Using P-Cable Technology for Offshore
Carbon Capture & Storage Surveys
What is Carbon Capture and Storage?
Carbon capture and storage (CCS), also known as carbon capture and sequestration or carbon capture, utilization, and storage (CCUS), is the process of capturing waste carbon dioxide (CO2) and depositing it in an underground geologic formation for long-term CO2 storage. The aim of CCS is to prevent the release of CO2 into the earth’s atmosphere as a means of mitigating the contribution of carbon dioxide emissions to global warming and ocean acidification. A key component of a successful CCS project is ensuring that there is no leakage of CO2 from the subsurface geologic formation selected for storage.
Carbon Capture & Storage, Pre-Injection Site Evaluation:
Prior to CO2 Injection – Confirm Seal Integrity
The first step of a successful carbon capture and storage project is to evaluate the suitability of a candidate storage formation for securely storing CO2 beneath the seafloor. Due to its unrivaled spatial and vertical resolution, the P-Cable Ultrahigh Resolution (UHR) 3D marine seismic system is an ideal technology to use for the evaluation. Of particular importance is the ability of the P-Cable UHR3D system to image small scale faults which may compromise the seal of the target formation, forming permeable pathways for the CO2 to potentially migrate upwards towards the seafloor and into the seawater above.
The P-Cable system is a unique, streamer-based 3D marine seismic data acquisition technology. P-Cable is designed to provide full 3D seismic imaging of the sub-seafloor with a much higher temporal and spatial resolution than conventional marine 3D seismic systems. The advantage of this approach lies in the comprehensive three-dimensional structural framework in which all ancillary datasets can be integrated while providing ultrahigh resolution imaging for interpreting subsurface geology. The P-Cable system is comprised of many closely spaced, short-offset multichannel streamer cables with small receiver group intervals. A typical P-Cable system configuration is shown in Figure 1.
Figure 1 – P-Cable Ultrahigh Resolution 3D (UHR3D) Seismic System Diagram. The number of streamer cables is generally between 12 and 24, with 3.125 to 12.5-meter crossline separation between cables. The individual streamers are connected to a cross-cable (known as the “P” or perpendicular cable) and typical cable lengths range from 50 to 100 meters with a receiver group interval of 3.125 or 6.25 meters. This very compact receiver array allows seismic vessels to operate in close proximity to the shoreline or offshore infrastructure.
The following P-Cable data example from a survey in the Barents Sea, offshore Norway, dramatically illustrates the ability of P-Cable data to provide extremely detailed images of complex, small-scale fault systems. Referring to the vertical seismic sections in Figure 2, the P-Cable dataset (bottom) clearly indicates a series of small-scale faults in the Cretaceous section just below the Upper Regional Unconformity (between 640 and 700 msecs TWT). This series of faults is completely absent in the conventional dataset, primarily due crossline spatial aliasing of the steeply dipping fault planes as a result of the 25 meter crossline bin. This difference in resolution is even more strikingly seen in a time slice through the polygonal fault system (Figure 3).
Figure 2 – Comparison of conventional streamer 3D (top) with P-Cable UHR3D (bottom) in the Barents Sea. The conventional data has been reprocessed at 2 msec sampling, and both datasets are processed through pre-stack time migration processing flow using broadband techniques for maximum resolution. Data courtesy OMV.
The ability of P-Cable data to delineate small-scale faults is an important consideration for selecting a suitable offshore geologic formation for storing CO2. Because these faults can act as permeability pathways that allow carbon dioxide to migrate towards the seafloor, they have vital significance to the assessment of whether an effective seal is in place to ensure that CO2 does not leak into the seawater or atmosphere above the geological storage formation.
Figure 3 – Time slice comparison of conventional streamer data (left) and P-Cable data (right) through the faulted zone in the Cretaceous section underneath the regional unconformity. The denser inline and crossline sampling of the P-Cable data, in combination with the finer temporal sampling, accounts for the ability of P-Cable to image the complex polygonal fault system with such incredible detail.
