Applications of VP/VS and AVO modeling for monitoring heavy oil cold production
Duojun (Albert) Zhang
Simultaneous extraction of oil and sand during the heavy oil cold production generates high porosity channels termed "wormholes". The development of wormholes causes reservoir pressure to fall below the bubble point, resulting in dissolved-gas coming out of solution to form foamy oil. Both foamy oil and wormholes are believed to be two key factors in the enhancement of cold heavy oil production. In enhanced oil recovery, it is important to map cold production reservoir changes due to wormholes and foamy oil. It is the purpose of this thesis to use seismic monitoring methods to map cold production footprints.
The presence of small amounts of gas trapped in the foamy oil can dramatically decrease the fluid bulk modulus, reducing the P-wave velocity of saturated sands, while slightly increasing the S-wave velocity. The Vp/Vs ratio and Poisson's ratio have a subsequent reduction.
The viscosity of heavy oil is primarily a function of oil gravity and temperature. Increasing the temperature will decrease sample's viscosity, causing both bulk and shear moduli to decrease approximately linearly with increasing temperature. Moreover, the frequency also plays an important role for seismic waves in heavy oil. For heavy oil in the 10-20 API range at ambient temperature of 20 oC, the shear modulus is negligible and heavy oil still acts like a liquid at seismic frequencies, especially after cold production. Gassmann's equation can still help us understand the seismic response of heavy oil reservoirs for pre- and post- cold production.
The Vp/Vs ratio is a function of both fluid bulk modulus and porosity. For unconsolidated sands with high porosity, pore fluids have a significant influence on final Vp/Vs ratio. Due to the dramatic reduction of fluid's bulk modulus after heavy oil cold production, the Vp/Vs ratio will have a detectable reduction, even though the increasing porosity from wormholes slightly increases the Vp/Vs ratio. For unconsolidated sands, the lower pore pressure and increasing differential pressure will also tend to decrease the final Vp/Vs ratio.
Interpreting multicomponent seismic data to get Vp/Vs ratio maps from traveltime measurements on vertical and radial component data is straightforward. Error analysis and practical mapping tell us that the calculated Vp/Vs ratio will not be overly sensitive to the choice of picking surrounding formations. Traveltime interval mapping of Vp/Vs ratio provides a robust method for us to monitor the reduction of Vp/Vs ratio due to heavy oil cold production. Although traveltime picking is relatively insensitive to spectral differences between components, bandpass filtering can provide some improvement to the quality of final Vp/Vs ratio map, by enhancing the similarity between PP and PS seismic volumes.
The difference of Poisson's ratio between pre- and post-production will create different AVO responses. The calculated result from fluid substitution reveals that there is about 10% reduction of P-wave velocity, about 30% reduction of saturated bulk modulus and about 20% reduction of Poisson's ratio due to heavy oil cold production. Further calculations indicate that there is about 20% reduction of the Vp/Vs ratio after heavy oil cold production. Meanwhile, there is no detectable difference between the pre-production and the wet case. Hence, we cannot readily use Vp/Vs ratio and AVO analysis to differentiate heavy oil and brine saturated sands.
Synthetic seismograms from the results of fluid substitution reveal that all the AVO responses for pre- and post-production and the wet case belong to Class IV AVO anomalies, as described by Castagna et al. (1998). The AVO response for post-production is separated from the other two cases. Although using the product of intercept and gradient is difficult to discriminate Class IV AVO responses, the fluid factor is useful to interpret Class IV AVO response. Because Vp/Vs ratios vary with time, a calibrated time varying gain function g(t) will give a better estimate of the fluid factor for the target zone.
For the in-situ well, four methods to do fluid substitution are performed, one of them using available S-wave sonic log data, others not using available S-wave sonic log data. The Greenberg-Castagna calculation gives the closest calculated S-wave log data to the actual S-wave log data with using available original S-wave log data. Assuming Castagna's equation is correct for the wet case, the calculations give a relatively small S-wave velocity, while assuming dry rock Poisson's ratio, the calculations give a relatively high S-wave velocity. But overall, all of the methods give the similar AVO response from the top of the target zone, which are Class IV AVO responses, and the AVO responses for post-production are separated from other two cases.