Appendix 1 Methodology for planning a monitoring program
R.J. Chalaturnyk & W.D. Gunter, APEC Capacity Building in the APEC Region, Phase II
Monitored decision framework
A monitored decision framework is a planned approach to decision making over time that draws on long-term field measurements for input, with planned analysis of the measurements and appropriate contingent actions (D'Appolonia, 1990). The framework is designed to deal with uncertainties in the geological storage system and making project design decisions with the knowledge that planned long-term observations and their interpretation will provide information to decrease these uncertainties. The framework is also designed to provide contingencies for all envisioned outcomes of the monitoring program.
Some elements of the monitored decision framework are already implemented as normal practice in subsurface waste disposal or oil and gas activities. The difference in the context of geological storage of CO2 is the need (and likely a requirement soon) to confirm the "science" of storage and to ensure adequate storage permanence. These drivers demand monitoring programs that inform operating practices but provide value-added knowledge on the evolution of the CO2 storage processes.
Systematic approach to planning monitoring programs
The purpose of geological storage monitoring is to:
- "Truth" the predictive capability of the simulators;
- Validate the physics of the storage process;
- Reduce uncertainty associated with parameters;
- Identify and validate hydrodynamic, ionic, mineral, solution or residual gas storage mechanisms in oil and gas reservoirs and aquifers;
- Correlate operational issues with aquifer and caprock behavior (e.g., high injection pressures leading to caprock hydraulic fracture); and
- Satisfy regulatory and public safety concerns.
These general attributes of monitoring can be classed into three distinct mandates:
- Operations – involve monitoring / controlling actual in situ processes by changes in the injection/production strategy based on the measured variables. There are minimal regulatory requirements; and the need for additional operations monitoring is determined by the complexity of the injection/production scenario. Operational monitoring is generally concerned with migration;
- Scientific or Verification – involves measurements that improve the understanding of complex processes occurring in situ. Scientific or verification monitoring is generally concerned with migration and leakage. This currently is a major focus of effort in geological storage research; and
- Environmental – involves monitoring aimed at safeguarding against health, safety and environmental risks. Depending on the risk level of the project, aspects of environmental monitoring may be part of operational monitoring scenarios. Environmental monitoring is generally concerned with seepage.
Figure 9.13 provides a schematic illustration showing the progression from operational monitoring through to environmental monitoring. Inherent within the monitored decision framework, is an effective fully integrated monitoring program, whether for operational, verification or environmental reasons. Planning a monitoring program should be a logical and comprehensive engineering process that begins with defining an objective and ends with planning how the measurement data will be implemented (Dunnicliff, 1998). The following sections provide a brief description of the steps that should be followed in developing the monitoring programs embodied within the monitored decision framework.
Figure 9:13: Operational, verification and environmental monitoring levels (after Chalaturnyk and Gunter, 2004).
There are a number of steps involved in designing a systematic approach to planning monitoring programs:
- Defining project conditions;
- Predicting mechanisms that control behaviour;
- Answering technical questions;
- Selecting parameters to be measured and identifying their role in answering technical questions;
- Determining the magnitude of expected change in parameters;
- Selecting instrumentation and monitoring approaches/systems;
- Selecting instrument or monitoring locations; and
- Determining timeframes and the depth for monitoring.
Each of these will be explained in more detail below.
Definition of project conditions
In the context of geological storage projects, defining project conditions is a necessary first step in the regulatory application process. For example, in Alberta, applications for acid gas injection operations must conform to a set of specific requirements given in Chapter 4.2 of EUB Guide 65 (2000). These requirements include geological interpretation of the acid gas disposal formation and bounding formations; analysis of reservoir fluids and injected stream; geological properties of the formations, and so on. In essence, the regulatory process can provide the majority of the information required to plan the monitoring program. Depending on the reasons for monitoring, however, additional information may be required and should be collected at this stage in the planning process.
Prediction of mechanisms that control behaviour
This second step involves developing working hypotheses about the important mechanisms that control the behaviour of injected CO2. This has been studied extensively over the last decade and from aperspective. This step is similar to the features, events and processes (FEPs) identification stage prior to performing risk analyses.
Technical questions to be answered
This step is perhaps the most critical step in the systematic development of a monitoring program for a particular geological storage project. This is because selection of an appropriate measurement method and/or the selection of instrumentation is based on whether it can provide the data necessary to answer a particular technical question. If there is no question to answer, there should be no instrumentation. This applies equally to all three monitoring scenarios: operations, verification and environmental. For example:
- Operational questions can be as simple as "what are the wellhead injection pressures?" or "what is the distribution of CO2 within the reservoir?"
