Technology - Technical specifications

A COMPARISON OF SURFACE GEOCHEMICAL TECHNIQUES

Summary

Vertical migration of volatile compounds from oil and gas reservoirs can be sensed using surface geochemical techniques.  For over 50 years, geologists exploring for oil & gas have developed and tested such methods which can image hydrocarbon filled reservoirs by measuring near surface parameters.  These traditional techniques have included direct soil analysis, active soil gas measurement, and microbial techniques. Unfortunately, these traditional methods suffer from some limitations.  These limitations include the poor adsorptivity of soils in 1 of 5 exploration sites, sampling difficulty due to poor soil permeability, low analytical sensitivity, limited data sets of C1-C5 hydrocarbons (methane-pentane), problems resulting from variability in the site soil and meteorological conditions, and interference from biologically generated methane.  For these reasons, these techniques offer limited utility and hence limited value.

More recently a revolutionary technique has been developed, the GoreÔ Survey for Exploration, which overcomes the shortcomings of these other methods, and when combined with other exploration tools, greatly reduces risk and enhances exploration success. This survey uses a sampler consisting of a hydrophobic engineered adsorbent inside a GORE-TEXÔ membrane tube, deployed for about 17 days, using a special TD/GC/MS analytical method with sensitivity in the ppt range, and using multivariate statistical interpretation techniques.  O&G Majors and Independents who have evaluated all of these techniques exclusively rely on the Gore Survey for their surface geochemical surveys.

Detailed Discussion

Vertical Migration

All near-surface methods rely on documented physical phenomena of vertical microseepage of hydrocarbons from the reservoir (Klusman 1996).  This is different from macro seepage where a reservoir is breached and hydrocarbons are shunted along a fault.  With microseepage, an effective reservoir seal is still in effect but the reservoir pressure and high hydrocarbon concentration create a natural driving force.  Laws of nature dictate that all systems move from high concentration and high pressure found in the reservoir toward the low concentration and pressure found at the surface of the earth.  While the reservoir seal is effective in holding 99+% of the hydrocarbons in the reservoir, the seal is not completely impervious, and grain boundaries and micro-cracks throughout the overburden above the reservoir, are pathways for movement of the hydrocarbons.

There are likely several physical mechanisms that aid in driving these hydrocarbons toward the surface of the earth at rates on the order of meters/day.  Two that may be most significant include:

• Micro-buoyancy: relative density differences between hydrocarbons and water or soil,
• Gas entrainment: Gases rising toward the surface carry hydrocarbons
  
  

Direct Soil Analysis

In direct soil analysis, a soil sample is collected over the site and soil bound hydrocarbons, typically in the range of C1-C5 (methane-pentane), are measured.  This technique has the advantages of simplicity in collection and analysis and is relatively inexpensive.  However it has four major shortcomings:

• First, many soil types are very poor collectors of hydrocarbons due to their lack of organic content, mineral types, pH, and other soil parameters (Conant, Pignatello).  In studies looking for C13/C12 isotopes, the soils of only 1 in 5 exploration sites had the minimum required nano-mole (12 nanograms) of hydrocarbons required for this analysis.  Most direct soil analysis techniques require orders of magnitude more bound hydrocarbon than 12 nanograms.  Thus few exploration sites will favor this technique simply because the soils are not good hydrocarbon collectors.  In addition, over medium to large exploration sites, the soil characteristics and adsorbtivity can vary dramatically making it difficult to identify patterns conforming to the reservoir.

• Second, more than 50% of the hydrocarbons in the soil may be lost during the sampling because they devolatize during the soil handling (Hewitt).

• Third, the analytical techniques used to measure hydrocarbons in soil are not very sensitive.  Values in the micrograms (10E-6 grams) are typical.

• Fourth, this technique is generally limited to finding only C1-C5 hydrocarbons.  This is a very limited data set even when exploring for dry gas, but especially for reservoirs containing wet gas, condensate or oil.  Additionally, some of these compounds, C1 (methane) in particular, is often generated by near-surface microbial activity and thus may not be reflective of reservoir migration.

In summary, direct soil analysis, while simple and relatively inexpensive to generate, has limited utility because soil has poor adsorbancy, the technique is not very sensitive, it generates a limited hydrocarbon data set, and is subject to influence by near-surface methane generation.

