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.
Of the key reservoir properties, permeability seems to be the most elusive. Since the middle of the 90s, various seismic attributes have been proposed to map permeability by using detailed analysis of the frequency content of reflected wavetrains. Some attributes are expected to show a relative increase of high-frequency content with increased permeability; other attributes assume the opposite. Actually, both these trends were observed. A possible explanation of these observations is here derived from an effective model of a permeable dual-porosity layer enclosed by impermeable rocks. For such a model, the reflected wavetrain can be regarded as a sum of three components, one of which is related to acoustic-impedance contrasts, another to extra compliance caused by P-wave-induced fluid flows between fractures and intergranular pores with a high aspect ratio, and a third to the fluid-flow-induced, inelastic attenuation. In layered reservoirs, all the components tend to be frequency dependent, and the well-known dependence of the first component on the reflecting-layer thickness may strongly dominate the effects of permeability. Hence, predicting the behavior of a permeability attribute in a particular environment requires a modeling formalism that can be called permeability substitution by analogy with the widely used fluid-substitution technique.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.