Geochemistry solving production, ownership, and environmental issues

PART II: This is the final segment of a two part series, focusing on the use and effectiveness of geochemistry in different phases in the life of an oilfield. Part I dealt with geochemistry in exploration and development.

During every phase of petroleum asset life, information is critical for proper management of the asset. Asset managers have a number of tools at their disposal to address exploration and production questions including those provided by geophysics, geology, engineering, and geochemistry. Petroleum geochemistry has historically been applied to questions encountered during the early stages of asset life.

The techniques necessary for addressing questions during exploration, such as source evaluation, maturity modeling, hydrocarbon volume assessment, and prospect evaluation, though not always routine, are well established and widely used. Geochemical techniques for addressing questions during production and enhanced recovery have been developed much more recently and are not as widely used.

Application of geochemistry to production can address questions regarding unrecognized pay opportunities, pay allocation, reservoir continuity and communication, production decline, and monitoring of production performance.

Monitoring fluids

The fact that hydrocarbon fluid compositions are affected by geologic and reservoir processes is what makes fluid composition useful for studying hydrocarbon generation, migration, and entrapment history. As we have seen, production and enhanced recovery processes also affect hydrocarbon fluid compositions. However, production processes, in particular, affect hydrocarbon compositions in predictable ways.

Slow or sudden changes in the composition of produced fluids can indicate changes or problems with the production equipment. To recognize these changes, produced hydrocarbon fluids must be periodically sampled and monitored. In fact, periodic geochemical analysis of produced fluids has been successfully applied as a useful tool to a number of production issues.

In many wells, oils from different stratigraphic zones are commingled and produced from a single production string. In many cases, legal or ownership requirements will necessitate the monitoring of production from individual commingled zones. Even without those requirements, it is necessary to monitor the production from individual zones to optimize production of from the reservoirs. When there is sufficient (even a small) difference in the composition of the oils from the different zones, periodic geochemical monitoring can be used to determine the production from each zone.

The technique requires only that a small (a few milliliters) sample of the produced hydrocarbons be taken periodically for analysis. Changes in the composition can identify and quantify depletion of individual zones relative to one another and recognize changes in production related to reservoir or other production problems. The techniques can be applied to a larger number of commingled zones, provided there are sufficient differences among the oils in the various zones to distinguish each.

Initially, oils from each zone are fully analyzed and the differences identified. After the initial analyses, a smaller number of diagnostic analyses can be used for monitoring the composition of the commingled fluid during production. There are techniques available for monitoring even when samples from the individual zones are not available.

Periodic hydrocarbon monitoring data can also be useful when zones are not commingled. In cases where different zones are produced in separate tubing, periodic monitoring of the hydrocarbon fluid composition can detect failures in the production apparatus.

Containment breach

In one case, a deeper, higher-pressure zone and shallower, lower-pressure zone were being produced through separate tubing. Ownership interests were different for the two zones, so the integrity of the separate production was important beyond the production ramifications. Gas chromatograph (GC) analysis of the oils from the two zones showed that they were clearly different and easy to distinguish. Years later, another GC analysis of the two production oils revealed that they were the same and matched the oil from the high-pressure zone.

Further investigation revealed that corrosion had breached the containment of the high-pressure oil and was allowing it to flow into the lower pressure zone. This meant that the production from both strings had been from the higher-pressure zone since the breach and the owner of the higher-pressure oil had been "giving away" oil to the owner of the lower pressure zone.

Cementing failure

In another case from California, a well was producing solely from the Monterey formation, but the shallower Sisquoc zone had been cemented. During production, fluid was discovered behind the well casing. Analysis indicated that the fluid did not match the oil from the Monterey.

However, since samples had been taken and analyzed from the Sisquoc prior to cementing, the geochemists were able to compare the fluids and determine that the behind-casing fluid matched the Sisquoc. This indicated that the cement job had failed and the work was redone.

Problem prediction

Another problem encountered during production is that of solid deposition and plugging. Asphaltenes, paraffins, and gas hydrates can, under the right conditions, precipitate and cause plugging problems during production. Geochemical analysis of reservoir fluids can quantify and characterize asphaltenes and paraffins and permit prediction of precipitation problems during field appraisal.

