Anisotropic 4D seismic response inferred from ultrasonic laboratory measurements: A direct comparison with the isotropic response

Abstract

Pore-pressure depletion causes changes in the triaxial stress state. Pore-pressure depletion in a flat reservoir, for example, can be reasonably approximated as uniaxial compaction, in which the horizontal effective stress change is smaller than the vertical effective stress. Furthermore, the stress sensitivity of velocities can be angle-dependent.Therefore, time-lapse changes in reservoir elastic anisotropy are expected as a consequence of production, which can complicate the interpretation of the 4D seismic response. The anisotropic 4D seismic response caused by pore-pressure depletion was investigated using existing core velocity measurements. To make a direct comparison between the anisotropic 4D seismic response and the isotropic response based only on vertical velocities, pseudoisotropic elastic properties were utilized, and the two responses were compared in terms of a dynamic rock physics template. A comparison of the dynamic rock physics templates indicates that time-lapse changes in reservoir elastic anisotropy have a noticeable impact on the interpretation of 4D seismic data. Changes in anisotropy as a result of pore-pressure depletion cause a time-lapse amplitude variation with offset response as if there is a reduction in V P /V S (i.e., pseudoisotropic V P /V S decreases), although the vertical V P /V S increases. The impact of time-lapse changes in anisotropy on the amplitude variation with offset gradient was also investigated, and the time-lapse anisotropy was found to enhance changes in the amplitude variation with offset gradient for a given case.

Third‐order elasticity of transversely isotropic field shales

“…Transverse isotropy is commonly employed in the study of overburden shales (Asaka, 2023; Delle Piane et al., 2011, 2014; Dewhurst & Siggins, 2006; Hornby, 1998; Johnston, 1987; Jones & Wang, 1981; Sarout & Guéguen, 2008; Thomsen, 1986). However, to conclude, this needs to be experimentally verified.…”

“…The TOE model proposed by assumes isotropic symmetry of the third-order tensor, resulting in a strain sensitivity of the dynamic stiffness that is independent of direction. This model has been cali-brated to ultrasonic data obtained from different lithologies (Prioul & Lebrat, 2004; and has been utilized to predict the seismic response (Asaka, 2023;Herwanger & Koutsabeloulis, 2011;MacBeth et al, 2018). Apart from Duda et al (2020) and Bakk, Holt, Duda et al (2020), who studied a model with hexagonal symmetry of the TOE tensor, restricted to isotropic horizontal strains, only isotropic TOE tensors have been employed in the modelling of sedimentary rocks (Asaka, 2023;Donald & Prioul, 2015;Prioul & Lebrat, 2004;Rasolofosaon, 1998;Sarkar et al, 2003;Sinha & Kostek, 1996;Winkler & Liu, 1996).…”

“…(2004) assumes isotropic symmetry of the third‐order tensor, resulting in a strain sensitivity of the dynamic stiffness that is independent of direction. This model has been calibrated to ultrasonic data obtained from different lithologies (Prioul & Lebrat, 2004; Prioul et al., 2004) and has been utilized to predict the seismic response (Asaka, 2023; Fuck et al., 2009; Herwanger & Koutsabeloulis, 2011; MacBeth et al., 2018). Apart from Duda et al.…”

“…(2020) and Bakk, Holt, Duda et al. (2020), who studied a model with hexagonal symmetry of the TOE tensor, restricted to isotropic horizontal strains, only isotropic TOE tensors have been employed in the modelling of sedimentary rocks (Asaka, 2023; Donald & Prioul, 2015; Fuck et al., 2009; Prioul & Lebrat, 2004; Prioul et al., 2004; Rasolofosaon, 1998; Sarkar et al., 2003; Sinha & Kostek, 1996; Winkler & Liu, 1996). To our knowledge, a transversely isotropic (TI) symmetric TOE tensor has not been previously proposed for any application.…”

Stress‐induced anisotropy in Gulf of Mexico sandstones and the prediction of in situ stress

“…If the stress change is anisotropic, then the change in elastic wave velocities will also be anisotropic. Laboratory measurements of stress-dependent elastic wave velocities in sandstones have been reported, for example, by Han (1986), Holt and Fjaer (1987), Sammonds et al (1989), Sayers et al (1990), Freund (1992), Scott et al (1993), Winkler and Liu (1996), Khaksar et al (1999), Khazanehdari and McCann (2005), Bathija et al (2009) and Asaka (2022).…”