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

Abstract

The evolution of the elastic properties of organic-rich shale as a function of thermal maturity remains poorly constrained. This understanding is pivotal to the characterization of source rocks and unconventional reservoirs. To better constrain the evolution of the elastic properties and microstructure of organic-rich shale, we have studied the acoustic velocities and elastic anisotropy of samples from two microstructurally different organic-rich shales before and after pyrolysis-induced thermal maturation. To more physically imitate in situ thermal maturation, we performed the pyrolysis experiments on intact core plugs under applied reservoir-magnitude confining pressures. Iterative characterization of the elastic properties of a clay-rich, laminar Barnett Shale sample documents the development of subparallel to bedding cracks by an increase in velocity sensitivity to pressure perpendicular to the bedding. These cracks, however, are not visible through time-lapse scanning electron microscope imaging, indicating either submicrometer crack apertures or predominant development within the core of the sample. At elevated confining pressures, in the absence of pore pressure, these induced cracks close, at which point, the sample is acoustically indistinguishable from the prepyrolysis sample. Conversely, a micritic Green River sample does not exhibit the formation of aligned compliant features. Rather, the sample exhibits a largely directionally independent decrease in velocity as load-bearing, pore-filling kerogen is removed from the sample. Due to the weak alignment of minerals, there is comparatively little intrinsic anisotropy; further, due to the relatively directionally independent evolution of velocity, the evolution of the anisotropy as a function of thermal maturity is not indicative of aligned compliant features. Our results have indicated that horizons of greater thermal maturity may be acoustically detectable in situ through increases in the elastic anisotropy of laminar shales or decreases in the acoustic velocities of nonlaminar shales, micritic rocks, or siltstones.

Influence of In Situ Pyrolysis on the Evolution of Pore Structure of Oil Shale

“…Based on the condition of the in situ pressure, Nottenburg et al [46] analyzed the change in mechanical properties of oil shale during pyrolysis and found that 380 • C is a turning point for the deterioration of mechanical properties of Green River oil shale. Allan et al [47] described the pore structure change of oil shale qualitatively using a scanning electron microscope (SEM), and found that the lithostatic pressure caused partial pores to be compressed associated with the pyrolysis products escaping from oil shale. Eseme et al [15] found that the increasing temperature results in loss of strength and decrease in Young's modulus, and the response is correlated with organic matter content.…”

Insights into Fractures and Fabric of Shales from Microseismicity

“…Sixth EAGE Shale Workshop 28 April -1 May 2019, Bordeaux, France One unresolved question is to what degree VTI anisotropy in shales can be explained by CPO and what effect horizontally aligned cracks and pores might contribute. Recent rock physics modelling and laboratory investigations suggests that the conversion of load bearing kerogen to oil and gas during thermal maturation of laminated organic rich shales effectively results in an increase in horizontally aligned pores, and thus an enhancement of the VTI anisotropy (Allan et al, 2016;Carcione & Avseth, 2015). In the Fort Simpson shale Baird et al (2017), used modal proportions of minerals which a range of plausible textures to conclude that the anisotropy could be explained by CPO of clay minerals.…”

The Role of Texture, Cracks, and Fractures in Highly Anisotropic Shales

“…To this point we have focused on estimating the magnitude of the VTI anisotropy of the overburden, without consideration of its causes. The primary factors that can contribute to seismic anisotropy in sedimentary rock are the intrinsic anisotropy due to lattice-preferred orientation (LPO) of anisotropic minerals (e.g., Valcke et al, 2006;Vernik & Liu, 1997) and extrinsic anisotropy due to aligned fractures, cracks, pores, and grain boundary contacts, along with their infilling material (e.g., Allan et al, 2016;Kendall et al, 2007;Sayers, 2005). Figure 7 shows estimates of the modal fraction of minerals along one of the boreholes derived from X-ray fluorescence data as it passes from the Fort Simpson into the Muskwa formation, a contact which is marked by a sharp decrease in anisotropy parameters (both and , Figure S1b).…”

“…To this point we have focused on estimating the magnitude of the VTI anisotropy of the overburden, without consideration of its causes. The primary factors that can contribute to seismic anisotropy in sedimentary rock are the intrinsic anisotropy due to lattice‐preferred orientation (LPO) of anisotropic minerals (e.g., Valcke et al, ; Vernik & Liu, ) and extrinsic anisotropy due to aligned fractures, cracks, pores, and grain boundary contacts, along with their infilling material (e.g., Allan et al, ; Kendall et al, ; Sayers, ).…”

“…In practice, however, there is usually some sedimentary fabric that the fractures are overprinting producing an orthorhombic symmetry (Baird et al, ; Verdon et al, ) (i.e., three orthogonal symmetry planes). Alternatively, horizontal fractures, microcracks, or compliant pores may be present that would simply enhance the already existing VTI anisotropy (Allan et al, ; Sayers, ).…”