Should Vertical Seismic Profiles (VSPs) be used in carbon storage?

Introduction

We published a novel onshore seismic monitoring, measurement and verification (MMV) design philosophy for carbon storage earlier this year in the Canadian Journal of Exploration Geophysics (Hunt et al., 2024a, 2024b, and 2024c). Motivation for the new set of techniques arose from the potential wastefulness that would result from treating commercial onshore carbon storage MMV projects like pilot studies or offshore projects. At stake is the prevention of an excess use of capital and surface (environmental) disturbances. The new method was called Theseus 24D in reference to two of its key elements: treating the baseline 3D in a manner like that of the Ship of Theseus Thought Experiment, and the integration of 2D and 3D repeat surveys in its monitoring, which we termed 24D. The Ship of Theseus Thought Experiment concerns itself with the ancient Greek mythological figure, Theseus, who sailed the Mediterranean on a ship made of many wooden planks. The thought experiment considers the identity of Theseus’s ship when one of the planks rots and is replaced. Is the ship still the Ship of Theseus after a plank is replaced? How about when many of the planks are replaced? This metaphor is applied to repeat seismic in carbon storage. In this metaphor, the baseline 3D survey is the Ship of Theseus, and repeat seismic surveys are considered plank replacements. The criticism of Theseus 24D is that repeat seismic surveys may be too large, that they replace not just the rotten planks (or image what is needed at each monitor time) but also image areas that have not seen CO2 yet. In effect, the entire ship may be replaced instead of a few rotten planks.

The Theseus philosophy also leveraged the differences between onshore and offshore carbon storage complexes, particularly the tendency for onshore storage reservoirs to exhibit significantly reduced seismic response to carbon dioxide (CO2) saturation. These design elements led to several specific recommendations, including:

• A redefinition of 2D and 3D repeat seismic imaging areas and the associated claim that AVO analysis is unnecessary in those repeats.
• The minimization of repeat 3D in favour of repeat 2D, supported by the baseline 3D and the use of wavefield reconstruction to produce baseline 2D lines anywhere within the original baseline area (the 24D method).
• The abandonment of short term active seismic monitoring methods like multi-offset, 2D and 3D vertical seismic profiles (VSPs).

Theseus 24D was created to take a critical look at onshore seismic monitoring for carbon storage and ensure that our approach is thoughtful, adequate and flexible, but not wasteful. The goals of MMV must be met in Theseus, but unnecessary methods or techniques should be ruthlessly abandoned.

Every one of the redefinitions and recommendations of Theseus are worthy of discussion and challenge. While we invite readers to review the source papers in the Canadian Journal of Exploration Geophysics, we have noted that one of these recommendations has engendered enough controversy to merit a separate conversation here: the ‘No VSPs.’ The suggestion that 2D and 3D VSPs are unnecessary in commercial onshore carbon storage monitoring came up specifically in conversation with industry experts at the Carbon Capture Canada Conference this year. We take up the subject in this short note.

What is a VSP?

VSP surveys have been well described in the literature (Stewart and Disiena, 1989, Stewart, 2001). They have value because they tie the wellbore with seismic, having aspects of both well log and surface seismic (Stewart, 2001). Their primary value is in subsurface and wave propagation evaluation very near to the wellbore. Figure 1 shows a cartoon of a multi-offset VSP. It includes a multi-level receiving array within the wellbore and several sources on the surface at different offsets. The receiving array could be comprised of 3 component geophones or distributed acoustic sensing (DAS) fiber. These receivers are typically stationed just above the target of interest. In the case of carbon storage, the placement might be above the top of the storage complex. With additional sources on the left side of the observation well, the VSP could become a 2-dimensional or 2D VSP. Adding sources along other azimuths can create a multi-azimuth set of 2D VSPs or even a 3-dimensional or 3D VSP. Going in the other direction, if we reduced the figure to a single source near the wellbore, we would represent a zero offset VSP experiment. Figure 1 only depicts the direct and primary arrivals from a single layer. The direct and primary arrivals from the other layers could be included, somewhat increasing the complexity of our simple cartoon. To this we could add the collection of numerous upgoing and downgoing multiples in permutations only limited by the number of elastic layers within the illuminated area. Finally, it should be noted that the VSP experiment can also be used to evaluate mode-conversions, which would crowd the figure with even more raypaths. The aggregate of this complexity is that the VSP experiment has the potential to evaluate certain wave propagation phenomena, rock and fluid properties in ways that may exceed that of surface seismic. The fact that the VSP experiment only goes through the overburden once is a well-known advantage over surface seismic within the VSP area of illumination (Stewart, 2001). It follows that there are many situations in which VSP surveys have value. For Carbon storage pilots, the ability of the VSP experiment to act as an easily repeatable control experiment to identify CO2 saturated rock is helpful. However, this potential value is only in the near wellbore area.

