Deformation Experiments on Bowland and Posidonia Shale—Part I: Strength and Young’s Modulus at Ambient and In Situ pc–T Conditions

Abstract

The production of hydrocarbons from unconventional reservoirs, like tight shale plays, increased tremendously over the past decade. Hydraulic fracturing is a commonly applied method to increase the productivity of a well drilled in these reservoirs. Unfortunately, the production rate decreases over time presumably due to fracture closure. The fracture closure rate induced by proppant crushing and embedment depends on mechanical properties of shales and proppants that are influenced by confining pressure (p c), temperature (T), and shale composition. We performed constant strain rate deformation tests at ambient and in situ conditions of a typical shale reservoir (p c ≤ 100 MPa, T ≤ 125 °C) using European shale samples exhibiting variable mineralogy, porosity and maturity. We focused on a comparison of Posidonia shale with Bowland shale, which is believed to be the most prospective shale formation in the United Kingdom. Compression tests were performed perpendicular to bedding orientation. Stress-strain curves show that Bowland shales are relatively strong and brittle compared to Posidonia shale which display semibrittle deformation behavior. Brittleness estimated from elastic properties is in good agreement with the recorded stress-strain behavior but shows no clear relation to composition. Compressive strengths (σ UCS = uniaxial compressive strength, σ TCS = triaxial compressive strength) and static Young's moduli, E, reveal a strong confining pressure and mineralogy dependence, whereas temperature and strain rate only have a minor influence on σ TCS and E. The coefficient of internal friction for both shales is ≈ 0.42 ± 0.03. With increasing amount of weak minerals (e.g., clay, mica) σ UCS , σ TCS and E strongly decrease. This may be related to a shift from deformation supported by a load-bearing framework of hard minerals to deformation of interconnected weak minerals at about 25-30 vol% of weak phases. At the applied conditions, the triaxial compressive strength and Young's moduli of most shales deformed normal to bedding are close to the Reuss bound. To our knowledge, this is the first study, which presents results of experimental investigations carried out to characterize the mechanical behavior of Bowland shale. The observed results are helpful to estimate the potential of the Bowland reservoir with respect to the economical extraction of hydrocarbons. Keywords Shale • Compressive strength • Young's modulus • Brittleness • Effective medium theories • P c-T conditions List of Symbols ϕ Porosity ρ Density p c Confining pressure T Temperature T a Absolute temperature σ UCS Uniaxial compressive strength σ TCS Triaxial compressive strength E Static Young's moduluṡ Strain rate ε max Maximum axial strain before failure of specimen µ i Coefficient of internal friction M Specific mechanical property (e.g., triaxial compressive strength, Young's modulus) f Volumetric fraction J Scaling parameter K Bulk modulus µ Shear modulus B min Brittleness determined from mineralogy B E Brittleness determined from Young's modulu...

Creep Deformation in Vaca Muerta Shale From Nanoindentation to Triaxial Experiments

“…The Voigt‐Reuss‐Hill value for E seems closer to the HS bounds than for C . A detailed comparison between such averaging methods is beyond the scope of this paper; however, we notice that our triaxial values are closer to the Reuss bound (the case for vertical loading of a layered sample), consistent with published experiments (Herrmann et al, ; Sone & Zoback, ). The Voigt‐Reuss bounds of Young's modulus are also broadly consistent with the Vaca Muerta triaxial moduli obtained by Ambrose (), that is, 27 ± 11 and 18 ± 7 GPa in horizontal and vertical samples, respectively.…”

“…The Voigt (isostrain, upper bound) and Reuss (isostress, lower bound) averages (Mavko et al, 2009) also provide elastic bounds of our ternary G1-3 groups, albeit looser than the HS bounds (Herrmann et al, 2018;Kumar et al, 2012;Sone & Zoback, 2013a;Vanorio et al, 2003). These bounds have been found useful to estimate shale anisotropy (Sone & Zoback, 2013a).…”

“…The triaxial Young's moduli for U.S. shales seem to have a lower bound of ~15 GPa (Sone & Zoback, ; Yang & Zoback, ), so the Vaca Muerta (Figure ) moduli lie at the lower end of this range and within the range of immature Kimmeridge shale (Kumar et al, ). European shales such as Alum, Posidonia, and Bowland shale also show softer Young's moduli of ~10 GPa (Herrmann et al, ; Rybacki et al, ). The presence of soft indentation group G1 in Figure , specific to Vaca Muerta shale, might explain the low stiffness of Vaca Muerta.…”

Deformation Experiments on Bowland and Posidonia Shale—Part II: Creep Behavior at In Situ pc–T Conditions

“…All experiments were performed with loading direction normal to bedding orientation. The triaxial compressive strength, σ TCS , and static Young's modulus, E, of investigated samples were determined in previous constant strain rate experiments performed at similar confining pressures and temperatures (Herrmann et al 2018).…”

Stress Transfer From Opening Hydraulic Fractures Controls the Distribution of Induced Seismicity

“…These values have been used in previous studies on induced seismicity (e.g., Schoenball et al, ; Catalli et al, ; Pennington and Chen, ) and are consistent with laboratory measurements of the frictional and mechanical properties of shales (Kohli and Zoback, ; Islam and Skalle, ). These values are also similar to those found from studies of the Bowland shale, the formation targeted by PNR‐1z (Herrmann et al, ).…”