Pressure‐dependent seismic response of fractured rock

Abstract

Effects of external stress and pore pressure variations on the seismic signature of fractured rocks remain of interest to geoscientists and practicing geophysicists. Commonly, the effects are modeled theoretically, assuming fracture faces to be rough surfaces contacting each other via the surface asperities. The model proposed here differs from other models of this kind in that (1) fracture roughness is described by a single parameter and (2) a controlled degree of hydraulic connectivity between fractures and equant pores is introduced. This adds to the model's convenience and makes it applicable to a wide variety of reservoirs. The model predictions of seismic velocities in fractured rock at variable stress are consistent with experimental data. For fixed effective stress, the model predictions coincide with those obtained using the model with ellipsoidal fractures of certain average aspect ratio and the same fracture porosity. Apart from known effects, the model introduced predicts an amplification of the stress variation influence on fracturing‐induced anisotropy with an increase of connected equant porosity, a decrease of VP/VS with effective stress, and implicit frequency dependence of the VP/VS relation. It is also shown that amplitude versus offset (AVO) anomalies caused by fluid replacement can be seriously distorted if the fluid replacement is accompanied by significant variations of pore pressure, as, for example, at intense gas production. Neglecting these effects can lead to erroneous conclusions on shear modulus dependence on the pore fluid type. Qualitatively, in rocks with azimuthally aligned fracturing, the increase of effective stress affects AVO gradient in about the same way as the increase of water saturation parameter Vw. In contrast, the AVO intercept is not affected by variations of effective stress, while fluid replacement effect on the intercept is significant. Potentially, this can help distinguish the effects of pore pressure variations and fluid replacement on the AVO attributes.

Numerical study of the anisotropy of wave responses from a fractured reservoir using the grid-characteristic method

“…We will maintain the distance of the fractured reser voir constant, but the number of the cracks will be varied. Figures 7-9 show the fields of speeds for various densities of crack location in the reservoir (N = 3,4,5,7,9,13,21). Figure 10 shows the anisotropy of the response depending on the number of cracks.…”

“…For the mathematical modeling of seismic responses from frac tured hydrocarbon containing geological rocks, averaged models or models with effective coefficients of the medium are conventionally used in seismology [2][3][4][5][6][7]. This makes it necessary to introduce some empirical coefficients (e.g., coefficients of rock yielding [3][4][5][6]), additional hypotheses (e.g., the hypoth esis of linear sliding [3]). In our work, we investigate the possibility of complete numerical modeling of wave fields in the rocks with cracks based on the system of deformed solid mechanics equations, without introducing any empirical parameters.…”

Pressure-dependent seismic velocities based on effective compliance theory and an asperity deformation model

“…Furthermore, they proposed the relation between stress and stiffness is nonlinear, and they showed that their theory is in good consistency with laboratory experiments (Yoshioka and Scholz, 1989b). Kozlov (2004) derives the normal and tangential compliances for dry fractures based on spherical contact Hertzian theory and by assuming regularly distributed asperity contacts separated by equal distance. Sevostianov and Kachanov (2008) apply and extend the results of Mindlin (1949) on normal and tangential compliances, which are described by the shape factor of the asperity contacts beside some the Young's modulus, shear modulus, and Poisson's ratio of the contact asperities.…”

“…King et al (1997) measured pressuredependence of the acoustic wave propagation in fractured rocks with aligned fractures and report the correlations between permeability and seismic velocities under a varying stress environment. Kozlov (2004), Duffaut and Landrø (2007), and Herwanger and Horne (2009) studied anisotropic seismic velocity changes caused by stress variations and their influences on reservoir monitoring and time-lapse AVO analysis. Among many other works, these studies demonstrate the importance of an accurate model that can effectively reproduce the velocity-stress relation for fractured rocks.…”

High-performance computer simulation of wave processes in geological media in seismic exploration

“…The mathematical simulation of seismic responses of fractured rock with hydrocarbon reservoirs is tra ditionally based on averaged models or models with effective medium coefficients [2][3][4][5][6][7], which imply the introduction of certain empirical coefficients (e.g., rock compliance coefficients [3][4][5][6]) and additional hypotheses (e.g., of linear slip [3]). In this paper, we explore the possibility of full numerical simulation of wave fields in fractured rock based on solid mechanics equations without introducing any empirical parameters.…”