Anisotropic and heterogeneous mechanical properties of a stratified shale/limestone sequence at Nash Point, South Wales: A case study for hydraulic fracture propagation through a layered medium Nathaniel Forbes Inskip (1,2), Philip Meredith (2), and Agust Gudmundsson (1) (1) Royal Holloway, University of London, Earth Sciences, United Kingdom (nathaniel.forbesinskip.2014@live.rhul.ac.uk) (2) University College London, Earth Sciences, United Kingdom 1. ABSTRACT While considerable effort has been expended on the study of fracture propagation in rocks in recent years, our understanding of how fractures propagate through layered sequences of sedimentary rocks with different mechanical and elastic properties remains poorly constrained. Yet this is a key issue controlling the propagation of both natural and anthropogenic hydraulic fractures in layered sequences. Here we report measurements of the contrasting mechanical and elastic properties of rocks from Lower Lias at Nash Point, South Wales, which constitute an interbedded sequence of shale and limestone layers, and how those properties may influence fracture propagation. 2. STUDY AREA The area is characterised by interbedded limestone and shale units from the early Jurassic (Lower Lias) which are expected to have major differences in their mechanical properties. Jurassic shales are of interest for shale gas production in many areas (Weald basin, Haynesville shale). As such Nash Point is an ideal location to study how fractures propagate through layered media of varying mechanical properties. 3. OBJECTIVES • Further the understanding of how induced fractures propagate in a layered sequence with varying mechanical properties • Further the understanding of how induced fractures interact with existing faults (and other discontinuities), in an aim to minimise the risk of induced seismicity • Assess the risk of the contamination of aquifers • Apply the knowledge gained to promote a more targeted approach to shale gas exploration and production P-wave velocity measurements demonstrate that Nash Point shale is highly anisotropic, while Nash Point limestone is essentially isotropic (Figure 2 and Table 1). Nash Point shale Nash Point limestone Mean vertical P-wave velocity (Vp-zz) 2.4 km/s 5.8 km/s Mean horizontal P-wave velocity (Vp-xx) 3.9 km/s 5.9 km/s P-wave velocity anisotropy 55% 2% Mean vertical dynamic YouŶg’s modulus 12 GPa 79 GPa Mean horizontal dynamic YouŶg’s ŵodulus 35 GPa 81 GPa 7. CONCLUSIONS • Nash Point shale and limestone have widely different mechanical properties, including: • Anisotropy • YouŶg’s modulus • Tensile strength • Tensile fractures propagate much more easily along the weak layers of the shale at ambient pressure • Observations from fractured samples suggest that the scatter in tensile strength at intermediate angles in the shale is due to stepping between the loading direction and the weak short transverse plane • Similarly, scatter of the tensile strength in the limestone is potentially due to veins and variation in fossil content Figure 1: Location and photo of interbedded limestone/shale units at Nash Point. 4. METHODS To date, the following methods have been used to characterise Nash Point shale and limestone: • Ultrasonic wave velocity measurements (Sample anisotropy and dynamic Young’s modulus) • Indirect Tensile Strength (ITS) tests (Tensile Strength) [1] 5. RESULTS 0 1.5 3 4.5 6 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210 220 230 240 250 260 270 280 290 300 310 320 330 340 350 P-wave velocity (km/s) of Nash Point limestone as a function of azimuth (degrees) 0 1.5 3 4.5 6 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210 220 230 240 250 260 270 280 290 300 310 320 330 340 350 P-wave velocity (km/s) of Nash Point shale as a function of azimuth (degrees) Vp-zz Vp-xx Vp-xz Table 1: Mean values for mean P-wave velocity, P-wave velocity anisotropy and dynamic Young’s modulus for Nash Point shale and limestone Figure 3 demonstrates the three principal crack orientations with respect to bedding planes. The tensile strength of both Nash Point shale and limestone were measured in these three principal crack orientations. Tensile strength of Nash Point shale was also measured at 15° intervals between the short transverse and arrester (Figure 4). Figure 2: P-wave velocities of Nash Point shale (left) and Nash Point limestone (right), where Vp – xx, zz, xz are the P-wave velocities in a particular orientation. zz is vertical (normal to bedding), xx is horizontal (parallel to bedding) and xz is from horizontal to vertical (parallel to bedding through to normal to bedding). Figure 3: Three principle crack orientations with respect to bedding planes [2] 0 2 4 6 8 10 12 14 16 18 0 15 30 45 60 75 90 105 Tensile strength (MPa) Angle to bedding (degrees), where ST = Short Transverse, A = Arrester, D = Divider Tensile strength of Nash Point shale and limestone as a function of angle to bedding plane Shale Limestone ST A D 6. RESULTS - CONTINUED Figure 4: Tensile strength of Nash Point shale and limestone as a function of angle to bedding where ST, A and D are the three principal crack orientations with respect to bedding planes, Short Transverse, Arrester, and Divider, respectively. Figure 4 demonstrates that the tensile strength of Nash Point shale is highest in the arrester and divider orientations, and lowest in the short transverse orientation. There is a monotonic and significant increase in tensile strength between the short transverse and arrester orientations. The tensile strength of the limestone is higher than that of the shale in all measured orientations, and is essentially isotropic. 8. FUTURE WORK References [1] International Society for Rock Mechanics, 1978. Suggested Methods For Determining Tensile Strength of Rock Materials. International Society for Rock Mechanics Commission on Standardization of Laboratory and Field Tests, 15, pp.99–103 [2] Chandler, M.R. et al., 2016. Journal of Geophysical Research : Solid Earth Fracture toughness anisotropy in shale. , pp.1–24. [3] Bieniawski, Z.T. & Bernede, M.J., 1979. Suggested methods for determining the uniaxial compressive strength and deformability of rock materials. International Journal of Rock Mechanics and Mining Sciences & Geomechanics Abstracts, 16(2), pp.138–140. [4] Kuruppu. M. D., Obara. Y., Ayatollahi. M. R., Chong. K. P., and Funatsu. T., 2014. ISRM-Suggested Method for Determining the Mode I Static Fracture Toughness Using Semi-Circular Bend Specimen, Journal of Rock Mechanics and Rock Engineering, 47. 267–274 • Measure uniaxial compressive strength both parallel and normal to bedding [3] • Calculate static YouŶg’s modulus for comparison with dynamic values • Measure the fracture toughness of both Nash Point shale and limestone, in different orientations, using the short rod and semi-circular bend methods [4] • Use the mechanical data set in numerical models to study how fractures propagate in a layered sequence, such as at Nash Point Arrester Divider Short Transverse