Brazilian Tensile Strength Test of Orgainc-rich Shale

Abstract

AbstractTensile strength is a critical parameter for hydraulic fracturing, predicting crack initiation and propagation in reservoirs, especially in shale reservoirs with complex natural fractures and fissures. The tensile strength of conventional rocks, such as sandstone and limestone, has been well studied and documented. There are many studies of the tensile strength of laminated shale, which focus on scale effects, loading direction, and temperature effects; however, the studies on effects of mineralogy and the water content on tensile strength of organic-rich shales are very limited.The objectives of this paper are to: 1) Critically review the key parameters that affect the tensile strength of shale; and 2) Experimentally examine the effects of water content, mineralogy and lamination on tensile strength. To do so, a rigorous workflow is followed: 1) Each 1 in. long shale sample is cut into two sub-samples, A and B, of similar length; 2) X-ray computed tomography scan is performed to diagnose pre-existing cracks and defects inside the core plugs; 3) Nuclear magnetic resonance (NMR) is then used to measure the air dry samples’ water content; 5) Sample A is placed on a load frame to measure the tensile strength; 5) Sample B is vacuumed and then saturated; 6) NMR is used to measure the water content after saturation; 7) Tensile strength of the saturated sample B is measured; 8) Samples are crushed, and X-ray diffraction (XRD), scanning electron microsope (SEM) and pyrolysis methods are used to estimate the mineralogy and total organic content (TOC).A total of 70 Mancos and 48 Eagle Ford shale samples have been tested. The experiment results show that: 1) Bedding plane/lamination has a significant effect on Eagle Ford tensile strength, but no pronounced impact observed for the Mancos shale; 2) The imbibed water significantly reduces the tensile strength by 4.4% to 51.7% as water content increases from 4.45% to 11.7%; 3) Pre-existing detectable micro-fractures can significantly reduce the tensile strength by up to 66%; 4) Eagle Ford exhibits typical brittle hard rock failure configuration, with primary facture and secondary fracture being observed, while for the Mancos shale, only primary fractures are observed; 5) Acoustic velocity test results confirm that Eagle Ford is mechanically transversely isotropic, and Mancos is likely mechanically isotropic.