Applications of Petroleum Exploration and Environmental Geochemistry to Carbon Sequestration

The following is an executive summary of a paper written by Robert J. Pirkle, Ph.D. of Microseeps Inc. and Victor T. Jones, Ph.D. of Exploration Technologies, Inc.

Recent interest in carbon sequestration has generated a need to monitor forcarbon dioxide leakage over a variety of subsurface storage reservoirs.  The determination of whether and how much sequestered CO2 may be leaking from a subsurface reservoir requires that the measurements be made within natural geologic conduits that can be defined by mapping the distribution of thermogenic hydrocarbon seeps associated with the subject reservoirs.  Soil gas data from exploration and environmental surveys are presented as a series of case studies that provides effective guidance for locating these natural conduits.  An extensive soil gas data base is presented that includes surveys conducted over many different petroleum reservoirs, underground gas storage reservoirs, a coal gasification reactor, natural macro-seeps, earthquake-related gases and even environmental site investigations.

These case studies define the relationships between reservoir and basin-wide seepages, and even more importantly, they demonstrate the very heterogeneous nature of the geologic strata overlying the reservoirs that cause the natural seepage patterns to take on dendritic patterns that can appear to be somewhat random. This is particularly true when the seepage pattern is under sampled.  An effective monitoring system must take into account this heterogeneity, and in our experience soil gas surveys offer the most cost-effective and efficient method for meeting this requirement.  Evaluation of these natural seeps shows that the most useful gases to measure are the ethane through butane (C2 – C4) hydrocarbons because they provide the most sensitive and accurate indicators of deep-source thermogenic reservoir fluids. They are not produced by biological processes in near surface sediments, and because they are unique deep source gases they provide natural reservoir tracers that can be used for identifying leakage of sequestered carbon dioxide.

Carbon dioxide and methane are very significant gases for carbon sequestration monitoring, however, their sources can be difficult to discern since they are also generated by microbially facilitated degradation of all types of organic materials, whether natural, or petroleum based (crude oil, gasoline, diesel, kerosene, chlorinated solvents, or just methane).  Because of this biological relationship, these two gases are generally the largest concentration gases in the vadose zone, making the determination of their subsurface source even more difficult.  This dual-source relationship is compounded for a large portion of the carbon sequestration monitoring programs because many of the reservoirs selected for carbon sequestration are abandoned oil and gas fields, where contamination may be present due to spills, pipeline leakage and abandoned well casings.  Gridded soil gas surveys conducted at the outset can help in interpreting the sources of these gases because environmental monitoring experience has demonstrated that biogenic methane and CO2 have well-established, stable and predictable relationships with their buried contamination sources. These biological relationships can also be defined by the soil gas surveys used to locate the optimum leakage pathways.

Well casings provide a special case of focused migration channels that must always be considered in the operation of a carbon sequestration survey.  This is particularly significant since old oil and gas fields contain numerous plugged and abandoned wells.  In the case of plugged and abandoned (P&A) wells over a petroleum reservoir, the potential for leakage is not only related to casing cement, but could also result from improper P&A procedures that may have left large portions of the casing open.   Carbon dioxide leakage along cementation defects will significantly increase corrosion and shorten the lifetime of abandoned well casings.  Channels behind casings provide avenues for gas and brine displacement from deeper to shallower formations.  Casing failures will likely increase over time as corrosion proceeds and must be monitored diligently to avoid catastrophic failure.

The examples presented demonstrate the ability of soil gas sampling to locate vadose zone anomalies associated with subsurface reservoirs and/or buried contamination.  Given an adequate density, soil gas data has the ability to vector the direction toward the largest magnitude seepage.  Flux chambers have been employed for this purpose, but have not provided good results because they are designed for measuring diffusive rather than advective flux.  Flux chambers must be located directly over an advective seep in order to make the required flux measurement.  In order to achieve useable flux results, without having a very large number of individual flux stations, it is imperative that the flux chamber locations be guided by soil gas data.

Surveys conducted over fields selected for carbon sequestration should begin with a regional soil gas survey designed to determine the overall pattern and composition of seeps located within the general area.  Exploration examples integrated with available geological/geophysical data can provide initial guidance for these regional grids.  Regional survey results should be followed by more focused infill surveys for refinement of any associated deep source seeps selected as possible monitoring sites.  These selected monitoring sites may require even more detailed sampling in order to define any biogenic CO2 and CH4 gases that might be associated with the thermogenic hydrocarbon seeps, or with any subsurface contamination plumes that might overlap the selected monitoring sites.  Once the thermogenic hydrocarbon and biogenic CO2 and CH4 concentrations have been mapped, experience has shown that they are very stable and can be used as a background against which any changes associated with carbon sequestration related leakage can most easily be measured.  Long term monitoring can then be conducted by establishing permanent monitoring stations based on the soil gas survey results.  Alternatively, selected portions of the soil gas surveys can be rerun on a periodic basis.