Hydraulic fracturing of the transverse isotropic argillite from Tournemire in a hollow cylinder triaxial cell: experimental study and analytical modelling

To study the effect of hydraulically induced damage in a caprock (argillite from Tournemire), hydraulic fracturing tests were conducted in a hollow cylinder triaxial cell on core samples with inner borehole, oriented parallel to the bedding plans. Acoustic Emission (AE) monitoring and strain measurements during hydraulic fracturing tests, as well as post-mortem X-ray CT analysis were carried out. Samples were subjected to stresses representative of the in-situ stress state, and different injection rates of low-viscosity fluid (water) were tested.

Overall, the breakdown pressure is higher and AE activity due to microcracking damage (which increases almost simultaneously with the pressure drop) appears earlier when the injection flow rate increases. During the second injection phase, peak pressures are lower, pressurization rates are higher and stress-strain behaviour is stiffer. The analysis of values of Rise Angle (RA) and Average Frequency (A-FR) indicates that increasing the injection flow rate generates more tensile cracks and increases the severity of damage. The cumulative damage variable, calculated from AE activity, increases significantly just before the first pressure drop and this increase is stiffer when the injection flow rate increases. This confirms that the use of a low-viscosity fluid (water) induces the propagation of unstable cracks, which initiate almost simultaneously with the pressure drop in the borehole, and that this propagation is faster for higher injection rates.

The breakdown pressure and the crack orientation are analysed thanks to the elastic theory for transverse isotropic materials and chemo-hydro-mechanical coupled processes at the borehole wall. The analysis of core samples deformations and X-ray images, and post-mortem visual observation of samples surface indicate that cracks are primarily oriented parallel to the bedding planes and the core sample axis.