When evaluating and monitoring carbon capture and storage (CCS) sites, seismic is a handy tool in all stages of project development. Solutions come in different vintages and dimensions. This latest article in our CCS series explores old and new seismic approaches used for CCS projects.
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2D seismic (seismic data acquired along a line) has been around for a long time. This imaging method can be an undervalued approach since the development of 3D seismic surveys, but it just might be the most important survey type in CCS today. In this evaluation stage, geophysicists must interpret a large quantity of data to select an appropriate site for long-term storage. 2D is the preferred data type because it is readily available, less expensive than 3D, and allows the interpreter to map a large area relatively quickly. The current preference for 2D reflects the vast number of projects in the early stages of development.
2D seismic has its challenges. Much of the data was acquired long ago. Retrieving it in large volumes can be time-consuming, particularly when the data is stored on tapes (or even paper!) instead of a more modern digital format. The data were likely processed with oil and gas extraction in mind. As a result, the data may focus on a shallower interval rather than the deeper intervals we seek for storage—similar to a camera that focuses on the foreground and blurs the background.
The other challenge in interpreting a vast number of 2D lines is that they all require alignment relative to each other – a process called mistie analysis. Geophysics by SeisWare’s automatic mistie analysis and 2D auto-picking help accelerate this step, which can be otherwise time-consuming.
3D seismic is acquired through a grid or surface network of sources and receivers to produce a 3D volume of data. Geophysicists can interpret horizons (rock layers) and structures in three dimensions. This helps to understand variation in the storage targets and the integrity of the caprock that confines the CO2. It is also a critical tool to evaluate risk. 3D seismic data also acts as a baseline for future plume monitoring using 4D seismic. For more on seismic – read The Importance of Seismic Data in Carbon Capture and Storage.
While this method sounds more attractive than interpreting individual seismic lines over a large area, 3D data is more expensive than its 2D counterpart, and existing surveys may not cover selected storage sites. New data acquisition typically occurs later in the project life, once a specific site has been selected. Conducting a 3D survey requires a significant investment and necessitates long-term planning due to the need for regulatory approvals, access to land, and the availability of seismic crews.
SeisWare’s depth conversion (turning data in time to depth) and attribute analysis are just a few tools that expedite the interpretation of 3D data.
After a project becomes operational, operators may repeat 3D seismic surveys (or portions of them) to search for changes resulting from the injected CO2. This type of survey is known as time-lapse or 4D seismic. The injected CO2 fills the pores in the storage reservoir. This creates an expanding plume and causes observable changes in the seismic data that can be mapped over time. To observe changes, operators will re-acquire surveys every few years. Geophysicists will compare successive surveys to map changes over an extended time period (though relatively short compared to the life of the storage reservoir). This approach does require consistent processing to ensure that the data accurately reflect reservoir changes.
Vertical seismic profiles (VSP) are another approach that geophysicists use to look for changes at specific points in the reservoir. This approach uses sensors in a borehole and sources at the surface to create an accurate profile near the well bore. VSPs can be a quick and inexpensive solution for localized measurements and are repeatable. Unfortunately, this approach does not give much insight into the storage reservoir attributes and variability beyond the wellbore. Much like 4D, repeated surveys require consistent processing. Repeated surveys also requires lowering and raising tools in the wellbore, which can risk damaging the wellbore. As an alternative to traditional VSP surveys, fiber optic monitoring is a more permanent solution and an approach that is gaining popularity. Fiber allows companies to supplement or replace VSP, but the technology can be expensive to install along the wellbore.
Reservoir characterization tends to use active seismic approaches. This means that companies perform long-term observation for seal and reservoir integrity using passive methods like microseismic or fiber optic monitoring. Real-time monitoring can alert project operators of hazardous events like fault activation or caprock failure, which could jeopardize long-term storage. This approach can be resource-intensive, but improvements in automation have reduced this workload.
CCS projects may utilize more than one type of geophysical survey. Overlapping survey types add additional information but also unique challenges. For example, VSPs and 2D seismic data may contain different frequencies that make comparison difficult – but not impossible. For a case study on how Carbon Management Canada worked through these challenges to visualize a CO2 plume, download our whitepaper on our CCS page.
Several options for geophysical data exist for CSS project evaluation. Different methods may be more appropriate depending on the project stage. While seismic data can be a costly investment, it is one that is necessary to reduce risk and uncertainty when evaluating storage targets and ensuring safe and effective long-term storage.
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