Improved technique for toughness testing of shale rocks

Mode I fracture mechanics tests of mostly calcareous Vaca Muerta shale rocks were performed in a new experimental device. A hydraulic system allows the injection of pressurized fluids inside of cracks to generate applied KI and measure fracture toughness in room pressure and temperature conditions. Multi-notched 1.5″ plugs were tested with different polar and non-polar fluids (waterbased fluids and organic solvents). Some samples were subject to previous saturation in order to activate multiple micro crack initiation via various embrittlement mechanisms. The methodology allows to test a large number of plug samples with low cost and times, and with a comparatively low uncertatinty and scatter of results with respect to other techniques. Results are discussed in terms of the two stress terms at the crack tip: Stress Intensity Factor (KI) and T-stress, and according to the Extended Maximum Tangencial Strain criterion (EMTSN). The variation of toughness in dry and saturated samples are discussed in terms of damage mechanisms, as well as in terms of the stochastic nature of toughness results in these rocks. The convenience of repeating these tests at reservoir pressure and temperature conditions is recognized.Fil: Antinao Fuentealba, Fabián Jorge. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata; Argentina. Universidad Nacional de La Plata. Facultad de Ingeniería. Instituto Malvinas; ArgentinaFil: Otegui, Luis Jose. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata; Argentina. Universidad Nacional de La Plata. Facultad de Ingeniería. Instituto Malvinas; ArgentinaFil: Bianchi, Gustavo Luis. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata; Argentina. Universidad Nacional de La Plata. Facultad de Ingeniería. Instituto Malvinas; Argentin

Accurate experimental determination of rock fracture toughness under simulated reservoir confining pressures

Rock toughness under confining pressure is recognized as a key mechanical property to understand brittle fracture in oil and gas projects. This article discusses a linear elastic fracture mechanics method to assess the critical stress intensity factor (KIC) of shale rocks subjected to confining pressure. Linear elastic fracture toughness (KIC) is determined in Round Bar Straight-Notch (RBSN) samples by a novel experimental set up. The device uses a primary chamber to apply a fluid-driven load to induce Mode-I opening. A secondary and independent chamber ensures variable hydraulic confinement pressures, up to 80 MPa, and both systems are isolated by O-rings. This allows analyzing the influence of fluid composition, temperature and confinement pressure. Early testing and finite element modelling allowed validating experimental results and identifying possible sources of experimental errors. Experimental assessment is discussed in terms of KIC tests. The effects of confinement on KIC in terms of crack closure mechanisms are recognized, pointing out a plausible cause for increased toughness as confinement rises.Fil: Antinao Fuentealba, Fabián Jorge. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional de La Plata. Facultad de Ingeniería. Instituto Malvinas; ArgentinaFil: Bianchi, Leandro N.. Solaer Ingeniería Argentina; ArgentinaFil: Otegui, José Luís. Universidad Nacional de La Plata. Facultad de Ingeniería. Instituto Malvinas; ArgentinaFil: Bianchi, Gustavo Luis. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata; Argentina. Universidad Nacional de La Plata. Facultad de Ingeniería. Instituto Malvinas; Argentin

Fracture toughness tests of shale outcrops: Effects of confining pressure

Brittle rock fracture is a core concept in oil and gas and other rock mechanics projects. However, the understanding of fracture behavior under mechanical well-bottom conditions remains insufficient. This article aims to analyze experimental results for the critical stress intensity factor (KIC) of outcrops from Vaca Muerta carbonate-rich shale rocks, tested under a range of crack depths and confining pressures (0–70 MPa). Fracture toughness (KIC) is determined in multi-notched 1.5″ plugs using a novel experimental set up, in which the crack-driving-force KI and the confinement pressure are both applied by hydraulic systems. Finite fracture-mechanics-based models are used to calculate KI. Our experimental results show that tests carried out at well-bottom pressures lead to apparent rock toughness doubling those for tests at atmospheric pressure. Stress analysis demonstrates that the size of the tensile stressed zone ahead of the crack tip tends to decrease as confining pressure increases. Additionally, compressive deviatoric stresses are developed ahead of the tensile zone, with their magnitude being dependent on the level of confinement. Moreover, triaxial stress states induced by confinement could lead to microcracking ahead of the crack tip. The mechanisms of crack closure and deviatoric stress-induced microcrack initiation are combined to assess a plausible mechanism for rock toughness enhancement under confined conditions. It is concluded that increasing triaxial pressure confinement allows to accurately model the mechanical response of shale rocks under reservoir conditions.Fil: Antinao Fuentealba, Fabián Jorge. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata; Argentina. Universidad Nacional de La Plata. Facultad de Ingeniería; ArgentinaFil: Blanco, Gonzalo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata; Argentina. Universidad Nacional de La Plata. Facultad de Ingeniería; ArgentinaFil: Bianchi, Leandro Noe. Universidad Nacional de La Plata. Facultad de Ingeniería; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata; ArgentinaFil: Otegui, Luis Jose. Universidad Nacional de La Plata. Facultad de Ingeniería; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata; ArgentinaFil: Bianchi, Gustavo Luis. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata; Argentina. Universidad Nacional de La Plata. Facultad de Ingeniería; Argentin