Carbon Capture & Storage, Injection Time-Lapse Monitoring:
During CO2 Injection – Confirm Containment
During the second step of a successful offshore carbon capture and sequestration project, carbon dioxide is injected into the storage formation. When the CO2 injection happens, it is typically in a supercritical state and provides a strong acoustic impedance contrast compared to originally brine filled formation. This strong impedance contrast makes CO2 sequestration an ideal candidate for time-lapse, or 4D seismic monitoring of CO2 storage. It is important to note, however, that acoustic impedance changes significantly as the formation fluid changes from pure brine to brine with even a low CO2 saturation, and that the impedance change as CO2 saturation increases is much less. As a result, seismic data is an excellent tool for CCS monitoring containment and potential leakage (see next section), but less applicable for determining the magnitude of CO2 saturation.
An excellent example of the ability of marine seismic data to monitor injecting CO2 is the Sleipner field in the North Sea, offshore Norway. Operated by Equinor and partners, CO2 has been injected into the originally saline, highly porous Utsira sandstone formation since 1996, and a seismic monitoring program consisting of a series of towed streamer surveys has been conducted starting in 2001. Although the seismic monitoring surveys were not identical in terms of spread configuration and wavefield sampling, a common time-lapse processing sequence was used to enhance repeatability albeit with some sacrifice in terms of bandwidth and resolution to match the pre-injection baseline survey from 1994. Despite this, the utility of the seismic data in monitoring the migration of the CO2 plume and confirm containment within the geological CO2 storage formation is compelling, as can be seen in Figure 4.
Figure 4 – Time lapse images of the Sleipner CO2 plume. North-South inline through the plume (top) and plan view of total reflection amplitude in the plume (bottom). Figure from Chadwick, A. and Williams, G., et.al., Quantitative analysis of time-lapse seismic monitoring data at the Sleipner CO2 storage operation, The Leading Edge, February 2010, pp 170-177.
P-Cable data has been successfully utilized for time lapse monitoring of producing reservoirs, in a much more challenging geophysical setting, and therefore is an ideal candidate for CCS monitoring where the target storage formations tend to be shallower, and therefore able to more fully benefit from the enhanced resolution that P-Cable UHR4D data provides. An example of one of our successful seismic monitoring projects is an oilfield offshore Louisiana operated by Shell (Figure 5). The survey area for the time-lapse 4D seismic monitoring program is located in water depths of approximately 1000 meters, and the target reservoir characterization is approximately 2600 meters beneath the seafloor. This field is being produced by water injection, and as can be seen in the upper images in Figure 5 the amplitude change as a result of water replacing hydrocarbons (which causes a hardening of the formation) is quite subtle compared to the dramatic amplitude increases associated with injecting CO2. Despite this relatively modest amplitude change, the P-Cable datasets are able to successfully extract the 4D signal (shown in the dRMS displays) and delineate the movement of water and hydrocarbons within the target reservoir characterization.
Due to the depth of the target reservoir, and the effects of earth filtering which preferentially attenuate high frequencies as the seismic signal travels through the subsurface, the resolution of the reservoir target does not show the dramatically enhanced resolution of the previous P-Cable examples. If, however, we focus on the near surface geology (Figure 6), we can see that compared to conventional 3D seismic data the P-Cable data again shows a significant uplift in terms of resolution in general, and fault imaging in particular.
Figure 5 – P-Cable time lapse seismic monitoring example from offshore Louisiana. The pre-stack depth migrated images in the upper left are the 2016 baseline and 2017 monitor surveys with the dark blue line indicating the location of the water inject well. Despite the relatively modest change in reflection amplitude as water replaces oil, the P-Cable data is able to reliably extract the 4D signal and delineate the relative movement of water and oil.
