K-Series Gas-Sieve Survey of the Arlington/Trimbley  Waterflood
Reno County, Kansas

The Arlington/Trimbley Waterflood was chosen to be a part of a program that was funded by a 1999 grant to Pangaea Geochemical Technologies from the Kansas Technology Enterprise Corporation.  Pangaea offered K-Series gas-sieve, light hydrocarbon soil vapor surveys in the Red Top Program to participating independent oil operators and in exchange Pangaea provided the operators survey data at no charge.  The chosen areas were already slated for the drilling of at least one test well in 1999 or early 2000.  In most cases the drilling locations were already when the geochemical survey was conducted.  The results of the surveys were provided to the operators prior to drilling activity.  The data from the program was used by Pangaea to develop interpretation models for gas-sieve geochemical surveys.

Background and Geology:   A waterflood had been initiated in this one-zone Pennsylvanian Lansing Limestone reservoir several years ago.  On primary production this stratigraphically isolated reservoir had been a prolific producer of high gravity oil.  Volumetrically, the waterflood had not yielded the expected secondary oil.  The operator concluded that there was by-passed oil located somewhere within the boundaries of the stratigraphic limits of the reservoir.  Geologic mapping, well cuttings, pressure tests, logs and intuition were the tools available for the decision about infill drilling. Pangaea offered a geochemical survey to add to the other tools.  The geochemistry survey was intended to locate the stratigraphic boundaries the reservoir so that wells were not misplaced out side the reservoir limits.

Gas-Sieve Survey:  A 29 point K-series gas-sieve survey was conducted by Pangaea in the summer of 1999.  Sampling depths were more shallow than normal, 4' BGL rather than 8' BGL due to shallow groundwater in this broad floodplain of a river.  The samples were analyzed and mapped.

Results:  Map 1 shows the total hydrocarbon concentrations (ethane + propane + butane concentrations in ug/L) and Map 2 shows the ethane concentration/propane concentration ratio.  This ratio data is unitless and only shows where the hydrocarbons, emanating from the reservoir below, have similar compositions.  This data seems to show where the reservoir is developed, while the total hydrocarbon shows where the remaining oil is located within the reservoir.  Methane was not considered in the data set because the data is noisy due to an unknown quantity of bacterially produced methane.

Interpretation:  Following the geochemical survey, the operator revealed that the waterflood is actually two waterfloods with different pressures in each compartment.  The northern compartment has a pressure of 1300 psi and the southern compartment has a pressure of 2500 psi.  The location of the dividing line between the two is not defined. Knowing this, it was expected that the total hydrocarbon data would have higher values over the higher pressure compartment of the flood.  This did in fact develop.  Each compartment's geochemical data was interpreted separately due to this pressure difference.

One new well was drilled into the northern compartment in October.  In looking at the total hydrocarbon map and the ratio map, this location appears to be well situated and it was predicted to be a good producer.  Drilling data revealed that it had a zone thickness of 5' rather than the 10' maximum found in the heart of the reservoir.  It was intended that the well come close to the edge of the reservoir oil pushed by the flood water.  The new well flowed for several weeks and was then put on a pump and the production has stabilized at 130 BOPD.

The well was successfully reentered.  It was treated because it was found to have poor permeability, as was expected, and its production has stabilized at 45 BOPD.   It is interesting to note that the washdown was expected to be in the higher pressured southern compartment of the flood but it is actually connected to the northern compartment.  Now that this is known, the 5x well will now be completed as an injection well to increase oil flow to the washdown.  Because the washdown was not in the high pressure compartment, there is no producing well to withdraw oil from that pressured up area and thus no point of withdrawal.  A new well will need to be drilled to produce that compartment.

Consideration of the ratio map data suggests that the best areas of the reservoir are located in the northwest quadrant of the project with ratios of 2.5-3.1.  The 6X well in the southwest quadrant is a substandard producer and its ratio is considerably lower than the high values.  The washdown, has a ratio of 1.86 suggesting that the reservoir quality is increasing over the 6X but still inferior to the heart of the reservoir.  

Update - November 2000:  Production from the AR-32 well and the washdown at location AR-7 has stabilized at 90 BOPD.