Active Soil Gas Meneasuremt

With active soil gas measurements a shallow hole is placed in the ground and a sample of the gases contained within the soil porosity (not the hydrocarbons adsorbed onto the soil)

is extracted through a probe with vacuum and then analyzed for C1-C5 compounds typically.  As with soil samples, this method has the advantages of easy collection except in bedrock, easy analysis, and is relatively inexpensive.  However, it suffers from some of the same limitations as soil analysis namely, collection problems, poor sensitivity, limited compounds, and interference from surface biological methane generation.

• The most significant limitation of active soil gas collection is the difficulty, or in some cases, impossibility of being able to draw a sample through soils with low permeability, such as clays or rock, or through water saturated soils (Klusman 1993).  Even in soils with some permeability, it can be difficult to seal the extraction probe to the hole resulting in a sample diluted by simultaneously drawing of ambient air.  As with soil sampling, over medium to large exploration sites, soil permeability and water saturation can vary significantly leading to data variability potentially biasing the image of the reservoir.

• Active soil gas sampling is also influenced by atmospheric changes and solar heating.  Because the active drawing of a soil gas sample represents a single point in time, low pressure fronts moving through the area prior to the sampling will yield low concentrations as the front pulls the soil gas into the atmosphere.  Likewise, soil gas concentrations vary significantly during the day as the sun heats the surface of the earth creating a thermal pump.  It becomes difficult to compare results of samples taken on different days or even different hours of the day due to these meteorological variations.

• As with soil analysis, active soil gas suffers from poor sensitivity in the ppm-ppb range and a limited hydrocarbon list of C1-C5 at best.  It is also impacted by near surface biologically produced methane and other light gases.

In summary, active soil gas measurement has limited utility in exploration due to sampling difficulties in low permeability soils, effects of solar heating and atmospheric pressure variations, low sensitivity to a narrow range of volatile hydrocarbons, and the influence of near surface biologically generated compounds.

Microbial Analysis

For microbial analysis, soil samples are collected over the exploration site typically at 0.2 meters beneath the surface.  Here the interest is measuring the mass of microbes in the soil that have been growing on light gases moving through the soil.  In the laboratory, the naturally occurring microbes in the soil are cultured by feeding them light gases or they are cultured on an agar of n-butanol.  The colonies are counted after a prescribed time (Klusman 1993). This bacteria count is used as an indicator of the mass of hydrocarbons that the soil has been exposed to.  This is a relatively inexpensive analysis with multiple serious disadvantages.

• The most serious limitation of microbial sampling and analysis is its indirect nature.  There is no attempt to measure actual hydrocarbons but only the bacteria count that may represent the feeding of this bacteria colony by gases in the soil. 

• Another almost equally serious limitation relates to the generated data set.  By feeding either specific light gases or n-butanol, the culture favors microbes that thrive preferentially on a few compounds, for example C4 (n-butane).  Thus the result is not only indirectly related to vertical migration from the reservoir, but consists of only a single value per sample that may relate most specifically to soil exposed to C4.  Such a limited data set precludes any robust multivariable analysis.

• Of course, the third very serious limitation with the microbial technique is identical to that of soil analysis, namely that most soils are poor collectors of hydrocarbons and will vary in nature significantly over medium to large exploration sites.  This is further complicated in very dry environments such as deserts, where despite available hydrocarbons, the soil moisture is inadequate to support microbes.

In summary, microbial techniques, although one of the least expensive surface geochemical techniques, has the least value due to its indirect nature, its univariate data set (C4 content), its limited utility in about 1 of 5 sites where soil is a reasonable adsorbent, and the inhomogeneous nature of the soil at the site.

A Revolutionary Exploration Technique

After examining the disadvantages of direct soil analysis, active soil gas, and microbial surface geochemical techniques, it is easy to understand why these techniques are not widely accepted exploration tools by the industry.  Adequate hydrocarbons will be found in less than one out of five exploration areas, they have limited data sets with poor predictability, they have poor sensitivity, and they are subject to variances in soil character over the site.  Even though they may be relatively inexpensive, their value is too limited to be cost effective.

After examining the deficiencies of these early surface geochemical techniques, Gore has developed an Amplified Geochemical Imaging Survey, which eliminates or minimizes all of these limitations. To do this, Gore has focused improvements in four areas:

• The collector was designed to eliminate issues with soil adsorbtivity and non-uniformity, and
• The deployment method eliminates the effects in variation in both soil conditions and ambient weather conditions, and
• The analysis method was improved to expand the data set allowing for multivariate analysis and the method significantly boosts sensitivity allowing detection of hydrocarbons through thick sections of volcanics, salt and anydrite, and
• A multivariate analysis technique was developed to utilize the robust data set and improve the imaging of charged hydrocarbon reservoirs of all types
  
