Pangaea Geochemical Technologies
K-Series Gas-Sieve Survey of the Arlington Waterflood
Reno County, Kansas

The Arlington Waterflood was chosen to be a part of a gas-sieve soil vapor exploration method-testing program funded by a grant from the Kansas Technology Enterprise Corporation to Pangaea in 1999. The grant program was entitled the “Red Top Program” and participation was open to E&P companies that would be drilling new wells in 1999-2000. Pangaea gained permission from program participants to perform soil vapor surveys on their leases prior to drilling activity. Once the gas-sieve samples were collected and the soil vapor analyzed, the analytical results were provided to the participating operators prior to any wells reaching T.D. The resultant data from the 30 + Red Top surveys were used to develop models of interpretation for gas-sieve geochemical surveys that would be applicable to basins worldwide.  Pangaea’s success at predicting well outcome in the Red Top program was over 80%. Some fields are still being developed by drilling additional wells, working other well over and modifying waterflood parameters.

Background and Geology: The Arlington Waterflood, located in Sec 34-T24S-R8W Reno County, Kansas produces oil from a single zone Pennsylvanian Age Lansing limestone from a depth of about 3500’ BGL.  Successful wells must encounter both porosity and structural position. The thickness of the porosity zone varies from 0-10’. Primary production of high gravity oil from this reservoir had been prolific.

A waterflood had been initiated in this one-zone reservoir several years ago, which after purchase, the current operator reengineered. As secondary production continued the operator realized that volumetrically the waterflood had not yielded all of the expected secondary oil and that there was likely to be by-passed oil located somewhere within the undefined boundaries of the field.

Geologic mapping, well cuttings, pressure tests, logs and intuition were the tools available for the decision about infill drilling locations to recover the additional oil.  Pangaea offered a geochemical survey to add another tool for reservoir evaluation so that the operator could judge where to place an additional edge well in the reservoir. All the data combined was successfully used to again increase secondary production.

Gas-Sieve Survey: Pangaea performed a 29 point K-series gas-sieve survey at the Arlington Waterflood in the summer of 1999.  Sampling depths were shallower than normal, 4’ BGL rather than 8’ BGL, due to shallow groundwater in this broad floodplain of the Ninnescha River. Following collection, the gas-sieve soil vapor samples were analyzed for C1-C4 (methane, ethane, propane, and butane) concentrations.

Data: Map 1 depicts the total hydrocarbon concentrations (ethane + propane + butane in ug/L) at each sample station. Map 1 reveals where hydrocarbons are gassing off in the near surface in the highest concentration. Map 2 depicts the ethane concentration/ propane concentration ratio data. This ratio data is unit-less and only shows where the hydrocarbons, emanating from the reservoir below, have similar compositions. Both sets of data are informational.

Interpretation: Map 1 reveals the location and orientation of two compartments in the waterflood. Note that the southern compartment has higher total hydrocarbon levels than the northern compartment leading to speculation that this was the best compartment from which to produce the by-passed oil. Additional information provided by the operator put a caution on this interpretation.

Following the geochemical survey, the operator revealed that the waterflood is actually two waterfloods with different pressures in each compartment. At the time of the gas-sieve survey the northern compartment had a pressure of 600 psi and the southern compartment had a pressure of 2500 psi. The operator used static fluid levels in wells within the two compartments to determine this. The location of the dividing line between the two was not known and there was speculation that there is a permeability change rather than a fault separating the two. Once this information became available, Pangaea was able to suggest that the relatively higher hydrocarbon concentrations above the southern compartment were likely due to reservoir pressure differences and may or may not suggest higher oil cut in the southern compartment.  The operator added that the input well in the southern compartment had been shut down because the compartment was “full” and no more water could be injected without first removing some fluid. If water input, with resultant increase in pressure, had never occurred in the southern compartment then the high hydrocarbons above the southern compartment would have been interpreted as an untapped reservoir compartment and a good place to site a new well.

Generally ratio data provides information about oil gravity, gas cap locations, and can sometimes be used to predict the potential for several zones to develop. At the Arlington Waterflood there are higher ethane/propane ratios in the soil vapor of the northern compartment relative to the southern compartment (Map 2). This translates into more ethane in the soil vapor above the northern compartment. This information suggests that the oil is high gravity and that there was a gas cap over part of the northern compartment at one time. Low gravity oils generally have an ethane/propane ratio closer to 1.0.

The operator’s comments about Map 2 are that this data looks very similar to a contoured map of the primary production volumes, areas of reservoir quality and varying reservoir thickness.  The wells in the northern compartment did indeed produce most of the primary oil, with a higher gas cut, from a thicker zone of porosity development. The remnants of these occurrences are still expressing itself via soil vapor composition above the reservoir today.

Results of Drilling and Recompletions: One new well was drilled into the northern compartment in October 1999 at gas-sieve sample location AR-32.  The well was situated here so as to be within the confines of the reservoir and far enough away from the reservoir edge that radial drainage could occur. The risk was that it would be structurally acceptable but outside the confines of porosity development.  Data from both Map 1 and Map 2 suggest that this was a good drilling location. The AR-32 well encountered a 5’ porosity zone and now there are two withdrawal wells in the north compartment at the 3X and AR-32 positions. The AR-32 well began production at 130 BOPD and has stabilized after 1 year at 45 BOPD. The old 3X well has maintained its long established production of 45 BOPD.

Next, the operator decided to reenter a P&A well located at AR-7 in hopes of establishing production from the southern compartment. Log data from the old hole suggested that the reservoir was 2’ thick at this location. Although this was not ideally situated it was deemed an inexpensive way to withdraw fluid from the southern compartment. This compartment was fully pressured via input at the 7X. The well was successfully reentered, fraced because it was found to have poor permeability, and put on line at 45 BOPD. The operator was unsure which compartment the AR-7 would be connected to. Interestingly after the OWWO of the AR-7 well, it was thought to be connected only to the southern compartment until which time the 5X was converted to an input well. Since the conversion of the 5X, it has been concluded that the AR-7 may be connected in a small way to the northern compartment as is the 5X.

Other interesting information from the operator suggests that the 6X well in the southwest quadrant of the survey area is a substandard producer. The operator suggests that it has a very slight connection to the remainder of the reservoir. Its ethane/propane ratio value is substantially lower than the 3.1 encountered in the best part of the reservoir where the gas cap once was. The AR-7 washdown has a ratio of 1.86 suggesting that the reservoir quality is increasing somewhat over the 6X but still inferior to the heart of the reservoir. Fracing the AR-7 OWWO likely helped address this deficiency.

Waterflooding creates a dynamic reservoir. Changing one variable can effects the entire waterflood. A gas-sieve soil vapor survey is one tool that can aid the operator in capturing a snapshot of the reservoir’s dynamics.