Geochemical Analysis Method 
Put To Test In Red Top Program
Article appeared in THE AMERICAN OIL & GAS REPORTER
September 1999
By J. Lynne Davison

Editor’s Note:  The following is an update on a new gas-sieve geochemical technology that is now being field tested in a drilling project that is partially funded by the Kansas Technology Enterprise Corporation.  The technology was featured in “Drilling To Test new Geochemical Tool,” Which appeared in the February 1999 issue of The American Oil and Gas Reporter.  The author extends her appreciation to Eagle Creek Corp., Jamie Hess, Bart Armsfield and John Darrah for their contributions to the project.

                WICHITA, KS – Applying cross-over technology from the environmental industry, a new gas-sieve technology for performing surface soil vapor surveys for oil and gas exploration and production management has been developed and is now being applied in a multiple-well drilling program.

                Using the gas-sieve method, the soil vapor sample is acquired by driving the vapor sampling tools to the soil-bedrock interface (typically to a depth of 7-10 BGL) using a hydraulic hammer mounted to an all-terrain vehicle.  After reaching TD, a tubing adapter is treaded into the base of the downhole tools and 1,000 milliliters of soil vapor is lifted through the tubing to the surface, where it is allowed to flow across an in-line gas-sieve-a molecular sieve-filled tube that filters and traps hydrocarbons.

                The gas-sieve is then capped and transported to the lab, where it is desorbed into a gas chromatograph.  Ethane, propane and butane (C2-C4) are separated and the concentrations of each analyte are determined in micrograms per liter of soil vapor (ug/L).  The data are ready for mapping without the need for statistical manipulation.

                Introducing the large, 1,000-milliliter sample into the gas chromatograph significantly reduces the analytical noise that has plagued “prior art” geochemistry.  Traditionally, geochemical surveys collected only 2-10 milliliters of soil vapor in a small syringe and then injected them directly into a gas chromatograph.  The small sample size is responsible for the noise seen in these earlier direct inject geochemistry methods.

                Another positive feature of the gas-sieve is that once collected, a sample has a holding time of more than 30 days.  In addition, the sampling device is small and easily transportable, and sampling time is quick (about eight minutes per location).  In fact, a single field crew can collect 25-30 samples in one day.

Red Top Program

                Field testing is now underway in various geologic environments – from locating bypassed oil in a mature waterflood to delineating targets in wildcat drilling.  Testing is funded, in part, by the Kansas Technology Enterprise Corporation through a 1999 Applied Research Matching Fund (ARMF) grant.  Initially, the grant was to fund three test wells, but is has been expanded to support a new testing initiative called Red Top Program.

                The Red Top Program is designed to generate a large volume of gas-sieve survey data in diverse geologic environments.  Oil and gas operators are being contracted for inclusion in the program if they have a drilling program for 1999, have chosen their drill location, the well is fully funded, and drilling is imminent.  In particular, the program is seeking operator participation in waterflood projects to locate by-passed oil, infill drilling projects, workovers where there is pay behind the pipe, wildcats and trend projects.  The operator is allowed input on the gas-sieve survey design, and if necessary, confidentiality is assured.  There is no charge to the operator for participating in the program.

                The operator arranges for Pangaea’s ATV access to the lease.  One low-impact ATV is used for each two-man crew, and it is used to transport the hydraulic hammer and electronically-controlled vacuum pump that lifts the soil vapor to the surface.  The rig has been granted a “casual use” permit for conducting surveys on Bureau of Land Management leases.

                A multi-point gas-sieve survey is then conducted over all or part of the lease to be drilled.  After sample collection, the gas-sieves are analyzed for C2-C4+ (UG/L) and the hydrocarbon concentration data are mapped and made available to the operator prior to drilling.  The operator is not asked to change a predetermined drill location based on the geochemical survey results.

                It is important to emphasize that the wells are not being drilled based solely on the geochemical data acquired using the patented approach.  The operators base well selection on a variety of data types and sources, and the geochemical data is collected in a lease where the operator has already decided to drill.  In this way, the results of the geochemical survey can be compared to the geological, seismic and drilling/logging/drill stem testing data after the well is drilled.  So far, the predictions made by the gas-sieve technology have matched drilling results.

