Robert D. Benson

Multicomponent seismic characterization and monitoring of the CO2 flood at Weyburn Field, Saskatchewan

High-resolution, time-lapse (4D), multicomponent (9C) seismic monitoring of a tertiary oil recovery project is being conducted by the Reservoir Characterization Project of the Colorado School of Mines. The project involves a miscible CO2 flood in a thin (30 m) carbonate reservoir at 1450 m depth in Weyburn Field, Saskatchewan. Multicomponent time-lapse seismic images illustrate the influence of fracture zones on the flood. These fracture zones are very important to characterize and monitor in order to manage the flood successfully and to achieve the desired improved recovery efficiency. The monitoring project offers insights into the future of dynamic reservoir characterization with 4D, 9C seismic technology.

Multicomponent 3-D characterization of a coalbed methane reservoir

Methane is produced from fractured coalbed reservoirs at Cedar Hill Field in the San Juan Basin. Fracturing and local stress are critical to production because of the absence of matrix permeability in the coals. Knowledge of the direction of open fractures, the degree of fracturing, reservoir pressure, and compartmentalization is required to understand the flow of fluids through the reservoir. A multicomponent 3-D seismic survey was acquired to aid in coalbed methane reservoir characterization. Coalbed reservoir heterogeneities, including isolated pressure cells, zones of increased fracture density, and variable fracture directions, have been interpreted through the analysis of the multicomponent data and integration with petrophysical and reservoir engineering studies. Strike-slip faults, which compartmentalize the reservoir, have been identified by structural interpretation of the 3-D P-wave seismic data. These faults form boundaries for pressure cells that have been identified by P-wave reflection amplitude anomalies. The analysis of polarizations, traveltimes, and reflection amplitudes from the shear-wave seismic data has allowed the identification of zones of variable fracture direction and fracture density. There is good agreement between stresses inferred from the structural interpretation and those indicated by the shear-wave polarizations. Reflection amplitudes have been calibrated to seismic velocities and reservoir pressures through the use of petrophysical data taken from core samples. New methods have been developed for the statistical analysis of prestack shear-wave polarizations, poststack polarizations, and the accurate determination of traveltime anisotropy. The prestack polarization analysis method allows for rapid and efficient determination of a dominant polarization direction. Shear-wave anisotropy has been quantified over the reservoir zone using both traveltime and thin-bed reflection response with excellent agreement between the two methods. Crack densities computed from the anisotropy show two regions of high crack density, one coinciding with a sealed overpressured cell and the other in the region of the Hamilton #3 well. This indicates the potential for monitoring production of coalbed methane reservoirs using multicomponent seismology.

Dynamic reservoir characterization of Vacuum Field

This article briefly describes the 4-D, 3-C technique and a research application of it at Vacuum Field, New Mexico, by the Reservoir Characterization Project (RCP) at the Colorado School of Mines. RCP, a consortium of 32 oil companies, has been involved in 4-D, 3-C research since 1995 because this technique has the potential to provide the most complete information needed for efficient reservoir characterization. For example, when three components of source are used, nine times the data of a conventional 3-D P-wave survey are recorded.

Morrow sandstone reservoir characterization; a 3-D multicomponent seismic success

A 3-D, 3-C seismic survey over a clastic valley‐fill reservoir in the Pennsylvanian Morrow Formation at Sorrento Field, Colorado, could mark a major turning point in the use of seismic technology for reservoir characterization in this area.

Time-lapse Analysis And Detection of Fluid Changes At Vacuum Field, New Mexico

Carbon dioxide (CO2) injection is currently used as a tertiary recovery method in the Permian San Andres Formation, Central Vacuum Unit, Lea County, New Mexico. Multicomponent, 3D seismic surveys were conducted prior to and 8 months after carbon dioxide injection in an attempt to monitor the fluid front. Poststack cross-equalization techniques were applied to the time-lapse data volumes to enhance the interpretability of the compressional and shear wave velocity and amplitudes, and their variation over time. Fluid porperties for a carbonate rock invaded by CO2 displacing oil suggest a complex interplay between density and compressibility of the mix which is mostly governed by pressure conditions. Time-lapse, crossequalized images also reveal a complex behaviour of the rock-fluid interaction with areas of decrease in shear wave velocities surrounding high fluid pressure, CO2 injectors, and areas of S2 velocity increase surrounding low fluid pressure producer wells. This result, validated by time-lapse amplitude anomalies, has important consequences for shear waves in the detection of fluid changes associated with CO2 flooding.

