Gennady Goloshubin

Seismic low-frequency effects in monitoring fluid-saturated reservoirs

There is a complex relationship between seismic attributes, including the frequency dependence of reflections and fluid saturation in a reservoir. Observations in both laboratory and field data indicate that reflections from a fluid-saturated layer have an increased amplitude and delayed traveltime at low frequencies, when compared with reflections from a gas-saturated layer. Comparison of laboratory-modeling results with a diffusive-viscous-theory model show that low (<5) values of the quality factor Q can explain the observations of frequency dependence. At the field scale, conventional processing of time-lapse VSP data found minimal changes in seismic response of a gas-storage reservoir when the reservoir fluid changed from gas to water. Lowfrequency analysis found significant seismic-reflectionattribute variation in the range of 15‐50 Hz. The field observations agree with effects seen in laboratory data and predicted by the diffusive-viscous theory. One explanation is that very low values of Q are the result of internal diffusive losses caused by fluid flow. This explanation needs further theoretical investigation. The frequencydependent amplitude and phase-reflection properties presented in this paper can be used for detecting and monitoring fluid-saturated layers.

Reservoir imaging using low frequencies of seismic reflections

There are numerous laboratory and field examples in which low-frequency components of reflected seismic waves show surprising imaging capabilities. Ironically, such components are often filtered out as useless in conventional data processing. However, as we demonstrate below, this part of the signal contains the most important information about the reservoir.

Poroelastic analysis of amplitude-versus-frequency variations

Although significant advancement has occurred in the interpretation of seismic amplitude-variation-with-offset (AVO) anomalies, a theory is lacking to guide the interpretation of frequency-dependent seismic anomalies. Using analytic equations and numerical modeling, we have investigated characteristics of the normal-incident reflection coefficient (NI) as a function of frequency at an interface between a nondispersive medium and a patchy-saturated dispersive medium. Because of velocity dispersion, the variation of NI magnitude is divided into three general classes. These classes are (1) low-frequency dim-out reservoirs, in which NI magnitude decreases toward lower frequencies; (2) phase-shift reservoirs, in which NI is a small negative value at low frequencies but becomes positive at higher frequencies; and (3) low-frequency bright-spot reservoirs, in which NI magnitude increases toward lower frequencies. This classification could provide insight for frequency-dependent seismic interpre-tation.

Reservoir permeability from seismic attribute analysis

In case of porous fluid-saturated medium the Biots poroelasticity theory predicts a movement of the pore fluid relative to the skeleton on seismic wave propagation through the medium. This phenomenon opens an opportunity for investigation of the flow properties of the hydrocarbon-saturated reservoirs. It is well known that relative fluid movement becomes negligible at seismic frequencies if porous material is homogeneous and well cemented. In this case the theory predicts an underestimated seismic wave velocity dispersion and attenuation. Based on Biots theory, Helle et al. (2003) have numerically demonstrated the substantial effects on both velocity and attenuation by heterogeneous permeability and saturation in the rocks. Besides fluid flow effect, the effects of scattering (Gurevich, et al., 1997) play very important role in case of finely layered porous rocks and heterogeneous fluid saturation. We have used both fluid flow and scattering effects to derive a frequency-dependent seismic attribute which is proportional to fluid mobility and applied it for analysis of reservoir permeability.

Seismic low-frequency effects from oil-saturated reservoir zones

We consider the frequency dependence of seismic reflections from a thin (compared to the dominant wavelength), fluid saturated reservoir for the cases of oil and water saturation. Reflections from a thin, water or oil-saturated layer have increased amplitude and delayed travel time at low frequencies if compared with reflections from a gas-saturated layer. This effect was observed for both ultrasonic lab data and seismic field data. One set of field data revealed high correlation of low frequency processed image for two different production horizons represented by fractured shale and sandstone. Another set was processed for the purpose of contouring of oil/water contact, and reveal very good correlation with available well data. The frequency dependent amplitude and phase reflection properties can be used for detecting and monitoring thin liquid saturated layers.

Seismic low‐frequency effects from fluid‐saturated reservoir

The normal incidence of Pelastic wave reflected from a thin porous dry and water saturated layer is considered. The effect of stronger reflections and travel time delays from water saturated layer is observed at low frequencies. We compared the results of laboratory modeling with “frictional-viscous” theoretical model and found that low (< 5) values of attenuation parameter Q and its approximate proportionality to frequency can explain the effect. The values of Q were determined in a separate experiment using recordings of a transmitted field for a thick porous layer, where water saturated layer had attenuation about two times higher then in a dry layer. These findings can be used for detecting and monitoring liquid saturated areas in thin porous layers.

Using frequency‐dependent seismic attributes in imaging of a fractured reservoir zone

Author(s): Goloshubin, Gennady; Silin, Dmitry | Abstract: Normal reflection from a fractured reservoir is analyzed using frequency-dependent seismic attributes. Processing of 3D low-frequency seismic data from a West-Siberian reservoir produced an accurate delineation of the fracturedhydrocarbon-bearing zones. P-wave propagation, reflection and transmission at an impermeable interface between elastic and dual-porosity poroelastic media is investigated. It is obtained that the reflection and transmission coefficients are functions of the frequency. At low frequencies, their frequency-dependent components are asymptotically proportional to the square root of the product of frequency reservoir fluid mobility and fluid density.

Pressure diffusion waves in porous media

Pressure diffusion wave in porous rocks are under consideration. The pressure diffusion mechanism can provide an explanation of the high attenuation of low-frequency signals in fluid-saturated rocks. Both single and dual porosity models are considered. In either case, the attenuation coefficient is a function of the frequency.

Poroelastic analysis of permeability effects in thinly layered porous media

The mechanism of wave-induced fluid flow, which causes significant attenuation and dispersion of seismic waves in heterogeneous porous media, opens a potential opportunity to estimate fluid-mobility properties from seismic data. However, the permeability effects on seismic data vary for different reservoir environments, so rules of thumb are difficult to establish. Two types of reservoir models were selected. One represents shallow, unconsolidated, low-impedance reservoirs. The other represents deep, consolidated, high-impedance reservoirs. Using analytic equations and wave-propagation numerical studies, permeability effects were investigated on the magnitude and phase of normal-incident reflection coefficients from an interface between an upper nondispersive medium and a lower thinly layered porous medium. The study found that permeability can affect reflection amplitudes significantly. For high-impedance reservoirs, magnitude of the reflection coefficient decreases when permeability increases. However, for low-impedance reservoirs, reflection magnitude increases as permeability increases.

Frequency-dependent seismic reflection from a permeable boundary in a fractured reservoir

Summary The coefficients of normal reflection and transmission of a planar p-wave from a permeable boundary in a fractured reservoir are studied in low-frequency range. The coefficients are expressed as power series with respect to a small dimensionless parameter, which is the product of the reservoir fluid mobility and density, and the frequency of the signal. The coefficients of such series are functions of mechanical properties of the reservoir rock and fluid. The zero-order terms of the reflection and transmission coefficients have a form similar to the one derived from the classical theory, without accounting for fluid flow. There is a strong similarity to Gassmans effective medium model. The next power expansion term both for reflection and transmission coefficients is proportional to the square root of the small parameter described above. Unlike frequencydependent terms, the zero-order coefficient does not depend on the permeability contrast. The functional structure of the reflection and transmission coefficients provides opportunities for seismic inversion and suggests new seismic attributes.