Surface Seismic Survey

Both 2-D and 3-D seismic surveys use a seismic source and a set of receivers located at the surface to infer properties of the subsurface using seismic reflections. 2-D seismic surveys provide information about the subsurface along a single depth profile. Whereas, 3-D seismic surveys provide information about the subsurface within the volume underneath the seismic grid.¹

Two-dimensional seismic surveys deploy the receivers in a single line, send seismic waves into the ground, and record the seismic reflections that come back. Three dimensional seismic surveys use the same technique, but instead of arranging receivers in a single line, they deploy a grid layout (seismic array) to measure seismic reflections in a plane at the surface.²

Both 2-D and 3-D seismic surveys are frequently part of monitoring programs because they provide data for large regional volumes of the subsurface. Multiple seismic surveys can be collected over time; and the change from one survey to the next represents a fourth dimension: time. These repeat seismic surveys are termed “time-lapse” or “4-D” seismic surveys. The 4-D seismic technique can shed light on the changing reservoir environment through time, particularly with regard to the movement of CO₂. For example, the presence of CO₂ will change the fluid properties within the reservoir, which shows up as a change in the seismic amplitude between the baseline (pre-injection) survey and later operational surveys. As a result, areas of the reservoir that contain CO₂ will have a greater amplitude difference in the operational 4-D seismic survey. The magnitude of the amplitude difference will be proportional to the amount of CO₂.³

Surface Seismic Reflection 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 surface seismic reflection, 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. Surface seismic surveys can assess large areas and large thicknesses completely (as compared to point measurements).
Challenges: Repeatability of seismic survey needed for time-lapse surveys may be difficult under varying surface conditions. Geologic complexity and a noisy recording environment can degrade or attenuate surface seismic data and the presence of gas in baseline fluids can reduce detection of CO₂. Surface seismic data generally have lower spatial resolution than borehole seismic data and may not image thin zones.⁴

Application

A seismic reflection survey (3-D survey) can be used for site characterization prior to injection and can serve as the baseline against which repeat surveys can provide time-lapse monitoring (4-D surveys). Changes in reflectivity between surveys can be interpreted as the result of the migration of a CO₂ plume in the subsurface, and in some cases increase in pressure. Analysis of time-lapse 3-D seismic surveys is a well-established oil industry tool, so developments for geologic storage to some extent track oil industry practice.⁵

These techniques were applied to monitoring of CO₂ injection as part of a study of storage associated with a large-scale commercial enhanced oil recovery project at the Denbury-operated Bell Creek Oil Field in Montana. Carbon dioxide is delivered via pipeline from two gas processing plants in Wyoming, where it is separated out during natural gas refinement.⁶

Map depicting the location of the Bell Creek oil field in relation to the Powder River Basin and the 232-mile Greencore pipeline route to the Bell Creek oil field. CO₂ is sourced from the ConocoPhillips-operated Lost Cabin Gas Plant and the ExxonMobil Shute Creek Gas Plant in LaBarge, Wyoming.⁷

Multiple surveys have been conducted over time and provide a means of tracking the progression of the CO₂ plume in the reservoir, which has helped better manage injection operations, as well as the timing and location of other monitoring techniques.⁸

4-D seismic map showing change in amplitude (Δamp) of the injection zone at the Bell Creek CO₂ enhanced oil recovery project between seismic surveys. Warmer colors indicate regions that have experienced greater change in CO₂ saturation since the baseline seismic survey. The CO₂ response outlines a buildup of CO₂ banking against a barrier to permeability (contours are close together near ‘I’), and then a communication Pathway between two Permeability Barriers.⁹

Image Credits

2 dimensional seismic survey 1: National Energy Technology Laboratory
2 dimensional seismic survey 2: National Energy Technology Laboratory