DV4.4 Optimizing Fibre-Optic Monitoring: A case study in the Norwegian North Sea

This report provides an overview of monitoring technologies for CO2 storage being considered in the ACT SHARP Project. SHARP is a research project funded under the ERA-NET ACT programme for accelerating Carbon Capture and Storage (CCS). The overall aim is to “Improve the accuracy of subsurface CO2 storage containment risk management to a level acceptable to both commercial and regulatory interests”.\nWithin the SHARP project, Work Package 4 (WP4) has the overall goal of developing more intelligent methods for monitoring rock strain and fluid pressure. To focus on this effort, we set up a more specific objective to ‘Design improved monitoring schemes using right-time and right-place detection.’ The initial tasks in WP4 were to understand the rock failure risks for each case study site (Deliverables 4.1 and 4.2). These studies form the basis for the monitoring parts of WP4.\nDepending on the storage site at hand, CO2 storage monitoring might involve various techniques and methods such as seismic data acquisition, utilizing pressure and temperature sensors, gravity and magnetic surveys, electro-magnetic surveys, and/or well logging tools. This report provides an overview of novel fibre optic (FO) monitoring technologies and focus on how FO monitoring systems might be optimized combined with conventional technologies. We use examples from the Norwegian North Sea case study area to show how a ‘right-time and right-place detection’ system could work, utilizing both conventional data but especially new emerging FO sensing technologies.\nThe primary focus of this report is on monitoring for containment risk management. Conventional approaches to CO2 storage site monitoring often focus on monitoring the migration of the CO2 plume (i.e. saturation monitoring) which is an essential part of both containment and conformance monitoring. Here we shift the focus to how the rock system responds to CO2 injection, specifically monitoring of pressure, strain and temperature. As CO2 injection proceeds, changes in fluid pressure will result in various geomechanical effects, including elastic deformation and potentially permanent deformation (e.g. in-elastic strains and fractures). If several storage sites are located in the vicinity of each other, they might impact each other’s stress field due to changes in fluid pressure. There is a need for monitoring and handling this potential interaction. Initial assessments of stress and pore pressure effects can be done by rock mechanical testing (see SHARP WP3), but we also need to monitor how the rock system responds to pressure changes. This can be achieved using a combination of microseismic monitoring and downhole measurements of pressure, strain and temperature. It is important to emphasize that while monitoring of fluid displacement is usually a primary objective, in this report we mainly focus on monitoring the effects of geomechanical changes.\nContainment risk management encompasses leakage risk management, but it is broader than that, with seismicity monitoring playing a crucial role. Seismic monitoring is also essential for public perception. In the case of significant seismic activity (e.g., events with magnitudes above 2), it is vital to distinguish between CO2 injection-induced events and natural seismicity. Operators must localize such seismic events and determine, for example, if they occur within the reservoir or beneath it. Therefore, reducing depth uncertainty is a crucial risk mitigation step for maintaining public trust and regulatory approval.\nThe report demonstrates the benefits of integrating existing offshore installations, such as Permanent Reservoir Monitoring (PRM) systems and telecom and power cables, into the monitoring technology to reduce the depth uncertainty in seismic event localization.\nA Norwegian North Sea case study illustrates how earthquake detection and seismic monitoring capabilities of the Norwegian National Seismic Network (NNSN) improve with the integration of additional monitoring technologies. Selected stations from two PRM systems streaming data to the NNSN in near real-time significantly enhance earthquake location accuracy. Incorporating the HolsNøy Array (HNAR) and the concept of array processing into this integrated system can reduce seismic location uncertainties by about 50%. Furthermore, the integration of Distributed Acoustic Sensing (DAS) technology utilizing surface optical fibers in submarine telecom infrastructure further reduces depth uncertainty to about 2 km, compared to approximately 10 km uncertainty using the NNSN alone.\nIn general, the utilisation of FO sensing as a CO2 storage monitoring solution is emerging fast and can include fibres both at surface and downhole. FO sensing can be used in several ways: (a) changes in temperature can be measured using distributed temperature sensing (DTS), (b) direct changes in rock strain can be recorded downhole using distributed strain sensing (DSS) and distributed acoustic sensing (DAS); (c) passive detection of microseismic events can be done using FO DAS cables, as well as (d) active seismic monitoring and seismic imaging.\nWe aim to illustrate with examples from published research and field trials from various regions worldwide, how combining these various monitoring technologies can create an enhanced monitoring scheme that provides a holistic understanding of subsurface dynamics in the Norwegian North Sea region.\nFocusing on the feasibility of DAS and its detectability, it was demonstrated in a field trial in Canada that microseismic events with magnitudes lower than -0.6 are detectable at distances exceeding 10 km when the DAS fiber was deployed in an injection well cemented behind casing. Experience from the geothermal sector show that utilizing low-frequency DAS downhole allows for the direct observation of microseismic events linked to fracture openings, with observed strain changes of about 0.5 nanoStrain. Current DAS technologies measure strain changes with picoStrain resolution, enabling the detection of these fracture openings and other signals related to CO2 injection when deployed in a well. The overall strains which occur in response to CO2 injection are about 2 milliStrain in formations and typically in microStrain range when the fiber is deployed in or cemented between formation and casing.\nAdditionally, DAS in surface fibers can measure seismic anisotropy and by this infer stress field changes. This capability was demonstrated by measuring icequake-induced seismic events in Antarctica. It is shown that DAS surface fibre can image the arrival of both fast and slow shear waves, and thus the delay time can be measured with high temporal and spatial accuracy (e.g., with a slowness resolution of about 0.1 s/km)\nPermanent Reservoir Monitoring (PRM) systems provide high-resolution measurements capable of detecting subtle changes, such as variations of approximately 200 microseconds time shift, indicative of pressure, strain, and temperature alterations. Additionally, stress field estimates utilizing shear wave anisotropy provide crucial inputs for stress field estimations updating geomechanical models, facilitating the development of preventative monitoring strategies.\nThe primary advantage of FO monitoring lies in its continuous real-time monitoring and high-resolution measurements, facilitating the early detection of subsurface changes. Although real-time monitoring is valuable since it enables prompt decision-making, changes in the stress field may also result in delayed fault reactivation, which is not immediately evident in the real-time data. This means that merely including FO as advanced monitoring solution as such is not sufficient. It is important to monitor the right parameters at the right time and place to record important input parameters for predictive geomechanical modelling.\nFor instance, various studies demonstrate that human activities, such as fluid injection or withdrawal, can trigger earthquakes. This poses risks in industries like geothermal energy production where it was observed that horizontal stresses were shifting by tens of mega Pascals, potentially leading to prolonged induced seismicity even after operations cease. Emphasized in the report by Williams et al. (2022) are the stress history, seismicity, and rock mechanics parameters, which serve as crucial inputs for geomechanical modelling and predictive analysis. It is imperative to complement real-time monitoring with preventative monitoring to identify potential risks before their manifestation in real-time data.\nThe data obtained from FO monitoring can be integrated into geomechanical modelling workflows, providing calibration and ground truthing of models as projects develop. This will enable a more comprehensive understanding of the subsurface behaviour. Such predictive geomechanical modelling empowers operators to devise proactive strategies, enables early identification of potential issues and enables the implementation of preventive measures to mitigate adverse events.\nFurthermore, as technological advancements in CO2 storage and monitoring continue to evolve, especially the use of FO sensing, there is a growing recognition that existing regulations may need to be updated to incorporate these innovations. Integrating fiber optic technologies into regulatory frameworks can significantly enhance the accuracy and reliability of monitoring efforts.

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