Pyrolysis-induced P-wave velocity anisotropy in organic-rich shales

Abstract

The sources of elastic anisotropy in organic-rich shale and their relative contribution therein remain poorly understood in the rock-physics literature. Given the importance of organic-rich shale as source rocks and unconventional reservoirs, it is imperative that a thorough understanding of shale rock physics is developed. We made a first attempt at establishing cause-and-effect relationships between geochemical parameters and microstructure/rock physics as organic-rich shales thermally mature. To minimize auxiliary effects, e.g., mineralogical variations among samples, we studied the induced evolution of three pairs of vertical and horizontal shale plugs through dry pyrolysis experiments in lieu of traditional samples from a range of in situ thermal maturities. The sensitivity of P-wave velocity to pressure showed a significant increase post-pyrolysis indicating the development of considerable soft porosity, e.g., microcracks. Time-lapse, high-resolution backscattered electron-scanning electron microscope images complemented this analysis through the identification of extensive microcracking within and proximally to kerogen bodies. As a result of the extensive microcracking, the P-wave velocity anisotropy, as defined by the Thomsen parameter epsilon, increased by up to 0.60 at low confining pressures. Additionally, the degree of microcracking was shown to increase as a function of the hydrocarbon generative potential of each shale. At 50 MPa confining pressure, P-wave anisotropy values increased by 0.29–0.35 over those measured at the baseline — i.e., the immature window. The increase in anisotropy at high confining pressure may indicate a source of anisotropy in addition to microcracking — potentially clay mineralogical transformation or the development of intrinsic anisotropy in the organic matter through aromatization. Furthermore, the evolution of acoustic properties and microstructure upon further pyrolysis to the dry-gas window was shown to be negligible.

On the evolution of the elastic properties of organic-rich shale upon pyrolysis-induced thermal maturation

“…During experiments, the axial sample strain is computed from sample length changes continuously monitored (with a resolution of approximately 8 s) by three external linear potentiometers. The radial strain is not monitored, but is assumed to correspond to the axial strain of the orthogonally oriented plug as detailed in Allan et al (2014). These measurements are conducted in an unsaturated, drained configuration.…”

“…When open, and sufficiently coaligned, these microcracks may alter the elastic anisotropy of the shale. The slow deposition from suspension and burial-induced compaction of clay platelets typically generates a strongly anisotropic rock frame in shales, which is preserved in the absence of bioturbation (Kaarsberg, 1959;Jones and Wang, 1981;Vernik and Nur, 1992;Johnston and Christensen, 1995;Hornby, 1998;Wang, 2002;Vasin et al, 2013;Allan et al, 2015), although induced, coaligned microcracks can significantly alter this intrinsic anisotropy (Vernik, 1993;Johnston and Christensen, 1995;Dewhurst and Siggins, 2006;Allan et al, 2014). The magnitude of the crack-induced alteration of elastic anisotropy will depend on the number of cracks, the degree of alignment, and their aperture as a function of pressure, i.e., whether they are open or closed cracks.…”

“…The combination of rapid hydrocarbon generation and lack of effective pressure resulted in the formation of aligned throughgoing fractures and microcracks with apertures on the order of tens of microns (Kobchenko et al, 2011;Allan et al, 2014;Yenugu, 2014). This fracturing of the rock and subsequent nonphysical increases of up to 1.25 in Thomsen's anisotropic parameter epsilon (Thomsen, 1986;Allan et al, 2014) must be minimized (or ideally removed entirely) to more robustly investigate the thermal maturity dependence of elastic anisotropy in organic-rich shale. Additionally, Zargari et al (2011) conduct hydrous pyrolysis experiments on organic-rich Bakken samples; however, the retorting vessels used lacked any separation of the confining and pore spaces.…”

“…Previous attempts at this methodology (Allan et al, 2014;Yenugu, 2014) induced thermal maturation through anhydrous pyrolysis experiments without applied effective pressure (the confining pressure minus the pore pressure). The combination of rapid hydrocarbon generation and lack of effective pressure resulted in the formation of aligned throughgoing fractures and microcracks with apertures on the order of tens of microns (Kobchenko et al, 2011;Allan et al, 2014;Yenugu, 2014).…”

“…Previous attempts at this methodology (Allan et al, 2014;Yenugu, 2014) induced thermal maturation through anhydrous pyrolysis experiments without applied effective pressure (the confining pressure minus the pore pressure). The combination of rapid hydrocarbon generation and lack of effective pressure resulted in the formation of aligned throughgoing fractures and microcracks with apertures on the order of tens of microns (Kobchenko et al, 2011;Allan et al, 2014;Yenugu, 2014). This fracturing of the rock and subsequent nonphysical increases of up to 1.25 in Thomsen's anisotropic parameter epsilon (Thomsen, 1986;Allan et al, 2014) must be minimized (or ideally removed entirely) to more robustly investigate the thermal maturity dependence of elastic anisotropy in organic-rich shale.…”