The overall objective of the project PROTECT is to investigate caprock integrity associated with injection and storage of CO2 in an aquifer. CO2 injection always implies a pressure buildup in the reservoir, and at high pressure build-up, existing fractures and faults may open and create leakage pathways for the CO2 plume. New fractures may also evolve at higher pressures. The project will build new knowledge about caprock strength and its response to large pressure build-up. Recommendations will be developed that can be used in the planning and execution of safe storage projects.
Research methods used in the project are divided into three subareas: data collection, experimental studies and computational tasks. The studies are performed at small scale (scale of individual cracks, cm to m) and at large scale (reservoir or basin scale, 10 m to km). Large-scale tasks will require upscaling of fractured rocks. Integration of the various investigations will present challenges with respect to the synthesis of data and benchmark simulations.
The project has completed a suite of advanced laboratory testing of fresh samples of shales and mudrocks from the North Sea and Svalbard. The samples already contain natural fractures. Lab tests include standard rock strength analyses as well as experiments to understand the impact of supercritical CO2 on hydromechanical properties. The measured mechanical data are valuable for understanding whether these rocks will be strong enough to withstand similar stresses during CO2 injection. The presence of flowing water through existing cracks substantially decreases rock strength. However, the impact of CO2 is more complex. CO2 can absorb water from the shale, causing the rock to shrink. This leads to higher permeability when CO2 flows through fractures than oil or water.
Other activities are focused on monitoring the subsurface for detection of leakage. Better tools have been developed to invert seismic data more effectively by combining the inversion with other data types such as electromagnetic or gravimetric surveys. This improves the data interpretation and gives more reliable estimates of where the CO2 plume is located.
The PROTECT team also developed state-of-the-art computational tools to describe the behavior of the caprock when subjected to large difference in pressure and temperature caused by CO2 injection. These codes can reproduce a variety of processes related to fractures, including fracture creation and activation, fluid flow, elasticity and geochemical alteration. Different mathematical techniques for simulating these processes have been developed and tested within the project. These methods have been shown to be conservative, accurate and robust.
The project also developed software to investigate the response of the caprock under massive injection into deep storage reservoirs. These large systems are well beyond the scale of individual fractures, and therefore simpler techniques must be used in order to be computationally efficient. The project has compared different approaches and demonstrated that analytical estimates of seabed uplift are reasonably accurate and save time. This is especially important when many simulations are needed to evaluate uncertainty and sensitivity to input data.
New simulation studies in the project have focused on the capacity of the Utsira Sand for high-volume CO2 injection. According to the Norwegian Storage Atlas, the theoretical capacity of the Utsira for CO2 is exceptionally high. However, effective use of the theoretical capacity requires that CO2 be injected at rates tens or hundreds of times larger than at the Sleipner CCS project. As CO2 injected volumes increase, the Utsira will become pressurized, which could exceed the allowable pressure the caprock can withstand. Understanding the impact of these pressure limits of the Utsira requires very sophisticated simulation tools and a good estimation of rock properties. Using all available data for the Utsira, project team members have investigated the maximum allowable injection rate that can be sustained over a 50-year storage project. Different simulation methods lead to the same conclusion that the Utsira can potentially withstand injection rates 100 times the current Sleipner injection rate.
The capacity of saline aquifers in the Norwegian North Sea for large quantities of injected CO2 has been well established. However, our ability to unlock this theoretical capacity in a safe and economically feasible manner remains hampered by significant uncertainties during the operational phase, specifically low injectivity, excess pressurization and leakage risk. The ability to effectively assess and manage these risks is related to our understanding of physical and chemical properties of the storage complex, which includes the storage reservoir and surrounding formations. In particular, the impact on geomechanical integrity of the caprock is a critical research area that has been identified in which data collection, process understanding and modeling capabilities are still lacking. This lack of knowledge can lead to ineffective and uneconomical exploitation of a given storage site, or lead to costly operational changes such as well shut-in and remediation after failure or high risk of failure.
Determining the role of complex processes with laboratory and computational approaches is essential for predicting the behavior of these systems in CO2 storage operations. The PROTECT project will address these critical knowledge needs regarding caprock inte grity and ultimately contribute recommendations to help plan and execute safe storage projects.
This project will produce high-quality datasets from a suite of laboratory testing on caprock samples, including samples of North Sea mudstones and naturally fractured caprock obtained from the Longyearbyen CO2 Lab. The experimental work will be accompanied by modeling to interpret the data, verify the models and understand coupled mechanisms acting at larger scales. New modeling approaches will be developed t o account for chemical and thermal impacts on mechanical processes. This work will provide insights towards the attainable injectivity and capacity for North Sea aquifers.