- Verification questions may pose the identical question, for example "what is the distribution of CO2 within the reservoir?", with the only distinction being the degree of resolution required to answer the question.
Select parameters to be measured and their role in answering technical questions
The range of physical processes active in geological storage is large and identifying measurable parameters to help explain these processes is difficult. Parameters such as pressure, temperature, load, deformation, acoustic velocity, and resistivity are commonly selected. Rock-fluid parameters such as conductivity, pH, ionic strength, stable isotopes, and mineralogy begin to identify more complex parameters that aid in answering specific questions. These parameters are referred to as performance measures.
Determining the magnitude of change expected in parameters
Predictions or estimates of the maximum possible value of a parameter provide limits on the instrument range and an estimation of the minimum value of a parameter. This makes possible the selection of appropriate instrument sensitivity or accuracy. Parametric studies with the models or analysis tools that will be used throughout the project can provide valuable input to assist in establishing the range, accuracy and sensitivity required of an instrument. The uncertainty and variability expected in a performance measure must also be quantified.
Select instrumentation and monitoring approaches/systems
Instrument selection should recognize any limitations in skill or quantity of available personnel and should consider the implications of construction, installation and long-term needs and conditions. Criteria established for operations monitoring may be quite different from environmental monitoring and may entail selection of two different monitoring methods. Monitoring approaches or systems refers to the selection of techniques, rather than instruments, within a particular approach. However, instrument selection remains an important step.
Timeframes and Depth of Monitoring
The previous discussion raises a number of outstanding issues which must be addressed, namely:
- How should monitoring tools be assessed (geophysical versus geochemical)?;
- At what depth should monitoring occur?;
- How should the type of monitoring be identified (remote versus in situ)?; and
- How should the frequency of monitoring be defined?
The focus of monitoring depends on the phase of monitoring (operational, verification or environmental) and the particular mechanism (migration, leakage or seepage correlating with depth in the subsurface) being measured. Risks in all three have consequences.
The frequency with which monitoring is undertaken is also an important design element in the monitored decision framework. Currently, regulatory agencies focus mostly on 25 years. This is the time period approved for waste fluid injection (of which CO2 would be an example) into depleted hydrocarbon reservoirs or deep saline aquifers. The lifetime of the injection operation is limited by the reservoir capacity and the injection rate. During injection and abandonment, issues are safety, well integrity, caprock integrity and monitoring. Operating, shut-in and abandoned wells in the vicinity of the injection well which may be contacted by the waste fluids have to be identified and assessed for leakage potential both in the short-term (during injection) and the long-term (after abandonment of the reservoir or aquifer). The definition of 'long-term' is based on perceived risk of leakage, which is expected to decrease towards a stable condition as the pressure decays after injection ceases. Figure 9.14.and ECN (2003) provide certain guidelines to approach the time framework for monitoring, as illustrated in
Figure 9.14: Timeframe of monitoring (after TNO and ECN, 2003).
Most of the knowledge in remediation for geological storage comes from experiences in underground gas storage. If gas leaks, appropriate remediation measures should be taken to stop or reduce the leaks. Generally repairing or plugging the leaking wells is sufficient to eliminate the problem. If the leaks are not related to well damage (i.e., they are caused by high storage pressures or an inadequate geological framework), the pressure in the storage aquifer or reservoir might have to be reduced (Benson et al., 2002). Similar actions will be used in the case of a leaking CO2 storage project.
Bibliography for Appendix 1
Benson, S.M., Hepple, R., Apps, J. Tsang, C.F. and Lippmann, M. Comparative Evaluation of Risk Assessment, Management and Mitigation Approaches for Deep Geologic Storage of CO2. Earth sciences Division, E.O. Lawrence Berkeley National Laboratory, 133 p, 2002.
Benson, S.M.,.Gasperikova, E. and Hoversten, M. Overview of Monitoring Techniques and Protocols for Geological Storage Projects. IEAR&D Programme, 89 p, 2004.
Benson, S.M., Hoversten, M., Gasperikova, E. and Haines, M. Monitoring Protocols and Life-Cycle Costs for Geologic Storage of Carbon Dioxide. Proceedings 7th International Conference of Greenhouse Gas Technologies, GHGT-7, Paper 410, Vancouver, 10 p, 2004.
Chalaturnyk, R.J. and Gunter, W.D. Geological storage of CO2: Time frames, monitoring and verification. Proceedings 7th International Conference of Greenhouse Gas Technologies, GHGT-7, Paper 530, Vancouver, 10 p, 2004.
D'Appolonia, E. Monitored Decisions. Journal of Geotechnical Engineering. Vol. 116, No. 1, pp. 4-34, 1990.
TNO-NITG/ECN. CRUST CO2 Reuse through Underground Storage. 28 p, 2003.