                The goal of the Red Top Program is to complete surveys prior to drilling 40 wells, with a grant completion data of Dec. 31.  About half of the gas-sieve surveys are now complete, and six wells have been drilled to date.  Other wells will follow this fall and through the first quarter of 2000.

Click Field Results

                Three of the six Red Top Program wells are drilled back-to-back in July and August in the Click Field, located on the Ellsworth and Rice county line in central Kansas.  Before the drilling program was initiated, the field had five producing wells, with four producing from basal Pennsylvanian –age sandstone from a depth of 3,150 feet BGL and one well producing from the early Pennsylvanian-age Lower Kansas City-Odessa Limestone at 3,050 feet BGL (Figure 1-see below).  Sand production is controlled by structure and truncation to the west.  All oil produced in the field is high-gravity and gassy.  The field was discovered in 1996 by Wichita-based Eagle Creek Corp.  Prior to the 1999 drilling program, the field was producing 45 barrels of oil a day.

                A gas-sieve soil vapor survey was conducted at Click Field in July, immediately prior to the Eagle Creek drilling program.  In all, 28 locations were sampled and the gas-sieves were analyzed within several days of collection.  As shown in Figure 2, the results of the analysis are mapped as a total of thane concentration plus propane concentration plus butane concentration (C2+C3+C4 [ug/L]).

                The three proposed locations were drilled in the following weeks (the drilling results are posted on Figure 2-see below).  The first well drilled was a producer, as predicted by the geochemical data.  The second and third wells were staked and the operator was committed to drilling.  Both of these wells were dry holes, again predicted by the gas-sieve survey results.  The new producer, the Rolfe No. 3-4, falls within the 300 ug/L contour interval of the geochemical data, and the well’s initial oil production was 75 bbl/d.  The geochemical data suggests that there are several other locations within and above this contour interval that will result in positive test wells.

Observations

Observations and sampling events related to the interpretation of the geochemical data include several interesting points.  First, there appears to be several other prospective locations for additional wells to the north, northwest and northeast of Rolfe No. 3-4.  These additional locations have higher hydrocarbon concentrations (C2+C3+C4 [ug/L]) than does the new producer.  Future drilling is expected to establish production in these areas.

Second, soil vapor sample No. CF-24—the location of the new producer—may have had a greater (C2+C3+C4) concentration value if it had been taken at a deeper depth, like most of the other project samples.  Since CF-24 was taken near an active intermittent stream where the deeper soils were saturated with water at the 3-10 BGL interval, the sample was taken at 2 BGL.  “Atmospheric washing” can dilute these shallow samples by soil gases diffusing to the atmospheric sink, and also by the pumping action of barometric pressure changes.  Atmospheric washing greatly decreases with soil depth.  Even with this sampling irregularity, CF-24’s total hydrocarbon value was still well within the 300 ug/l contour interval used as a productivity predictor.

Third, it is common for producing wells to have less surface hydrocarbon expression than other areas of the oil-bearing reservoir.  One theory about this occurrence is that a producing well is a pressure sink with less ability to drive hydrocarbons in the soil voids today were first expelled from the reservoir long ago, but are part of the hydrocarbon plume advancing to the atmospheric sink at the surface.  When pressure from below is reduced, the plume’s advance is also reduced, diminishing the surface expression.  Notable exceptions to this are if shallower oil-bearing zones behind the pipe have not been produced and continue to drive the plume.

Therefore, C2+C3+C4 concentrations are lower where gas-sieve surveys are conducted in fields that have decreased pressure caused by production.  Red Top Program data suggest that infill drilling locations within a field may produce if the geochemistry data is only 2-3 times over the background levels obtained off of the field.  Background levels are defined as geochemistry survey values taken near production, but in low-lying parts of the reservoir, water-bearing areas, and impermeable areas that do not accumulate hydrocarbons.  Background values are lower overall in basins where there is no hydrocarbon source rock than in basins with actively-generating source rocks.  More data will be evaluated to confirm these occurrences.

Prior art geochemistry was likely to see producing fields as geochemical “halos” because the sampling method did not involve concentration, as does the gas-sieve method.  Therefore, the reduced geochemistry values seen with production actually disappeared to non-detect.

Other generalizations about gas-sieve survey data are beginning to emerge from the Red Top Program data, and these will be applied as they develop.