Geophysical Model and Report

Key Knowledge Document   |    NS051-SS-REP-000-00013

This is a document summarizing the methodology and results of the geophysical work applied to the proposed Endurance saline aquifer CO2 disposal site. The document was provided by BP Exploration as part of the Northern Endurance Partnership project.

What is a geophysical model and why is it important?

The geological structure which has been proposed to store CO2 in Endurance is 1000m or deeper below the seafloor. Therefore, the structure is not directly visible and mapping of the structure (size, depth, shape and thickness) is done by indirect means. This is the function of using geophysical techniques specifically seismic methodology. Seismic data involve generating acoustic (seismic) waves at the surface, the seismic waves travel into the subsurface and are reflected back from layers in the subsurface. The complicated arrays of reflections are recorded at the surface and processed through sophisticated geophysical interpretation software to build a model of the rock layers in the subsurface.  These data can show the top and base of formations, give an indication of shape and continuity and if there are any faults cutting through the layers. Seismic data can be shot as a line resulting in a cross section through the subsurface (2D seismic) or as an area which results in a three-dimensional model of the subsurface (3D seismic). Depending on how data are acquired, physical properties within the reservoir rock may be inferred from some seismic data and not just the top and base of the formation. These types of data have been used in other CO2 sequestration projects to detect where CO2 has been injected into the reservoir. If the same type of data is collected over set time intervals, for instance every 2 years, the size and location of where CO2 has travelled in the subsurface can be monitored (4D seismic). Where possible seismic data are always integrated with any well data drilled in the areas where the seismic has been acquired. The wells are usually the only data available where the rock has been analysed using well logs and/or directly sampled by taking a rock core. These data are used to calibrate and validate the seismic interpretations.

With respect to carbon sequestration activities, seismic data are key in assisting in the following:

  1. Identify locations where geological structures and reservoirs in the subsurface are suitable for CO2 This would include identifying the appropriate geologic intervals and geographic locations that are of the appropriate scale and quality to accommodate and contain the required volume of CO2 proposed to be sequestered. Essentially this is the size, shape and location of the underground storage container.
  2. If the seismic data are of sufficient quality some inferences can be made to the physical properties within the target formation such as porosity and degree of cementation.
  3. Identifying favourable geologic formations above the targeted reservoir which can act as seals preventing migration of CO2 out of the structure. Seismic data can also be used to identify any faults cutting across the formation which may also cause potential containment risks.
  4. Observe the effectiveness of CO2 storage through monitoring the injection and migration of CO2 in the storage reservoirs over time. Introduction of CO2 into a saline reservoir alters the seismic response allowing for the identification of areas where CO2 has been introduced.

Technical summary

The Endurance Geophysical Report consists of a thorough review of the geophysical data, specifically focusing on the use of seismic, and its application with respect to the Northern Endurance Partnership Project. How these data are utilized, and the methodologies employed to build and characterize the structural framework for the Endurance structure and its suitability with respect to carbon sequestration is discussed in detail. Limitations with respect to the data and the resulting interpretation were identified and quantified with respect to the impact on the subsurface model. Recommendations for potential future seismic acquisition, to assist in mitigating these limitations and monitor the effectiveness of the project were also discussed.

The Endurance project and regional structural understanding was defined using multiple seismic datasets. These datasets consist of:

  • Regional 2D seismic of varying vintages, used predominantly to assist in building a regional understanding of the basin
  • 1997 3D seismic (OBC), which consists of a sparse lower resolution 3D
  • 2013 3D seismic (Polarcus), which consists of a higher quality seismic 3D

Although it was recognized that these seismic datasets were designed and processed for deeper targets, and therefore not optimally designed to image the Triassic stratigraphy of the Endurance structure, the quality was sufficient to assist in building the structural framework for the project. Recognizing the limitations of the seismic datasets it was determined that to optimize the imaging of the key Triassic stratigraphy of the Endurance structure, reprocessing of the higher quality Polarcus 3D was warranted. Focusing on image quality and noise reduction at the Triassic level, CGG was engaged to undertake a post-migration reprocessing project. Although an improvement in data quality was observed following reprocessing, the challenges with respect to data quality from non-optimal design and acquisition could not be completely rectified.