  

The Gore™ Amplified Geochemical Imaging Survey for Exploration

To eliminate the problem of the soils in only 1 in 5 exploration sites having reasonable adsorption characteristics, Gore engineered a hydrophobic adsorbent specifically to collect C2-C20 hydrocarbons.  While compounds don’t need to be gases to have a measurable vapor pressure and exist as a component of soil gas, hydrocarbons greater than C20 typically have vapor pressures too low for molecules to be found in the gaseous state.  The engineered adsorbent’s hydrophobic nature minimizes competition with water vapor in sites with variable moisture content.  Because some exploration sites can have both dry, saturated, swampy, and embayment areas across the site, the engineered adsorbents are sealed inside a tube of microporous ePTFE  known as GORE-TEXÔ membrane.  This membrane has micropores sized to keep out water to depths of 25-50’ yet allow free diffusion of gases to the engineered adsorbent.

The sampler (module) is easily deployed by inserting it into narrow diameter holes pounded or drilled into the ground to about 0.6 meters depth.  Field installation and retrieval are easy and low cost and allows economical deployment over difficult terrain, with no disruption to landowners. The module is left in the ground for a period of about 17 days, during which it passively collects volatile compounds in the soil and vertically migrating from the reservoir.  This extended period smoothes out any potential variations due to atmospheric changes, solar heating, rain, or other meteorological events.  Additionally, the longer time boosts the hydrocarbon signal on the module by continually collecting vapors while in the ground.

To quantitatively and directly measure the hydrocarbon vapors collected by the module, Gore has developed the most sensitive analytical method available.  It uses a thermal desorber to transfer the compounds on the adsorbent into a gas chromatographic column, GC, to separate each of about 90 compounds from C2-C20 and then uses mass spectroscopy, MS, to positively identify and quantify each compound.  As a result of the engineered adsorbent, the long deployment time, and the sensitive GC/MS, this method can collect and measure concentrations equivalent to about 1 nanogram (10E-9 grams) or about 1 part per trillion (ppt).  This is 3 orders of magnitude better than the best alternative methods.

Rather than five compounds from C1 to C5 as measured in soils or soil gas, the 90 different compounds from C2 to C20 allow us to differentiate between the fingerprint of hydrocarbon compounds naturally found in the soil over uncharged areas from the fingerprint of hydrocarbon compounds found over charged reservoirs.  Further, the robust data set allows us to see compounds found in dry gas, wet gas, condensate, and all types of oils.

The revolutionary collector and higher sensitivity GC/MS analysis is important to the success of the GORE Survey in all exploration sites including desert, forest land, jungle, swamps, shallow bays, and even frozen tundra.  However, even more revolutionary is our multivariate statistical techniques employed to analyze the very extensive data set.  These techniques identify a fingerprint of compounds at the surface that relate to charged reservoirs versus background (uncharged reservoirs).  In frontier areas where productive and dry wells may not be available, Gore uses hierarchical cluster analysis to identify groups of compounds indicative of the hydrocarbon phase of interest and of background to use as end members for the modeling. Then each module location in the array over the exploration area is compared to these end members to determine a probability of being charged.  When plotted on a map, these images of the reservoir charge can be integrated with other G&G data to maximize drilling success. 

The value in use for the Gore Survey has been validated by many hundreds of surveys in 50 countries throughout the world generating a 94% success rate in predicting dry holes over low probability areas, and a 88% success rate in finding hydrocarbons over high probability areas (Potter).

References

Conant, B. H., R. W. Gillham and  C. A. Mendoza (1996), “Vapor transport of trichloroethylene in unsaturated zone: Field and numerical modeling investigations”, Water Resources Research, 32: 9-22

Hewitt, A. D. and N. J. E. Lukash (1996), “Obtaining and transferring soils for in-vial analysis of volatile organic compounds”, USA Cold Regions Research and Engineering laboratory, Special Report 96-5.

Klusman, Ronald  W., “Comparison of Light Hydrocarbon Microseepage Mechanisms, 1996, AAPG Memoir 66, p. 157-166

Klusman, Ronald W., Soil Gas & Related Method for Natural Resource Exploration,  1993, ISBN 0-471-93892-0

Pignatello J. J. and B. Xing (1996), “Mechanisms of slow sorption of organic chemicals to natural particles”, Environmental Science Technology, 30(1): 1-11

Potter, R. W., et al, “Significance of Geochemical Anomalies in Hydrocarbon Exploration: One Companies Opinion”, 1996, AAPG Memoir 66, p. 431-439

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