1

Application of Seismic Trace-Based Analysis to Detect the Extent of Injected CO2 Using Modeled Seismic Data from Aquistore

Amanda J. Livers-Douglas, Olarinre Salako, Shaughn A. Burnison, Cesar Barajas-Olalde,

Jose A. Torres Rivero, Lawrence J. Pekot, and Charles D. Gorecki

University of North Dakota Energy & Environmental Research Center 15 North 23rd Street, Stop 9018, Grand Forks, ND 58202-9018

Abstract An important factor in developing carbon capture and storage (CCS) monitoring programs is to find cost-effective methods that are viable for commercial deployment. Current industry standard methods include various forms of 4-D surface and borehole seismic that can provide high-resolution images of changes in the reservoir due to injected CO2. However, 4-D methods are expensive, and the turnaround time for acquisition, processing, and interpretation can be significant. The Energy & Environmental Research Center (EERC) has been developing new monitoring technology that offers incremental information with a smaller surface impact and at a lower cost to complement current monitoring methods. A new method developed by the EERC as part of the Scalable Automated Semipermanent Seismic Array (SASSA) project funded by the U.S. Department of Energy (DOE) National Energy Technology Laboratory (NETL) under Award No. FE012665 (Burnison et al., 2017) could be used to monitor the extent of injected CO2. This trace-based analysis method uses the seismic method to monitor individual reservoir reflection points for signal character changes that could be attributed to the passing of a CO2 saturation front. Data from a single stationary remotely operated seismic source and a permanent receiver array are used to track the lateral migration of injected CO2. Mapping the amplitude change for each individual receiver and applying kriging may provide a useful and intuitive periodic image of the plume on a more frequent basis than full 3-D seismic monitor surveys and at less expense. The incremental results can help determine when and where the CO2 saturation front has progressed, provide actionable information to the field operator, help to improve the accuracy of reservoir simulations, and inform the timing of acquisition of more expensive conventional 3-D repeat surveys. When applied to an array with evenly spaced receiver geometries, the results from the trace-based analysis can be used to create a map of the CO2 plume or CO2 saturation front. The SASSA trace-based analysis was applied to synthetic seismic data generated using two layered-earth models built to simulate the pre-CO2 injection baseline conditions and post-CO2 injection conditions coincident with the timing of the 3-D seismic monitor survey and pulsed-neutron logging (PLN) in February 2016 at the Aquistore CCS site. The 17-layer 3-D models were built using interpreted horizons from depth converted seismic data collected at Aquistore and elastic properties from well log data and the Roach et al. (2017a) Aquistore model. Rock physics calculations were used to determine the elastic properties of intervals affected by injected CO2 for the post-CO2 injection model. The geometry of the CO2 plume in the post-CO2 injection model was defined by tracing the interpreted plume image from traditional time-lapse seismic analysis of the baseline and monitor surface 3-D surveys collected at Aquistore that employed the permanent array and dynamite

2

sources (Roach et al., 2017b). PNL logs and dynamic reservoir simulations were used to determine the CO2 saturation of the plume in the 10-m-thick “Perforation 2” interval in the reservoir that showed breakthrough at the monitor well. Synthetic seismic records were generated by 3-D raytracing for receivers and a single source consistent with the Aquistore permanent array geometry and the permanently installed orbital vibrator source (White et al., 2015). Different levels of noise were added to the synthetic seismic data to simulate real-world conditions. A simple trace-based seismic processing workflow developed as part of the SASSA project was applied to the data. Root mean squared (RMS) amplitudes were calculated over the reservoir interval to compute difference volumes generated by subtracting the pre-CO2 injection data from the post-CO2 injection data. RMS amplitude difference maps were then created by kriging the calculated RMS values in ArcGIS (Figure 1). A comparison of the interpretation of the plume extents from the RMS amplitude difference maps from the model data and the interpreted plume outline from the traditional 4-D analysis was done. Results from the data set with no noise added indicates a good match between the plume from 4-D analysis and the plume generated from applying the SASSA trace-based analysis method to model data. The results for data sets with random noise added to simulate real-world conditions indicate that noise affects the shape of the kriged plume. Distinguishing between RMS amplitude values associated with the plume and noise becomes more difficult as the level of noise is increased. Confidence in the computed plume extent interpretation is dependent on noise level. Increasing the signal-to-noise ratio of the data by stacking more shots would help mitigate this problem. If the right signal-to-noise conditions could be achieved, applying the SASSA trace-based analysis method to real Aquistore data would produce interpretable maps showing the extent of the injected CO2. Application of this new method to periodic data collected at Aquistore using the permanent array and the permanently installed orbital vibrator source may extend the time between the 3-D monitor surveys and help reduce the cost of the Aquistore CO2 injection monitoring program. References Burnison, S.A.; Livers, A.J.; Barajas-Olalde, C.; Jin, L.; Salako, O.; Vettleson, H.M.; Hamling, J.A.;

Gorecki, C.D. Final Report of a Scalable, Automated, Semi-Permanent Seismic Array (SASSA) Method for Detecting CO2 Extent During Geologic CO2 Injection; U.S. DOE, 2017.

Roach, L.A.N.; Angus, D.A.; White, D.J. Assessment of the Limitations on the Seismic Detectability

of Injected CO2 Within a Deep Geological Reservoir. Energy Procedia 2017a, 114, 4008–4019. Roach, L.A.N.; White, D.J.; Roberts, B.; Angus, D. Initial 4-D Seismic Results after CO2 Injection

Start-Up at the Aquistore Storage Site. Geophysics 2017b, 82 (3), B95–B107. White, D.J.; Roach, L.A.N.; Roberts, B. Improved Time-Lapse Seismic Performance Using a Sparse

Permanent Seismic Array—Experience from the Aquistore CO2 Storage Site. Geophysics 2014, 80 (2), WA35–WA48.

3

Figure 1. Kriged RMS amplitude difference maps for the no-noise data, 20 SNR (signal-to-noise) data, 10 SNR data, and the 5 SNR data. The black outline is the outline of the interpreted plume

from 4-D analysis done using pre- and postinjection data collected at Aquistore using the permanent array.