Utilizing all available well ties, the key stratigraphic intervals including the primary seal of the Rot Halite and the targeted reservoir of the Bunter sandstone were identified and interpreted with high confidence. Although the interpretation resulted in a robust structural framework the data quality was determined to be insufficient to resolve and characterize subtle stratigraphic variations within the Bunter sandstone. Outstanding data quality issues due to multiples, 60Hz noise and acquisition footprint were specifically identified as issues that negatively impacted the seismic resolution. As a result, it is suggested that a new 3D seismic program be executed designed specifically to optimize the imaging of the Triassic stratigraphy. A new 3D seismic shoot would also be critical in developing a thorough MMV (Measuring Monitoring and Verification) plan to monitor CO2 plume migration after injection commences.

Identification of potential faulting was also key in quantifying potential containment risk for the project. Following reprocessing it was determined that the data could resolve faults with offsets down to 10-15m. Multiple faults could clearly be imaged in the stratigraphy above the Endurance structure. Although limited by the resolution of the seismic data, faulting does not appear to extend below the Rot Halite into the Bunter sandstone reservoir, minimizing risk of faulting impacting containment.

A layer-based depth conversion of the seismic data and interpretations was undertaken with the resulting structural framework used as input for the static geological model subsequently incorporated in the dynamic, geomechanical and geochemical models. A thorough review of the depth conversion methodology and associated uncertainty analysis was summarized and determined to be appropriate. Following a detailed uncertainty analysis, confidence in the Endurance structure is high in that variations did not have a large impact on the overall structure.

The impact of CO2 on the seismic response was also reviewed, focusing on the ability to detect CO2 using seismic data. The application would monitor both CO2 migration and containment within the targeted Bunter reservoir. Seismic modelling was undertaken specifically to evaluate what CO2 saturations are detectible seismically. Modelling strongly supports the ability to see low concentrations of CO2 in the reservoir. Although excellent at identifying the introduction of low concentrations of CO2 at a particular location, the ability to resolve variations in its concentration would be limited. The ability to resolve the presence of CO2 was also shown to be highly dependent upon seismic data quality, i.e., higher frequency and lower noise content required, with the ability to resolve CO2 migration in thinner beds requiring the acquisition of a higher resolution 3D survey.

Monitoring the migration of the CO2 would require the repeated acquisition of high-resolution 3D seismic over time. This technique is referred to as 4D seismic and would be of value not only to monitor CO2 migration but also could be used to confirm and monitor containment within the reservoir. Prior to the initiation of injection, a high-resolution 3D survey would be required to act as a baseline and the timing of any subsequent 3D’s would be based upon injection volumes and reservoir modelling.

Overall, the Primary Store Geophysical Model and Report is a very thorough document. The methodologies and analysis align with industry standard workflow and the resulting conclusions and recommendation can be supported.

Technical comments for possible future work on the geophysical model

Within the report it was recognized that the existing seismic data are not optimal for investigating the Bunter sandstone reservoir and associated sealing formations. The report concludes that a new 3D seismic survey be acquired designed specifically for this project. We strongly agree with that approach. A new survey specifically designed for this project could be used to better define the structural framework and possibly provide information about the physical properties and stratigraphic variability within the reservoir formation and of the adjacent sealing formations.

Modelling indicates high quality seismic should be able to resolve the migration of CO2 within the Bunter reservoir. This new 3D is critical to establish a baseline to compare any future seismic data that could be used to monitor and map CO2 migration and containment.

Document information

Document name: Primary Store Geophysical Model and Report

Reference number: NS051-SS-REP-000-00013

Document length: 69 pages

Topic area: Geophysical model for CO2 sequestration

Project: Net Zero Teesside / Northern Endurance Partnership

Original report date: August 2021

Original Author: BP Exploration Operating Company

Link to all original reports

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