The limited illumination volume of the VSP about the wellbore is a function of the nature of the experiment itself. The receiving array may be multi-level, but it is only in one location (the wellbore), and it is deep in the subsurface. The critical angle will be smaller than that of 2D and 3D surface seismic, and can be estimated through modeling (Schweigert, 2019). While VSPs have value, this limited area of illumination is an important drawback for commercial scale carbon storage monitoring, limiting the time in which such monitoring can be used to assess the growth of the CO2 plume. One of the principal reasons that the Theseus philosophy does not include VSP surveys is the likelihood that such surveying is only relevant for a small portion of a commercial project’s multi-decade injection life. We argue that repeat 2D and 3D surface seismic are sufficient to the tasks of monitoring for the entire period, and it is more efficient to restrict our attention to them, absent a particular site-specific project requirement for VSP surveying.

This argument suggests a redundancy or wastefulness to VSP surveying that requires further examination. Because repeat 2D and / or repeat 3D will be required anyway later in the project life as the plume grows, then the VSP information may be seen as redundant. In this sense the VSP exercise could be argued as wasteful. Arguments that VSP data has some data quality advantages within their area of illumination hinge again on the early life of the project. The question should be not whether VSP data has some advantages in a commercial scale project, but whether 2D and 3D surface seismic is sufficient to the task of monitoring. The VSP argument has value in the case where 2D and 3D seismic are insufficient, but if the 2D and 3D data is truly insufficient, the use of such methods—and Theseus—will not apply to the project.

Pilot Studies Use VSPs, Why Does Theseus Advise Against Them?

There have been a few notable pilot studies in onshore carbon storage. Aquistore in Saskatchewan, Canada and Otway in Australia are examples. The Quest project in Alberta Canada may not properly be termed a pilot study due to its scale and commercial nature, but we shall see that it does have some pilot-like characteristics. There can be some confusion between feasibility studies, pilot studies and even early commercial projects. Pilots are distinguished from these other small-scale preliminary or early studies by their efforts to test certain concepts for use in later work (Whitehead, 2014). We take the time to point out this important discriminating point because it is this aim to test concepts for future studies (or projects) that tell us a few critical things about pilots. The first of these points is that a pilot looks to the future, and biases to turn around results—and be able to report on them—quickly, if it can. This bias is a necessary connection to the future project(s) and the techniques being evaluated that are awaiting the timely result of the pilot. This short-time period bias influences experimental set-up to favor measurement techniques suited to early times, such as VSPs. Another critical element of a pilot study is that it indeed tests certain study features—some of which should be expected to fail. With their ability to discriminate certain types of wave propagation in the near wellbore area, VSP surveys have some utility in such an evaluation. Commercial projects are unlike pilot projects because they should only include features that project planners are certain will work and work efficiently. Further, the evaluation of a commercial project is aimed towards the goals of monitoring, not the evaluation of monitoring methods, which affects the need for certain tools like VSP surveys.

These influences partially explain why 2D and 3D VSP surveys have been used in onshore carbon storage pilots. The Otway pilot made use of 3D and multi-offset VSP surveys in addition to existing 3D surveys (Dodds et al., 2009). Aquistore included repeat 3D VSP surveys in addition to a baseline 3D surface seismic survey (White et al., 2019). Quest is not properly a pilot because it is a commercial scale project, however, the fulsomeness of the monitoring plan undertaken there—which may be partially a result of the significant government funding given for the project—included significant testing of concepts that are useful for other studies (Crouch, 2011). As a result, the Quest project includes a baseline 3D, multi-azimuth, multi-offset 2D VSP surveys, and 2D surface seismic along the same transects as the 2D VSP surveys (Harvey et al., 2021). VSP surveys are used in all these projects. We do not question the usefulness of the VSP data for each of them, however we maintain the Theseus argument that the VSPs would generally be a wasteful addition to a purely commercial monitoring project. The pilot study bias towards quick results and the fact that pilots test concepts likely influenced the designs of each study towards using VSP surveys—a survey type generally only useful at the early stages of monitoring or for projects with small injection volumes. Quest’s larger volume is only partially an exception as Quest data was still aimed to help future projects, and the program design therefore was likely affected by many of the same influences as that of the pilots.

