Prediction of pore fluid viscosity effects on p-wave attenuation in reservoir sandstones

Seismic wave attenuation (absorption or intrinsic attenuation) is underutilized by the petroleum industry mainly because of the difficulty of measuring it with sufficient accuracy using seismic reflection methods. However, the recent trend toward time-lapse 3D seismic monitoring of reservoirs means that absolute attenuation measurements are no longer so important. Instead, temporal changes in seismic amplitude and wavelet frequency content could hold the key to interpreting changes in reservoir fluid properties in response to certain production strategies. Seismic monitoring of heavy-oil reservoirs would be particularly appropriate because of the known link between seismic attenuation and pore fluid viscosity (Batzle et al., 2006a). Seismic wave attenuation in porous rocks is thought to be dominated by viscous fluid flow mechanisms such as those described by the unified Biot and Squirt (BISQ) model (Dvorkin and Nur, 1993; Dvorkin et al., 1994). By definition, the magnitude of attenuation is related to the viscosity of the pore fluid and the frequency of the elastic wave, as well as to other pore fluid parameters (density, bulk modulus) and pore geometry parameters (porosity, permeability, tortuosity, squirt flow length). This theoretical link between attenuation and pore fluid viscosity could be exploited if changes in heavy-oil viscosity caused by thermal stimulation, typically in the range 1–1000 cP (1 cP 1⁄4 10 3 PaAEs), give rise to sufficiently large changes in elastic wave attenuation for detection and monitoring by surface seismics. Also, an accurate model of frequency/viscositydependent attenuation as a function of lithology could help tie in sonic well logs at frequencies of 10–20 kHz to surface seismics at less than 200 Hz for the benefit of reservoir characterization. In this chapter, a laboratory ultrasonic data set of Pwave velocity and attenuation measured as a function of pore fluid viscosity in three reservoir sandstones is compared to the BISQ model. The three sandstones (Elgin, Berea, Northern North Sea) represent a typical spread of porosity, permeability, and intrapore clay contents for reservoir sandstones (see Table 1). The importance of clay content for increasing ultrasonic attenuation in sandstones is well established for P-waves (Klimentos and McCann, 1990) and S-waves (Best et al., 1994), but in the absence of a validated elastic wave model, it is difficult to predict how this behavior will manifest itself on surface seismic data sets. Hence, there is a need for careful crosschecking of theoretical models against well-constrained experimental data sets. The BISQ model is a strong candidate for further investigation because it uses input parameters that are readily available to the reservoir engineer or petrophysicist. The only ambiguity in the BISQ model concerns the value of the characteristic squirt flow length. However, in practice, this is found by matching model outputs to velocity or attenuation measurements at a given frequency (Dvorkin and Nur, 1993). The resultant calibrated model can then be used to explore frequencyand viscosity-dependent velocity and attenuation effects for a given lithology. After determining the BISQ squirt flow lengths for the three sandstones reported here, there was reasonable agreement between predicted and observed attenuation magnitudes. The BISQ model was then used to predict the likely sonic and seismic attenuation magnitudes for each sandstone as a function of pore fluid viscosity. The results indicate that significant attenuation changes due to thermal stimulation may only be detectable in clay-rich sandstones with low permeabilities at seismic and sonic logging frequencies.