Recent work also suggests that the evaluation of gas, water, asphaltene, resin, and other surfactant content can enable the prediction of hydrate formation. Prediction of precipitation problems early in the exploration process allows planning and inclusion of costs of specialized production techniques or avoiding problems by selecting production areas that do not exhibit tendencies to precipitate solids.

Environmental issues

Geochemistry is an important element in addressing environmental issues related to petroleum exploration, production and transportation. Keith Kvenvolden (Kvenvolden, 1993) of the USGS performed a study of residual tar in Prince William Sound, following the Exxon Valdez spill in 1989. He gathered tar from beaches and other areas around the sound and used carbon isotopic composition to determine the source.

In 1990, most of the tar samples matched Alaska oil that was spilled from the Exxon Valdez. However, a 1992 sampling yielded mostly tar from California oil. After some investigation, the authors concluded that most of the tar contamination remaining in Prince William Sound is the result of the destruction of an asphalt storage facility in Valdez by the earthquake of 1964 and subsequent tsunamis.

This event spilled large amounts of asphalt into the sound. Since asphalt is more resistant to evaporation and degradation than the much lighter Alaska oil, it has remained, while most of the Valdez oil has dissipated.

Litigation

Geochemistry can be useful in other areas of the petroleum business. For example, a few years ago, Four Star Oil entered litigation with the Internal Revenue Service (IRS) to recover overpayment of taxes. The issue involved whether oil produced from certain wells in California in the early 1980's was "tar sand" by the IRS definition. A major issue in the definition is whether the oil is produced in its "natural state."

An important point in showing that the oil was not produced in its natural state was to show that it was necessarily chemically altered during production. A geochemical approach was able to show that the oil was chemically altered during steam flooding.

The finding was based on laboratory tests on a number of asphaltenes and on the kinetics of conversion of California Monterey kerogen (which is similar to the asphaltenes found in the oil) to oil. It was shown that a substantial proportion of asphaltenes are converted to lighter, oil-like substances when exposed to the temperatures of the steamflooding for a few hours to a few weeks. This was an important part of the overall case by Four Star that led to a settlement prior to going to court.

Geochemistry is a proven tool for managing and monitoring hydrocarbon assets in every stage of asset life. From the earliest stages of exploration to the final stages of enhanced recovery and even beyond, geochemistry can provide vital information about almost every aspect of hydrocarbon asset management.

In addition, geochemistry is inexpensive, compared to the value of the hydrocarbon assets, investments in drilling and production hardware and even compared to many other sources of information currently used. We recommend that geochemistry be utilized at every stage of petroleum asset life as part of an integrated toolbox that includes geophysics, geology, engineering data, and other sources.

References

Ramos, H., Callejon, A., et al. (1999); "Reservoir Compartmentalization in the Arcabuz field, Burgos Basin: A Geochemical Approach;" Mexican Conference on Revitalization of Mature Exploration Provinces, Veracruz, Mexico.

Robison, C., Elrod, L., et al. (1998); "Hydrocarbon generation, migration, and entrapment in the Zhu 1 depression, Pearl River Mouth basin, South China Sea;" International Journal of Coal Geology 37: 155-178.

• Collect samples: During exploration and development, drilling sediment and all encountered hydrocarbon fluids should be sampled and stored. Sediment should be sampled with cuttings. Organic rich sediments and reservoir intervals should be cored, especially during the exploration phase. Drilling fluids, especially oil-based and those with organic additives, should be periodically sampled as well. All encountered hydrocarbon fluids should be sampled. When possible, reservoir-condition samples are best. Although this will add some cost to drilling, the samples can have enormous value in later stages of asset life.
• Analyze hydrocarbon samples when initially taken: Simple compositional and isotopic analyses are not expensive, but can be valuable. For example, an unexpected composition for a sample during development drilling could indicate an undetected barrier to reservoir communication, and indicate that the drilling program may need modification. Alternatively, it might indicate a phase segregation event that could lead to new discoveries in the field. In addition, the analyses provide a baseline for comparison with later production samples and can be useful for monitoring production effectiveness.
• Think geochemistry: Remember, virtually everything that happens to hydrocarbons during generation, migration, accumulation, and production affects their composition. Geochemistry measures these changes and can detect important variations. These variations and changes can reveal information that is critical to proper and efficient management of the hydrocarbon asset. Utilize geochemistry as a tool for decision-making during every phase of hydrocarbon asset life.