Ultimately, the differences between pilots and commercial projects are significant and create a set of circumstances where the use of VSP surveys in a pilot or pilot-like project could make sense whereas in a commercial project such use would be technically profligate.

Characterization Advises

The Characterization stage of carbon storage development involves developing a deep understanding of the storage complex prior to injection, often through the collection of new data such as seismic, well test, wireline data, core sampling and analysis, fluid and pressure data (CSA Group, 2022). As noted in Hunt (2024), this stage typically precedes a project’s final investment decision (FID), and helps guide and inform project development, risk management and monitoring. Depending on the project needs and data availability, various seismic could be purchased or acquired during this stage, including 2D, baseline 3D, VSP data, and other downhole monitoring. While the aims of Characterization remain largely the same from site to site, the technical work needed to assure them varies. Any of these data kinds—including VSP surveys—may be included to provide the required confidence of the project team, stakeholders and regulators. Sometimes the baseline 3D and downhole monitoring activities take place after FID, and sometimes before, even though they may well be argued as being an extension of Characterization.

A complete Characterization should provide the project team with the information they need to decide whether 2D and 3D seismic, as in Theseus 24D, is sufficient for active seismic monitoring, or if other seismic methods should be included. That is, by the end of Characterization, it should be well understood if the specific and unique wave propagation information from ongoing VSP studies are needed, or if the information from such VSP work would be redundant and wasteful in the monitoring stage. It should be known if ongoing VSPs are a luxury versus a necessity. In most cases of onshore carbon storage development—particularly in mature areas such as Alberta and Saskatchewan, Canada—we believe that VSP data is unnecessary during and after Characterization, however we acknowledge that the act of Characterization should inform this answer.

Site (Project) Specificity

Theseus advocates for a critical look at carbon storage monitoring, and that critical look begins at the site itself, its unique needs, risks, character, and opportunities. The needs of a specific site are of paramount concern and relate to the previous section on the learnings from Characterization. For example, sites with large injection volumes, and deep thin storage reservoirs may have very large plumes. In such cases, the redefinition of seismic imaging areas from Theseus will become crucially important. These savings terms depend on square and cross terms of the plume radius and the depth of the storage complex (Hunt et al., 2024a). In such cases, the more radical Theseus concept of ‘coring’ may come into play. VSPs are unlikely to be an efficient part of the active seismic monitoring program for such a site. Conversely, a site with a very thick, porous, multi-layered, storage reservoir combined with small injection volumes could be more apt for VSP monitoring than the previous example. We expect that repeat 3D or 24D surface seismic may still be more efficient than VSP monitoring, however, the comparative efficiency would come down to the particulars of the site itself. Each site has its own unique set of risks and opportunities to be considered as part of the rationale for an efficient monitoring program. If there is a reason, perhaps arising from risk management, that the near wellbore area needs continual seismic monitoring throughout the life of injection, a VSP element in monitoring could be warranted.

Storage projects utilizing depleted gas reservoirs, such as the pilot conducted in the Otway Basin of Australia, have implemented VSP analysis. Besides being a low-volume, 100,000 tonne, pilot project, it also made opportunistic use of existing wellbores for observation and involved a structurally controlled storage complex (Dodds et al., 2009). This is an example of a site where pilot-meets-opportunity supporting the use of VSP analysis in monitoring.

Ultimately, the monitoring program for every site should be considered carefully on the merits of the local geology, the risks, injected CO2 volumes, local seismic characteristics, nearby assets, and expected plume growth. While we are taking a critical look at monitoring, we must keep our minds open to circumstances.

