Case Study
Carbon Capture Utilization and Storage (CCUS) has become an essential operation in reducing greenhouse gas emissions globally, with the greatest concern being the environmental risk associated with long-term storage of captured CO2. Any containment breach and leakage would likely negate the initial environmental benefits of capturing and storing CO2emissions.
Shell’s Quest Carbon Capture and Storage (CCS) Facility located near Fort Saskatchewan, Alberta, commenced CO2 injection in August 2015. CO2 is injected into a deep saline aquifer, Basal Cambrian Sandstone (BCS), at a depth of about two kilometers below ground.
As part of this project, ESG installed a downhole array consisting of 8 triaxial geophones (15 HZ) in an observation well in 2014, which has been used to monitor the microseismic activity associated with the storage of CO2 (Harvey, O’Brien, Minisini, Oates, Braim, 2021). To maximize coverage of the area, the array is installed in the middle of the area of interest, which is central to the 3 injector wells.The Quest area is considered an extremely quiet tectonic location, and therefore microseismic activity within the Storage Complex was evaluated as a low risk to containment during the project.
Results & Obsevations
Over the 7 years of monitoring microseismic activity associated with CO2 injection at the Quest facility, a moderate number (over 500 events) of microseismic events have been detected, of which in-depth analyses has been conducted to better understand the nature and distribution of seismicity.
The moment magnitude of the detectable microseismic events ranged from -1.9 to +1 Mw with an average of -0.8 Mw. No dominate trend has been observed within a 10 km radius of each injection well (Figures 3).
Many clusters of events have been identified, locating both close and far from the injector wells. In the initial years of monitoring, it was unclear if the far field activity observed was related to CO2 operations. Through evaluating the long-term trends in microseismic data over the field, however, a correlation can be found between seismicity generated and the stress changes on a large scale. The spatial distribution of these events helps us to understand fluid migration and stress transfer around the reservoir, providing insight into the behaviour of the injected CO2.
Figure 4 shows the seismicity distribution over time, which highlights that no seismic activity was observed in the first year. Additionally, the distribution of magnitude demonstrates the importance of using the right sensor configuration for the project, such as a downhole array plus surface sensors. The average magnitude increases with time due to the increase of the number of the larger magnitude events. This indicates larger stress build-ups and releases are happening, which can result in damage-scale seismicity.
Conclusions from Quest and other long term injection operations
- Seismicity is expected (both within the reservoir and within the basement).
- Seismicity is contained within the basement, with no large magnitude events detected so far (Mw<1), showing no indication of compromising the storage seal.
- The migration of CO2 and the associated pressure front is much further than initially thought.
- Long term seismicity monitoring is a key factor to understand the response of the reservoir and for long term risk mitigation associated with CCUS operations.
- Using ESG’s HybridTM solution (surface Seismicity Monitoring Stations and a downhole geophone array) is highly recommended to monitor the full range of seismic activity associated with operation. It will provide the detection and the accuracy within a large area.
- Integrating Fiber and/or ESG’s Electromagnetic Imaging is beneficial to visualize the extension of the CO2 Plume and measure the strain variation over time which can help to properly interpret the observed seismicity.
- Establishing baseline seismicity prior to injection will help to distinguish between natural and induced seismicity.
References
Harvey, S., O’Brien, S., Minisini, S., Oates, S., Braim,M. 2021. “Quest CCS Facility: Microseismic System Monitoring and Observations.” 15th International Conference on Greenhouse Gas Control Technologies, GHGT-15.
