Site-Specific Standard in CCS Monitoring

Expert-Based Development of a Site-Specific Standard in CO2 Sequestration Monitoring Technology

Process for Quantitative Assessment of Tool Site-Specific Effectiveness

A number of variables determine whether a tool is effective. In this study, we focus on those variables that are site specific. However, to make a clear evaluation, we have prepared a list of factors that influence whether a tool is effective, so that those that are not site specific can be dealt with via specifying best available technologies.
Many variables interact to determine whether a tool is effective, adding complexity to our assessment. In this study we force a rather artificial single-variable approach on the assessment. In addition, we have initially selected sets of variables that we think will have a strong effect on tool sensitivity, which is subject to revision as the assessment matures.

One major difficulty in determining the appropriateness of tools is determining the appropriate signal threshold that it is desirable to detect. EPA rules provide no threshold below which a leakage or mismatch with model results are considered negligible.

The table below provides, for each tool type, a preliminary list of site-specific and non-site-specific variables that impact tool sensitivity. In the next year this matrix will be assessed more deeply via literature review, expert panel review, and where needed in a numerical modeling of tool sensitivity. This version of the table is assembled for the purpose of discussion.

Preliminary inventory of site-specific factors that influence tool sensitivity compared with general factors that are the same at all sites.

Compartment tested	Non-site-specific factors that influence tool sensitivity*	Site-specific factors that influence tool sensitivity*	Notes
CO2 concentration of air	Analytical detection limits Sample height Location of sample point with respect to leakage path	Ambient daily and seasonal variation Across-site variation in ambient CO2 emissions Dilution between emission point and sample point
Natural or introduced CO2 tracers sampled in air	Analytical detection limits Lab/sampler contamination Location of sample point with respect to leakage path	Difference between ambient composition and injectate composition Stability of tracer during leakage process, dissolution, sorption processes	Even introduced tracers have some background in environment
Percent CO2 in soil gas (relative to O2 and N2)	Analytical detection limits Well construction—atmospheric contamination	Ratio of leakage rate to natural cycling processes In situ CO2 production, release, dissolution, sorption processes	Collecting O2 and N2 provides process information
Natural CO2 tracers (δC¹³, noble gasses) sampled in soil gas	Analytical detection limits Lab/sampler contamination Location of sample point with respect to leakage path	Difference between ambient composition and injectate composition Stability of tracer during leakage process	These tracers may be more conservative and therefore more sensitive than CO2 itself.
Introduced CO2 tracers (PFT) sampled in soil gas	Analytical detection limits Lab/sampler contamination Location of sample point with respect to leakage path	Stability of tracer during leakage process, e.g., dissolution, sorption during transport	Even introduced tracers have some background in environment
Groundwater salinity	Well-completion issues, perforated interval, mixing along sand pack, Analytical detection limits Location of well sample point with respect to leakage path, considering density, mixing	Ambient variably in salinity Aquifer flow rate and process, mixing	Might use near-surface conductivity to sample a larger area
Groundwater/above-zone monitoring interval (AZMI) pH	Analytical detection limits Sampling process, outgassing Location of well sample point with respect to leakage path, considering density, mixing	pH change with respect to introduced CO2, buffering Ambient variably in pH Aquifer flow rate and process, mixing	pH is sensitive to sampling procedure, especially temperature, pressure, and outgassing
Groundwater/AZMI DOC/DIC	Analytical detection limits Location of well sample point with respect to leakage path, considering density, mixing	Ambient variably in DOC/DIC Aquifer flow rate and process, mixing Sorption and mineral reaction	Best used in combination with other measurements
Groundwater/AZMI Head-space gas	Analytical detection limits Location of well sample point with respect to leakage path, considering density, mixing	Ambient variably in free/dissolved CO2 Aquifer flow rate and process, mixing Sorption and mineral reaction	Head gas is sensitive to temperature, outgassing, sampling procedure Best used in combination with other measurements
Groundwater/AZMI major and minor elements	Analytical detection limits Location of well sample point with respect to leakage path, considering density, mixing	Ambient variably in major and minor elements Aquifer flow rate and process, mixing Sorption and mineral reaction	Sensitive to lab detection limits, proper sampling, stabilization and curation for various elements and compounds; best used in combination with other measurements constituents
Confined aquifer/ AZMI pressure	Gage sensitivity, depth error in positioning gage, land surface elevation error. Well completion, perforations, well fluids Location of well measurement point with respect to leakage path	Aquifer ambient variability to recharge, barometric pressure, tides, etc., fluid density; monitored zone thickness, transmissivity, storativity
Pressure in injection zone	Gage sensitivity, depth error in positioning gage Well completion, perforations, well fluids	Ambient variability to recharge, barometric pressure, tides, etc., fluid density; zone thickness, transitivity, storativity
Tubing pressure at wellhead	Noise from daily thermal variations Well completion, perforations, well fluids	Rate of breakthrough. Dissolved CO2 may complicate response Non-CO2 fluid changes will complicate response
Pulsed neutron logging to detect CO2 saturation in injection zone or in AZMI	Well completion, perforations, well fluids Tool construction and deployment Processing of sigma Near-well-bore perturbations	Salinity of ambient fluids Saturation of CO2 Noise Stratigraphic complexity, bed thickness	Complex processing is possible
Resistivity logging to detect CO2 saturation in injection zone or in AZMI	Well completion, perforations, well fluids Tool construction and deployment Processing Near-well-bore perturbations	Salinity of ambient fluids. Saturation of CO2 Noise Stratigraphic complexity, bed thickness	Not used in steel wells
Sonic logging to detect CO2 saturation in injection zone or AZMI	Well completion, perforations, well fluids Tool construction and deployment Processing of signal Near-well-bore perturbations	Change in impedance and velocity with change in CO2 saturation Stratigraphic complexity, bed thickness Saturation of CO2 Noise	Tool size can limit deployment
2-D seismic profiling to locate plume edge	Source selection, receiver deployment, many variations in set-up Processing	Static error Signal:noise ratio Change in impedance and velocity with change in CO2 saturation Stratigraphic complexity, bed thickness, depth Saturation of CO2	Multiple variables intersect complexly Focus area for CCP study
Walk-away VSP to locate plume edge	Source selection, receiver deployment, many variations in set-up Processing	Static error Signal: noise ratio Change in impedance and velocity with change in CO2 saturation Stratigraphic complexity, bed thickness Saturation of CO2	Multiple variables intersect complexly Higher vertical resolution; limited horizontal coverage Focus area for CCP study
3-D seismic survey for locate plume edge	Source selection, receiver deployment, many variations in set-up Processing	Static error Signal: noise ratio Change in impedance and velocity with change in CO2 saturation. Stratigraphic complexity, bed thickness, depth Saturation of CO2	Multiple variables intersect complexly Focus area for CCP study
Surface gravity	Consideration deferred		Focus area for CCP study
Surface electrical and magnetic techniques	Consideration deferred		Focus area for CCP study

* Sensitivity is quantifiable response to the specified signal

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