Larger Projects, Larger Plumes, Less Use for VSP

As noted in Hunt et al. (2023), carbon storage projects currently under development in Alberta, Canada have a range of size, with some more than an order of magnitude greater than that of Quest’s 1 million tonnes per annum. This tendency to larger project sizes will not be restricted to Alberta. The implications of the size of these projects must be carefully considered in the design of efficient seismic monitoring plans. The Theseus reduction of both outside and inside imaging areas is one approach to this, as is the use of adaptable, change-as-needed repeat 2D in the 24D method. Other approaches for large injection projects will also be developed, such as the Spotlight, semi-permanent single source-receiver pairs, focussed seismic or the sparse, nodal, methods being developed by Carbon Management Canada (CMG) or Microseismic’s CO2SeQure nodal method (Brun et al., 2022, Lawton and Osadetz, 2023, Duncan, 2024). We expect that the limited coverage of the VSP method will see it being used less for these large projects in favor of newer, more efficient, ideas.

Induced Seismicity Monitoring (ISM) is a Different Animal

The advice for ‘No VSPs’ from Theseus was not referring to ISM. CSA Standard Z741, and many other regulatory standards and guidelines require that induced seismicity from injection be monitored (CSA Group, 2022). This can be achieved through a variety of means, including through downhole geophones or DAS such as are also used for VSP recording, or using near surface recording (Eaton, 2024, Cooper et al., 2024). The Quest project uses downhole geophones in an observation well for ISM as well as for active repeat seismic VSP (Shell Canada, 2017, Harvey et al., 2021). It is understandable that the Theseus admonition against the value of VSPs in seismic monitoring might be conflated with the use of downhole equipment in ISM, however this was not the intention of the authors. ‘No VSPs’ refers to active seismic VSP programs as described previously in Figure 1.

Not Every Problem Needs a Seismic Solution

It is important for geophysicists to remember that seismic monitoring is only one element of the myriad methods used in MMV. Geoscientists should always be wary of the Maslow’s Hammer cognitive bias regarding the over-use of their favourite tools (Hunt, 2021). There are many other methods of assuring the containment and conformance of the injected CO2 (Shell Canada, 2017, Hunt et al., 2024a). Some of these methods are inexpensive, create little surface disturbance and are adequate to specific monitoring tasks. This has two implications: the first being that if we are inefficient in the seismic aspects of our monitoring proposals, seismic could lose favor or even be omitted, and the second being that we must always consider the use of other, more cost-effective methods to solve certain problems. For example, in an injector well, downhole instrumentation—including fiber for temperature monitoring—could be used to detect wellbore-related containment issues cheaply as opposed to activities such as active repeat VSP programs.

Conclusions on VSPs and Theseus 24D

While we enthusiastically acknowledge that the VSP experiment can in many ways add value to subsurface analysis, we stand by the advice from Theseus that VSPs are not generally needed in onshore seismic monitoring for commercial scale carbon storage. The VSP survey is only useful near the wellbore, which means it cannot be used to monitor the outer elements of the large CO2 plume growths that will typically occur after a short number of years into commercial injection. Theseus recommends an efficient approach for seismic in which 2D and 3D seismic are integrated with the baseline 3D survey (the Ship of Theseus), saving capital and environmental disturbances. In most well characterized onshore commercial projects the VSP experiment does not add enough unique information for its expense.

Just as the differences in onshore versus offshore carbon storage are important to the rationale for Theseus, it is paramount to also consider that pilot projects and early commercial projects are different from the commercial projects being actively developed. Pilot projects have a clear and well understood tendency to produce learnings quickly, and often with much smaller injection volumes than the new projects such as those currently being developed in Alberta, Canada (Hunt et al., 2023). This difference makes VSP surveys far more valuable to a pilot study than a commercial one.

VSPs may have uses in some storage projects. Site specificity must always be remembered in both characterization and monitoring for carbon storage. VSP surveys are sometimes required in the characterization stage, however the cautionary advice from Theseus is referring to monitoring, not characterization. Some sites may also have a subsurface motivation for near wellbore VSP monitoring throughout the injection period. The admonition against VSP surveys does not include downhole observation, particularly as it pertains to ISM, which must be adequately achieved in carbon storage monitoring.

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