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Viola Limestone Production in Southern Oklahoma
EDITOR'S NOTE: This abstract has been made from the article "Oklahoma's Fractured Viola Limestone Reservoir" by Mr. Skarda which appeared in the Oil and Gas Journal of February 27, 1956, page 109. The article discusses this reservoir in a more extended manner and shows a cross-section and representative logs. The structure and Viola distribution map shown with this abstract is also from that article.
The increasing demand for petroleum requires that oil be found in the less obvious types of porosity. Fractured carbonate reservoirs form one of these types, and are probably among the greatest sources of undiscovered reserves. They are difficult to find and to produce, and consequently have attracted relatively little attention. A large fractured system of this type has been found in the Viola limestone in 5S-2W in southern Oklahoma. This fracture system is very complex and the usual methods of reservoir evaluation are of little value and actual production tests of wells are required for calculations for adequate and efficient spacing.
The Viola section unconformably underlies the lower Pennsylvanian beds on a folded segment overthrust, as shown on the accompanying geologic and structure map. The fracture system varies from megascopic channel-like fractures to thin hair-line partings, with crystalline calcite frequently occuring along the fracture planes. The fracturing appears to be associated with the overthrusting. Matrix porosity and permeability are generally below 2 percent and 5 md respectively. A method developed for measuring whole core porosity indicates an average of 4 to 5 percent, but even this low average through the average of about 1,000 feet of Viola makes the reserve picture satisfactory. No evidence of a water drive has been found and reservoir performance indicates a pure and also efficient solution-gas drive.
In putting down wells, different techniques of drilling and in the use of lost circulation material and cement have resulted largely only in impairing the permeability of the reservoir. Air drilling was excluded because of the danger of explosive mixtures, but an exhaust gas system has been developed which shows great promise. Essentially the formation is tested as it is cut by this method. Since the fracture system is non-uniform, a producing well indicates that the drainage area affecting it is large; if the zone indicates non-commercial fractures are present, costly production tests are not necessary. Drilling operations with this method have been carried below 8,000 feet.
Conditions in the reservoir are so heterogeneous that apparently where well performance can be improved by treatment, any treatment will do. Results are about the same for sand-oil fracturing or acid treatment. However, generally sand-oil fracturing has been used in the basal section where solubilities are in the lower ranges of 20 to 50 percent and inhibited acid in the upper zones where solubilities are in the 50 to 95 percent range. The problem of pressure maintenance is being studied.
To aid in the reservoir studies and reserve estimates a method of tracing oil-in-place has been worked out with Electrochemical Laboratories whereby injections of radioactive krypton (Kr85) have been made and traced to different wells across the field.
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