9C, 4D Seismic Processing For the Weyburn CO2 Flood, Saskatchewan, Canada

We present a summary of the 9C, 4D processing used for the seismic monitoring of a CO2 flood in the Weyburn Field, Saskatchewan, Canada. The resultant time-lapse anomalies for both the Pand S-wave volumes are coincident with the locations of the CO2 injection patterns. Furthermore, the anomalies that we observe are extremely robust and are typically observed far before the final processed sections were produced. That is, they are evident on the differenced brute stacks. We believe this is largely due to the similarity of source and receiver locations during the monitor survey as compared to the baseline survey. The primary prestack processing steps include appropriate shot and receiver edits, source-related phase corrections, independent refraction statics, common velocity models (between baseline and monitor), independent residual statics, and a common pilot for trim statics. A post-migration cross-equalization algorithm is used prior to analysis of the differenced volumes.

Monitoring production processes by 4‐D multicomponent seismic surveys at Vacuum Field, New Mexico

Multicomponent, time lapse seismology has great potential for monitoring production processes in reservoirs. The reason is simply the presence of fractures. Shear waves are much more sensitive than p 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 (see Martin and Davis, 1987).

Seismic Reservoir Characterization of the Morrow A Sandstone, Postle Field, Oklahoma

Three multicomponent (9-C) seismic surveys were conducted at Postle Field, Oklahoma. Interpretation of the surveys illustrates that the Morrow “A” sandstone can be detected. The sandstone was previously considered acoustically invisible, yet the combination of multicomponent and time-lapse seismic data has enabled us to detect the reservoir, with average thickness OF 38 ft (8.5 m) buried beneath a complex overburden at 6100 ft (1850m) depth. Even though the sandstone is thin, it has a greater elastic impedance contrast than acoustic impedance contrast. We have found that shear wave data enables reservoir mapping of at least half the minimum thickness seen on P-wave data. This is because the shear wave reflectivity contrast between the sandstone and adjacent shale is three times that of P-wave, thus enabling higher definition of the thin sandstone reservoir with shear wave data. Dynamic changes introduced by water and carbon dioxide flooding enables further delineation of the sandstones in the shale-dominated interval.

Tight-gas seismic monitoring, Rulison Field, Colorado

Time-lapse multicomponent seismic monitoring of pressure depletion in tight-gas development can be used to increase recovery efficiency. Tight-gas plays require technology to enhance the economic viability of resource development. Our study at Rulison Field, Colorado, confirms that time-lapse, multicomponent seismic surveys can monitor tight-gas depletion and be used for well planning and completion design.

An onshore time‐lapse (4‐D), multicomponent, data processing case history, vacuum field, New Mexico

An onshore time-lapse (4-D), multi-component (3-C), seismic data survey was acquired, processed and interpreted in conjunction with a CO2 Huff-n-Puff Enhanced Oil Recovery (EOR) project at Vacuum Field, New Mexico, USA. The CO2 injection program induced bulk rock property variations due to fluid saturation and pore pressure changes associated with reservoir production processes. Acquisition of the time-lapse (4-D), multicomponent data included re-occupying the vibrator source and receiver locations to approximately +/2 meters. Data acquisition of the surface seismic data was augmented with the simultaneous recording of a single, 3-C geophone, positioned in a wellbore centrally located in the survey area at a depth of 1000 meters. A data processing methodology for time-lapse, multi-component, onshore seismic survey was developed and applied to the P-wave and S-wave data volumes. Significant features of the data processing method are application of surface consistent processes, windowed cross-correlation of common-trace pairs between the initial and repeat surveys with subsequent decomposition to source and receiver components, and the use of information from the downhole 3-C geophone to constrain P-wave and S-wave processing parameters.