Selection of monitoring tools is a complex process that involves a number of factors, the site-specific characteristics of which influence only a few. The process of testing and monitoring (T&M) and monitoring, reporting, and verification (MRV) are still immature, with both active discussion of policy and regulatory needs and assessment via field tests and tool development.
Monitoring in the context of GS has been previously explored and considered, with a number of summaries of approaches, e.g., Nicot and Hovorka (2008), Benson and Cook (2005), GEO-SEQ (2004), DOE NETL Monitoring Best Practices, and WRI Best Practices, among others. Although these documents highlight and rank some of the benefits of various tool types, none of them provides rigorous guidance on the limits of tool sensitivity needed to select a site-specific monitoring procedure. In addition, no consistent view on how to monitor has wide acceptance among technical practitioners, leaving us uncertain as to how to deploy an adequate monitoring program.
Considering uncertainty in the variables that will drive monitoring tools selection, we considered three characteristics in selecting the first tools for assessment.
- Availability of a reasonable body of relevant experience from which the site-specific sensitivity of a tool can be assessed. We considered tools that had been tested in a list of past GS pilots that we considered “monitoring dense” and for which experts are available to consult.
- Interest in using a tool for monitoring in diverse sites. We considered suites of similar projects that we are working on, and attempted to imagine site types with a range of properties. For example, we imagined structural closures and regionally extensive dipping (with respect to buoyant trapping of free-phase CO2); hydrologically closed and hydrologically open boundary conditions (with respect to brine displacement and pressure build-up, stratigraphically simple, thick, uniform injection zones and highly heterogeneous injection zones); thick and thin injection zones; well-characterized seals and uncertain seals; injection at shallow (>2,000 ft) to deep (>10,000 ft); different surface conditions such as offshore, mountainous, wetland, urban, cropped, thickly forested, and different types of above-injection-zone geology. Next year, models construction will more formally assess this diversity.
- Tools shown to be useful, sensitive to measuring needed parameters, and durable in the field: We did not select tools thought to be of questionable value. In addition, commercial tools that can be purchased from service companies or vendors were preferentially selected for the first assessments. This choice should not be interpreted to imply that new methods now in development will rise to the front; in fact we hope that this is true.
We anticipate that during the course of this project and during the years following project completion, the list of tools selected will grow. However, for this first year and for the methods development proposal to be submitted to the expert panelists early next year, we proscribed the initial tools selected very narrowly so as to assist in methodology development. The following list summarizes our initial tool selection.
- Direct sampling of CO2 concentration of air
- Direct sampling of natural or introduced CO2 tracers sampled in air
- Direct sampling of percent CO2 in soil gas (relative to O and N2)
- Direct sampling of natural CO2 tracers (δC13, noble gasses) sampled in soil gas
- Direct sampling of introduced CO2 tracers (PFT) sampled in soil gas
- Direct sampling of groundwater/above-zone monitoring interval pH
- Direct sampling of groundwater/above-zone monitoring interval DOC/DIC
- Direct sampling of groundwater/above-zone monitoring interval head-space gas
- Direct sampling of groundwater/above-zone monitoring interval major and minor elements
- Confined aquifer above-zone monitoring interval pressure using a pressure transducer below water table with hourly readouts
- Pressure in injection zone using bottom-hole pressure and temperature gage in perforated well bore, isolated with packer with hourly readouts
- Surface pressure-based measurement of CO2 breakthrough using tubing pressure at wellhead
- Pulsed neutron logging to detect CO2 saturation in injection zone or above-zone monitoring interval using commercial wireline deployment and data interpretation
- Resistivity logging to detect CO2 satuaration in injection zone or above-zone monitoring interval using commercial wireline deployment and data interpretation
- Sonic logging to detect CO2 saturation in injection zone or above-zone monitoring interval using commercial wireline deployment and data interpretation
- 2-D seismic profiling to locate free-phase CO2 assuming standard commercial deployment and processing with best practices
- Offset or walk-away VSP to locate free-phase CO2 plume edge assuming standard commercial deployment and processing with best practices
- 3-D seismic profiling to locate free-phase CO2 assuming standard commercial deployment and processing with best practices
Additionally, the following tools mentioned by UIC class VI as examples represent future tool development. As these are novel techniques, consideration of these tools is currently deferred.
- Gravity survey to locate free-phase CO2 plume edge
- Electrical and magnetic techniques survey to locate free-phase CO2 plume edge