Drilling To Test New Geochemical Tool
Article appeared in THE AMERICAN OIL & GAS REPORTER
February 1999
By J. Lynne Davison and Bill Morris

Editor’s Note: The following is the first segment of a two-part article on a new gas-sieve geochemical technology that is being applied in a multiple-well drilling project.  Part Two will focus on the results of the drilling based on the geochemical data, and will appear in a future issue of The American Oil & Gas Reporter.

                 WICHITA, KS. – After three years of field testing and refinement, a new geochemical technology that locates subsurface hydrocarbons using a gas-sieve to sample surface soil vapors will be demonstrated in a four-well drilling program.

                The applications are varied-from locating bypassed oil in a mature waterflood to delineating targets in wildcat drilling-and were selected based primarily on the geochemical data acquired using the patent-pending Gas-Sieve Sample Concentration and Collection approach.  The operators are applying for U.S. Department of Energy funding for some of the demonstrations.  Others, developed as case studies using Kansas Technology Enterprise Corporation 1998 ARMF funding, will be drilled using other funds.

                Although the gas-sieve methodology is definitely on the cutting edge, the principal of surface mapping light hydrocarbon concentrations dates to 1920s-era European oil fields, where several significant discoveries were attributed to surface mapping.  Major oil companies made incremental improvements to the concept after it made its way to the United States, but kept their methods proprietary.

                Oil- and gas-bearing reservoirs are generally considered to be bounded above by a seal or a cap rock, but the bounding materials do not form a perfect barrier and some hydrocarbons are known to leak over time to the surface.  This migration to the atmospheric sink is the basis for using a near-surface light hydrocarbon survey to map and monitor drainage from reservoirs at depth.

New Geochemical Tool

                In recent years, the effectiveness and reliability of surface mapping and related geochemical approaches have been greatly improved.  In developing gas-sieve technology, the weaknesses inherent in conventional geochemical methods were systematically identified and addressed.  The result is a new geochemical tool that can be leveraged in both exploration and field development.

The major weaknesses in “prior art” surface mapping methods were:

·         Soil vapor samples were often contaminated by atmospheric dilution;

·         Soil vapor samples were not concentrated, so there were unacceptably high levels of analytical noise in the data;

·         A small, inexpensive collection/concentration device did not exist;

·         The collection/concentration device needed a long holding time so that the sample could be sent to the lab form anywhere in the world; and

·         Quality control methods were needed to ensure proper identification and quantification of the hydrocarbons and the separation of biogenic contaminates.

                The sample dilution problem was addressed using crossover technology from the environmental industry, and the optimal depth for sampling was determined with advice from soil scientists performing gas exchange studies for the agricultural industry.  Each of these answers was field tested over two years with observation in various geologic environments.  The resulting modifications were applied to the method.

                The single biggest problem associated with prior art soil vapor geochemistry was that only an extremely small sample could be physically introduced into the analytical equipment.  The small size (commonly 2-10 milliliters) resulted in excessive noise-to-signal ratio in the analytical equipment.  This amount of noise required researchers to use statistics to determine if there were any usable data, and the statistical interpretation often only confused the issue.  In geologic environments such as basins with thick sale, which acts as a natural earth filter to retard the rate of migration to the surface, there was no usable data collected.

                The solution to this problem came in the form of sample concentration-that is, collecting hydrocarbons from a 1,000-milliliter volume of soil vapor using the gas-sieve.  Smaller molecules such as carbon dioxide, nitrogen and water found in the soil gas sample are purged from the gas-sieve during sampling.  This effectively removed the uninteresting gases and leaves stripped hydrocarbons occupying space on the gas-sieve.

                The gas-sieve is then transported to the lab and its hydrocarbon contents are purged into analytical equipment, where the concentrations of methane, ethane, propane and butane are determined.  These concentrations are the mappable data collected during the survey.

Expanding Applications

                Applications for gas-sieve technology are varied and continue to increase as testing is expanded into new situations.  Applications identified so far include wildcat exploration; basin reconnaissance; trend and field extension; confirming two-dimensional seismic features-especially if fresnel zone effects are suspected; confirming that a 3-D seismic feature bears hydrocarbons; locating hydrocarbon-filled, acoustically invisible traps; locating infill drill locations in fields with decreased reservoir pressures or high reservoir pressures; locating by-passed oil in waterflooded fields; defining the limits of a trap; locating regional fracture patterns used to site wells in coalbed methane reservoirs and in unconventional gas reservoirs such as shales; and identifying faults as sealed or open.

                The four test wells slated for drilling based on data acquired using gas-sieve technology are diverse.  One well will demonstrate that by-passed oil can be located using the gas-sieve technology in a 300-psi waterflood reservoir (800 psi under the normal geogradient pressure), and a second well will demonstrate that bypassed oil can be located in a 2,000-psi waterflood reservoir (1,000 psi over the normal goegradient pressure).  It is thought that the high reservoir pressure in the second waterflood drives the geochemistry anomaly to exceed that of the first waterflood.

                Both waterfloods are adjacent and separated by a sealed fault.  Each has one Simpson sand reservoir and a thick salt section in the subsurface.  A Department of Energy BOAST II simulation was run on both waterfloods.  The results suggest that there are 100,000 barrels of oil in place and recoverable by the first well and 200,000 barrels of oil in place and recoverable by the second well.

                A third well will demonstrate the ability to site infill well locations in a low pressure field that has two productive zones.  Once of the zones is the Pennsylvanian Age Lansing/Kansas City, which has higher-gravity solution, gas-drive production.  The second zone is lower-gravity, water-drive Ordovician Age Arbuckle oil.  This field occurs on the Central Kansas Uplift, and has a thick section of salt in the subsurface.

                The fourth well will demonstrate using gas-sieve surveys for wildcat exploration in a basin that has 350 feet of salt between the reservoir and the surface.  The project was initiated using 1980s-vintage 2-D seismic leads.  Some successful wells were drilled in the 1980s using that data, but too many non-productive wells were drilled to use the 2-D data alone.  A gas-sieve geochemistry survey was run over many of the seismic leads and the surrounding areas so that background data could be collected.  All dry holes were samples and included in the data set.  The location of the demonstration well was chosen based on the strongest geochemistry anomaly situation over a good seismic lead.  Most of the seismic leads were condemned by the geochemistry survey results.

                Although evolved from decodes-old principals, gas-sieve geochemical technology is an advance for the next millennium.  As wells are drilled and the method becomes field-proven, it will help speed geochemistry’s maturation to a mainstream exploration and production tool – whether used standalone or in conjunction with geophysics.  The potential applications are broad.  In fact, research is already underway to adapt gas-sieve technology to offshore exploration using near-bottom surveys in place of soil vapor, and a collaborated effort is now being investigated that would take the method to ultra-deep water surveys.