4.2.1. Workflow Description

Deep Saline Formation storage projects may offer the most promising storage potential in terms of aggregate capacity. However, as compared to other types of CO2 storage, limited information is available for desk based assessment.

Desk based assessment:

The objective of the first desk based assessment is to collect and interpret existing and available geological information at a regional/sedimentary basin scale. Such information originates from past underground activities in the area: fresh water production, geothermal activities, O&G activities, Underground Gas Storage, mining, etc. However, data available from such activities are generally not sufficient to properly characterize the future storage site. Indeed, they are not necessarily targeting the same layers, are not geographically focused on the expected storage site location, and did not include the required information. Consequently, the acquisition of new local data is necessary. Figure 17 is showing the successive steps and potential loops that are supposed to reach bankability for a DSF onshore project from end of Phase 1 (see Table 1).

Figure 17: Development workflow up to bankability for Deep Saline Formation (DSF) onshore project

Please note that when a failure occurs in the workflow shown on Figure 17 above, it is considered that the suitability over the area has improved from its original status. Actually, new dataset has been acquired (at least one 2D seismic survey), therefore improving the overall “geological” knowledge of the area.

It is obviously impossible to create a workflow that maps all storage characterization possibilities for CO2 storage project. However, the assumptions enable modelling of 50 to 100 different scenarios.

2D Seismic survey(s)

As described in Part 3, the first step after the desk based assessment is to apply for a license to explore the area. The licensing process, when it is required, varies widely from one region to another in terms of duration and content. However, we assume that CO2 storage will develop within a stringent policy. The exploration license will be given for an area that can be very large (several thousands of square kilometres). One of the first new dataset supposed to be acquired is a 2D seismic survey, aimed at understanding the subsurface geometry, including fault patterns in the target area of the storage. The extent of this acquisition depends on the existing knowledge on the targeted area. It can range from a few tens kilometres to several hundred (See C.2.1.1 for further details).

Such 2D survey is used (after processing and interpretation) to acquire 3D detailed seismic data on the best potential area of the exploration license.

Figure 18: Deep Saline Formation Onshore Storage Project- Case of two successive 2D seismic survey failures

If the results of the 2D seismic survey are not positive (doubtful presence of cap rock, of reservoir, too many faults, etc.), the project developer would rather select a new area for performing a new 2D seismic survey, than going directly to well drilling. In the proposed workflow, the maximum number of times the project developer has the opportunity to acquire 2D seismic survey depends on the suitability (see section 3.2.2) of this area: two times for areas either highly suitable or suitable, and three times otherwise (Figure 18).

When suitability is high or good enough, the definition of the storage site area is based on better knowledge acquired in Phase 0 and Phase 1, and the probability of success of the 2D survey is greater (see comments in Table 1). Figure 18 shows the case of two successive failures of 2D seismic surveys: the effective actual workflow followed is orange coloured in this example.

3D seismic survey(s)

Whereas the 2D survey gives information on the surroundings of the future storage site, the 3D acquisition will detail information over the sedimentary pile, including potential reservoir characteristics (porosity, fractures, and detailed fault pattern). It might also serve as a baseline for 4D seismic monitoring23. This 3D survey area is supposed to be in the range of 100km2 to 200km2.

If the 3D seismic survey processing and interpretation (including inversion to obtain a porosity information) confirm the expected potential for CO2 storage (suitable reservoir and caprock, no faults in the expected area of injection, etc.), and after a necessary risk analysis, a first well can be drilled to calibrate the local properties of caprock and reservoir layers. When the 3D seismic survey is not positive (not good enough supposed reservoir properties, discontinuous caprock for instance), another 3D seismic survey can be acquired on a different location within the area of the 2D survey (see Figure 19). However, we consider quite unlikely that none of the two 3D surveys do not show a potential for CO2 storage, if the 2D showed such a potential. Therefore, the proposed workflow does not loop back to the 2D seismic level as shown in Figure 19.

Figure 19: Deep Saline Formation Onshore - Logic trail for 3D seismic survey

Exploration well drilling

If the data (well logs, core samples, production test, maximum pressure test for the caprock, etc.) acquired during the first well drilling show that the properties of the layers are good enough (the well has found a porous reservoir and the corresponding caprock both with satisfactory properties – i.e. mechanical, petrophysical...), a second well should be drilled to perform a production interference test, and ultimately the CO2 injection test (see section 4.1). In that case, we consider that no probability of failure should be linked to this second well if the first one has been successful.

However, if the first well is not successful, we assumed that a reprocessing of the whole seismic dataset (2D/3D) and an update of the 3D geological model will be carried out. These actions will define whether or not a suitable location can be identified in the surrounding area of the unsuccessful first well. Within the workflow, a negative result of this step of reprocessing and model update would mean that the selected area is not appropriate and the project has to be relocated (new exploration license).

If the step of reprocessing and model update shows that it is possible to find another location, then another well will be drilled. If the results of this second well are negative, the project will obviously have to be relocated. Nevertheless, if the new well is successful another 3D seismic acquisition might be needed on this new well area. Actually, the new well location may be offset from the original 3D seismic survey which may not cover anymore the future extent of the CO2 plume.

Injection test

These tests are essential to ensure administrative authorities and public acceptance of storage project.

Prior to the injection test authorization application, a final risk analysis has to be performed. It should allow identification of the different risks but also monitoring and verification measures to be implemented as well as foreseen mitigation/remedy actions.

No probability of success was associated with the injection test. It is considered that sufficient information has been gained from the seismic surveys, well drilling, production and interference test. The injection test purely serves at defining the appropriate number of injection wells for the commercial (industrial) scale deployment. One shall note that this assumption has been made for the sake of simplicity. In practice, a risk exists that the final injectivity of the site is not as good as expected. The consequence of such a poor injectivity is, besides the increased number of wells for industrial scale, a possibility of increased investment aimed at water (brine) production for releasing pressure inside the porous volume for the injected CO2.

Figure 20 shows one possible actual workflow (orange colour) to achieve bankability status for a project in a poorly explored area. The grey rectangle at bottom of Figure 20 highlights the route not considered in this instance.

Figure 20: Deep Saline Formation Onshore - example of workflow development for exploration in poorly explored geological province

23 4D seismic monitoring: repetition at fixed interval in time of a 3D seismic acquisition on a given area to monitor evolution of gases within the geological layer. This is one of the monitoring techniques used in the Sleipner case.