the fracture after it has been created. These, in turn, have implications on the mechanism of fracturing and potential operational considerations.
Several mechanisms play a role in the hydraulic fracturing process. There are two main ways to initiate hydraulic fractures in rock:
1. increasing the liquid pressure beyond the tensile resistance of the rock will result in tensile fractures.
2. decreasing the effective stress (by increasing the pore pressure) to have the Mohr circle tangent to the shear failure envelope will result in shear fractures.
After initiation, the propagation of hydraulic fractures depends on many factors. One of these factors that affect hydraulic fracture behavior is the solid-liquid interaction. A higher viscosity liquid pressurizing rock may result in different behavior than a lower viscosity liquid.
The motivation of this study was to investigate the solid-liquid behavior that occurs in the hydraulic fracturing, specifically in the experiments conducted by AlDajani (2022). These hydraulic fracture experiments are beyond the scope of this paper, but the materials used in the study are the same. The liquid used as a fracturing fluid was hydraulic oil and the rock being hydraulically fractured was Opalinus Shale.
