Cross-Well Seismic Imaging

Cross-well seismic imaging is a borehole approach that uses a seismic source located in one well and a receiver array located in an adjacent well. This method can deliver important information about formation properties such as acoustic velocity, seismic reflectivity, or electromagnetic resistivity between wells. A resulting velocity map between wells is referred to as a tomogram. Cross-well surveys require wellbore access, and careful planning is required in order to coordinate survey activities with other monitoring activities.¹

Cross-well Seismic Imaging Summary

Description: Seismic geophysical methods use acoustic energy to image the subsurface. Differences between the acoustic properties of CO₂ and other fluids enable plume monitoring by seismic methods. Active seismic methods, such as cross-well imaging, require a source and receiver.
Benefits: Substitution of CO₂ for brine under many conditions creates a strong change in seismic velocity ideal for time-lapse quantification from pre-injection baseline (brine-filled) pores to pores partly filled with CO₂. Reflection seismic under the right conditions is useful both for time-lapse monitoring of a CO₂ plume and for identification of any out-of-zone CO₂ accumulation indicating a release. Borehole seismic surveys, such as cross-well imaging, can provide higher-resolution imaging near or between wellbores.
Challenges: The presence of gas in baseline fluids can reduce detection of CO₂. Borehole seismic methods require a wellbore for monitoring, and for cross-well imaging, the distance between wells containing the source and receivers may limit success of the survey due to source strength constraints.²

The sources used in cross-well seismic surveys are deployed at selected depth, while the receivers are placed in the nearby well.³ Multiple receivers can be linked together in a chain and deployed in the borehole, as in these two photographs from the Ketzin Storage Project in Germany.⁴

Application

The Citronelle Project demonstrated a fully integrated, pulverized coal-fired carbon capture with saline storage. Carbon dioxide was captured from a coal burning unit at James M. Barry Electric Generating Plant, compressed and delivered through a ~ 12-mile pipeline to the storage location southeast of Citronelle, Alabama. The CO₂ was injected into a saline formation that overlies the oil production horizon of the Citronelle Oil Field, demonstrating saline storage in conjunction with oil field operations.⁵

Location of the Citronelle Project. The injection site is in the area of the Citronelle Dome, the structure responsible for trapping oil at the Citronelle Oil Field.⁶

The Citronelle team performed a cross-well survey and constructed a time-lapse image between an injection well and an observation well. The velocity difference image shows regions where seismic velocity changed over time. The time-lapse difference image indicates a decrease in seismic velocity in the upper injection zone of as great as 3 percent, suggesting an increase in CO₂ saturation. Negative velocity anomalies are not observed in or above the confining unit, implying no detectable leakage out of the injection zone.⁷

Time-lapse velocity difference showing a velocity anomaly corresponding to the approximate location of the CO₂ plume in June of 2014. Track displayed to the right is the injection well, whereas the track to the left is the observation well. The injection zone and confining zone are annotated on the diagram. Negative velocity differences are in green and blue and suggest the presence of CO₂. Yellow correlates to no significant negative velocity anomalies, indicating no CO₂ leakage into or beyond the confining zone.⁸

Image Credits

cross well 1: R. Giese
cross well 2: R. Giese