Hydraulic fracturing: Laboratory evidence of the brittle-to-ductile transition with depth

Understanding the propagation of hydraulic fracture (HF) is essential for effectively stimulating the hydrocarbon production of unconventional reservoirs. Hydraulic fracturing may induce distinct failure modes within the formation, depending on the rheology of the solid and the in-situ stresses. A brittle-to-ductile transition of HF is thus anticipated with increasing depth, although only scarce data are available to support this hypothesis. Here we carry out laboratory hydraulic fracturing experiments in artificial geomaterials exhibiting a wide range of rheology: cubic samples 50x50x50 mm3 in size are subjected to true triaxial stresses with either a low (σv = 6.5 MPa, σH =3 MPa, and σh =1.5MPa), or a higher (15 MPa, 10 MPa, and 5MPa) confinement. The 3D strains induced by hydraulic fracturing are monitored and interpreted; X-ray Computed Tomography (CT) imaging is used to document the HF geometry; and viscoelastic modelling of the tested materials is also conducted to explain the distinct geometry of hydraulic fracture subjected to the stress state. Finally, a correlation between the normalized fracture area (AFN) and the brittleness index (BI) of tested samples is introduced. Our results reveal that: (i)The intermediate stress plays a profound role in hydraulic fracture propagation subjected to the normal faulting regimes (i.e., the transitional intermediate strain observed from brittle to ductile samples); (ii) The orientation angle of hydraulic fracture is highly inclined to the maximum horizontal σH (or vertical σv) stresses in brittle/semi-brittle samples; as BI decreases, the angle inclination is reduced for that of semi-ductile samples, finally reaches to zero (parallel to σH and σv) in ductile sample. (iii) The normalized fracturing area (AFN) decreases as the decrease of BI among different samples under either low or higher confinement. The results of viscoelastic modelling explain the distinct characteristics of hydraulic fracturing induced deformation among the tested samples subjected to true triaxial stress state. This study reveals the importance of understanding the underground brittle-to-ductile behaviour of hydraulic fracture prior to the field implementation.