Time Lapse Shear Wave Anisotropy: A Tool for Dynamic Reservoir Characterization at Vacuum Field, New Mexico. Raúl Cabrera-Garzón, Thomas L. Davis*, Robert D. Benson, Department of Geophysics, Colorado School of Mines. Summary A carbon dioxide (CO2) flooding program consisting of a series of six injectors, was monitored at Vacuum Field. During this program, multicomponent baseline and repeat surveys were acquired providing the opportunity to dynamically characterize the San Andres reservoir. Analysis of the data at intervals above the reservoir show that the amount of observed anisotropy is small and that it can be considered as the limit of resolution for anisotropy estimations. The interpretation of these data at the reservoir level, shows a differential shear wave anisotropy anomaly that coincides with the tertiary flood bank. The results show that travel-time based attributes are a robust measurement to look for subtle differences caused by changing reservoir conditions. A comparison between the differential shear wave anomaly from the six injector program, and a differential shear wave anomaly obtained from a previous single CO2 injector program at the same area, provides support to the idea that shear wave anisotropy can be used to monitor secondary (water flooding) as well as tertiary (CO2) methods. Introduction Multicomponent, time lapse seismology has great potential for monitoring production processes in reservoirs, particularly in fractured rocks. Shear waves are much more sensitive than compressional waves to the presence of fractures or microfractures and the fluid content within the fracture network. Fractures introduce seismic anisotropy into a reservoir, causing two shear modes to propagate with different velocities and therefore different arrival times. The arrival time difference is referred to as shear wave splitting or birefringence and is a critical parameter for estimating fracture density (Martin and Davis, 1987). Vacuum Field is located 20 miles west of Hobbs on the Northwestern Shelf of the Permian Basin in the state of New Mexico. This field has produced oil from the San Andres (divided into upper and lower) and Grayburg (dolomite and sandstone) formations at an average depth of 4500 feet. The geological setting of these formations is that of the rim of a broad carbonate shelf province to the north and northwest and of a deeper intracratonic basin on the southeast and east. The static reservoir characterization at Vacuum Field established that the dominant reservoir heterogeneities include faults, fracture zones, anhydrite plugged zones linked to old fracture systems and the layered structure of the reservoir (Pranter, 1999)

The San Andres reservoir has been under production since 1940. In 1980 a water flooding program provided an increase on production from an average of 3000 BOPD to a peak average production of 17000 BOPD. After 10 years of water injection, production declined to an average of 4000 BOPD and it was decided to start frac jobs and an infill campaign. In 1995 the first CO2 injection program took place at a single well (phase VI) and in 1997 CO2 injection was expanded to a series of six injectors (phase VII) (Figure 1). Due to this tertiary recovery program, production has been projected to increase up to an average of 6000 BOPD by the year 2003. At Vacuum Field shear wave splitting is the key to monitoring production processes associated with carbon dioxide flooding. The behavior of the fluid property changes associated with CO2 flooding, gives rise to changes in the velocities of the split shear waves passing through the reservoir interval. Shear wave splitting can also be used to identify areas of anomalous reservoir pressure. Shear wave splitting and velocities are extremely sensitive to the local stress field because all rocks and especially carbonates contain incipient networks of micro fractures at a state of near-criticality (Zatsepin and Crampin, 1997). Multicomponent Seismic Data A baseline multicomponent seismic survey was acquired in December, 1997 prior to the start of injection of CO2 which began in April, 1998. The monitoring survey was conducted in December 1998, eight months after the start of injection. Large efforts were made during acquisition to reduce costs while increasing the quality of the shear wave data. Processing produced high resolution data with high signal/noise ratio by the application of prestack noise attenuation and surface consistence deconvolution. The high quality of the data was reflected in better statics control, improved velocity picking and better stack imaging. Given the interest to characterize shear wave anisotropy of vertically propagating shear waves that encounter azimuthally anisotropic media (Crampin, 1985), a four component rotation (Alford, 1986) was performed. The maximum horizontal stress direction, also known as the

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Time Lapse Shear Wave Anisotropy

natural polarization direction for fast shear waves (S1), and the direction of the minimum horizontal stress or natural polarization direction for slow shear waves (S2) were defined as 118 degrees and 28 degrees, respectively.

The final shear wave data consisted of four post-stack, Kirchhoff time migrated volumes (S1 and S2 wave from both pre- and post-CO2 injection) with a separation between traces of 55 ft. The data are sampled at 4 msec and trace length of 4 seconds. A band pass filter (6-10-25-35 Hz) was applied as a final processing step. A corridor stack from a multicomponent vertical seismic profile (VSP) was used to locate horizons in time in the seismic sections. Figure 2 shows a seismic section (inline 69) from the pre-CO2, fast shear wave volume.. The lines are some of the interpreted horizons above and at the reservoir level. Shallow Shear Wave Anisotropy Shear wave anisotropy is determined from time interval measurements made by differencing the slow (S2) and the fast (S1) shear wave isochrons computed from two particular seismic events. The differences are normalized by the fast shear wave isochrons. The measurements are sensitive to horizon time interpretation errors and therefore careful attention has to be given to the picking of shear wave seismic events. The estimates of the shallow shear wave anisotropy show that the rocks above the reservoir induce a very small amount of anisotropy on the seismic. For a time window of

approximately 700 ms the anisotropy values range from +/- 2 % and most of the area having a +/- 1 % anisotropy. Shear wave anisotropy estimates from the pre- and post-CO2 injection surveys (Figures 3a and 3b) show that the higher positive anisotropies coincide with thinning of an evaporitic interval (Mendez-Hernandez, 1999).