Figure 6 – Shallow subsurface imaging comparison of conventional OBN data (left) and P-Cable UHR3D baseline survey data (right) from the offshore Louisiana field shown in Figure 4. The P-Cable dataset shows a dramatic improvement in the imaging of small scale faults, and is even able to image the imprint of three plugged and abandoned exploration wells.
Carbon Capture & Storage, Injection Time-Lapse Monitoring:
During CO2 Injection – Monitor Leakage
In addition to confirming containment, an equally important objective of time-lapse seismic monitoring is to monitor for leakage of CO2 from the targeted storage formation. This leakage process is analogous to the leakage of hydrocarbons from oil and gas reservoirs. P-Cable technology has a proven ability to image this type of leakage for reservoirs under the sea floor with unprecedented accuracy. The example below (Figure 7) is taken from a geohazards survey in the US Gulf of Mexico, and shows the enhanced imaging that P-Cable data can provide for gas leakage and fluid expulsion features on the seafloor.
Figure 7 – Comparison of conventional high-resolution seismic data (left) and P-Cable Ultrahigh Resolution data (right). In addition to the overall enhancement in subsurface resolution, note in particular clearly delineated the gas chimney on the right-hand side of the P-Cable section is. Both the wipeout zone associated with gas leakage along the chimney, and free gas accumulations (“bright spots”) within the chimney are clearly imaged.
The significant “wipeout” zone that delineates the gas chimney in the P-Cable data in Figure 7 is due to the limited, near-offset trace distribution of the P-Cable receiver spread. For conventional marine seismic data (streamer or OBN), only a relatively few traces are impacted by a localized accumulation of gas, and as a result the traces with raypaths that do not intersect the anomaly are unaffected by the presence of gas (left side of Figure 8). In contrast, P-Cable data consists exclusively of near offset traces, so even localized gas anomalies result in wipeout zones as seen in the right side of Figure 8. These obvious wipeout features, in combination with “bright spots”, which indicate gas accumulations beneath a seal, are a distinctive feature of P-Cable datasets.
Figure 8 – P-Cable data, due to its limited, near offset trace distribution, sees local gas accumulations as “wipeout” zones because all the raypaths travel through the gas accumulation.
Another example of gas leakage is shown in Figure 9, which is taken from a P-Cable survey from offshore California. This data volume was collected in shallow water across a portion of the Hosgri fault zone, located offshore central California. The seismic amplitude volume is overlain with fault prediction and gas chimney prediction attributes derived using neural-network technology, and clearly shows the strong correlation of gas leakage with shallow faults within the data volume.
Figure 9 – A 3D perspective view of P-Cable seismic data offshore California showing neural network fault and chimney attributes. Fault attributes (black) and chimney attributes (color) are show on top of seismic amplitudes along a time slice at 174 milliseconds, illustrating the relationship between faults and gas chimneys. Note on the crossline vertical section the termination of the dense zone of high-chimney probabilities below the shallow, abruptly ending high-amplitude reflectors interpreted as bright spots. Kluesner, J. W., and D. S. Brothers, 2016, Seismic attribute detection of faults and fluid pathways within an active strike-slip shear zone: New insights from high-resolution 3D P-Cable™ seismic data along the Hosgri Fault, offshore California: Interpretation, 4, no. 1, SB131–SB148, http://dx.doi.org/10.1190/INT-2015-0143.1
Carbon Capture & Storage, Post-Injection:
After CO2 Injection – Confirm Successful Long-Term Storage
The final step of a carbon capture utilization and storage project is successful long-term storage. Ongoing CCS measurement, monitoring and verification of the geological CO2 storage is vital for managing these sub-seafloor reservoirs.
With its ability to detect and delineate both CO2 storage within the target formation and CO2 leakage above the target storage formation, P-Cable data obviously also has an ability to confirm long term storage after CO2 injection has been completed. This unique capability makes P-Cable UHR seismic data a valuable tool for surveys, assessment and monitoring of CO2 throughout the life cycle of a CCS project.