Such effect is reflected at the surface as small topographic depressions and the anisotropy anomalies tend to cancel each other when computing the anisotropy difference (Figure 3c). These results suggest that only anomalies due to changes in reservoir conditions will be reflected on the time lapse shear wave anisotropy observations. Some positive and negative differences are present but they occur at the edges of the seismic area where fold and signal to noise ratio decrease. Reservoir Shear Wave Anisotropy Shear wave anisotropy and anisotropy differences become much larger at the reservoir zone, the reason is the fluids (CO2, oil and water) associated with fractured intervals. The computed anisotropy differences were classified into four groups based on a combination of increases and

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Fault trace Figure 1: Basemap showing the array of water and CO2 injectors, and producers. Fault traces, interpreted from P-wave data as part of the static reservoir characterization, are overlain. Coordinate units are in feet.

Figure 2: Fast (S1) shear wave seismic section. Inline 69, running North-South and going through wells CVU-200, CVU-97 and CVU-194.

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decreases of the fast and slow shear wave velocities (Figure 4). For the cases where both S1 and S2 velocities either decreased or increased (Figures 4a and 4b), the resulting shear wave anomalies are very small and fall between the range of background anisotropy that was estimated from the anisotropy analysis above the reservoir. The combination of increasing one velocity and decreasing the other yields interesting results that spatially correlate to the zones where either CO2 injection took place (positive anisotropy differences) (Figure 4d) or oil and water were produced (negative anisotropy differences) (Figure 4c).

The CO2 anomaly matches the porosity and permeability trends in the reservoir and it also matches estimates of fracture density reported by DeVault (1997) suggesting a strong fracture control on the permeability. Carbon dioxide has a high mobility ratio and can move over relatively large distances particularly within a fracture network. This

anomaly shows that the CVU 97 well is contacted by the tertiary flood bank. Actually, breakthrough first occurred in this well in October of 1998, two months before the seismic monitoring survey was conducted. Interestingly, reservoir simulation had previously suggested it would be two years before the well would be contacted.

A comparison was made between anisotropy differences obtained from the two different CO2 injection programs at Vacuum Field (Figure 5). Magnitude and distribution of anomalies, at the center portion of the survey, are similar, indicating that CO2 causes the positive anomaly between the six injector area. The zone of negative anisotropy difference to the south of the injectors is present on both maps and correlates with the row of producers which rates of production where kept similar between surveys. On the other hand, there are large differences between anisotropy anomalies to the north that can also be explained with production information. After the post-CO2 seismic survey from phase VI was acquired, and prior to the pre-CO2

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Figure 4: Anisotropy differences for the interval Queen to Cycle1, computed as Pre- minus Post-CO2 injection. Each map shows anisotropy differences obtained from a combination of S1-S2 velocity changes. Only anisotropy differences larger than +/- 2% are displayed.

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survey from phase VII, the field experienced a decrease in average monthly production from 1996 to 1997 and then an increase from 1997 to 1998 in the northeast part of the survey area (Figure 6).

Conclusions Shear wave anisotropy has proven to be a robust method to image fluid changes within a reservoir. Monitoring of these changes gives insights into field behavior, due to characteristics such as the presence of sealing faults and bypassed oil, that are absent in predictions from static models alone.

By producing time-lapse anisotropy differences we have been able to track the tertiary flood bank and produce a spatial image of the bank. This enables us to monitor the lateral sweep efficiency of the reservoir. The economical impact of these results would be reflected on better incremental recovery without a large increase in field operation costs. These benefits can be achieved not only in mature fields as Vacuum but also in many new fields that might require a better dynamic characterization. Acknowledgments The authors thank industry sponsors of the Colorado School of Mines Reservoir Characterization Project for supporting this research. References Alford R. M., 1986. Shear data in the presence of azimuthal anisotropy: Dilley, Texas. Society of Exploration Geophysicists 56th Annual International Meeting, Expanded Abstracts. Crampin, S., 1985. Evaluation of anisotropy by shear wave splitting. Society of Exploration Geophysicists Geophysics, Volume 50, Number 1 De Vault. B., 1997. 3-D Seismic prestack multicomponent amplitude analysis, Vacuum Field, Lea County, New Mexico. PhD Thesis, Colorado School of Mines. Martin, M. A. and Davis, T. L., 1987, Shear wave birefringence: A new tool for evaluating fractured reservoirs: The Leading Edge, v. 6, no. 10, p. 22 – 28. Mendez-Hernandez, E., 1999, Influence of shallow heterogeneities on multicomponent 4D seismic data at Vacuum Field, New Mexico., Master of Engineering Report, Colorado School of Mines. Pranter, M. J., 1999, Use of a petrophysical-based reservoir zonation and multicomponent seismic attributes for improved geologic modeling, Vacuum Field, New Mexico, PhD Thesis, Colorado School of Mines. Talley, D. J., Davis, T. L., Benson, R. D., and Roche, S. L., 1998, Dynamic reservoir characterization of Vacuum Field: The Leading Edge, v. 17, no. 10, p. 1396 – 1402. Zatsepin, S. V., and Crampin, S., 1997, Modeling the compliance of crustal rock—I. Response of shear wave splitting to differential stress, Geophysical Journal International, v. 129, p. 477 – 494.

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Figure 5: Shear wave anisotropy differences from phase VI (single CO2 injector, well CVU-97) and phase VII (six CO2 injectors).

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Figure 6: Differences between the averages of oil, gas and water produced per month for the period 1996 to 1998. Oil and gas volumes were converted from standard conditions to equivalent volumes at reservoir conditions Units are barrels per month.

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