1965 Paper II-B

GRAPE* - A DEVICE FOR CONTINUOUS DETERMINATION OF MATERIAL DENSITY AND POROSITY

Hilton B. Evans

ABSTRACT

(*Gamma-Ray Attenuation Porosity Evaluator)

Basically, this device consists of a variable speed drive system to move geologic material between a shielded gamma-ray source and a shielded scintillation detector, an optical caliper to measure the sample thickness, and an analog computer to calculate density and porosity from the measured parameters.

The measured thickness and the computed density and porosity of the sample are recorded on a strip chart. Because the sample and the recorder chart are driven at the same speed, a direct comparison of the recorded parameters and the actual sample is possible. In addition, an integrated sample porosity is provided for comparison with laboratory core porosity measurements.

The GRAPE system is versatile. It is designed for either field or laboratory use and only one operator is required. The device handles core 1 inch to 4-1/2 inches in diameter, conventional core plugs, and slabbed core at drive speeds of over 3 inches per minute. At this speed, 100 ft of core can be analyzed for density and porosity in about 6 hours.

The error of the system, including statistical variations in source strength, is less than ± 1 percent porosity at the 20-percent porosity level. Several hundred feet of core (sandstones and carbonates) have been analyzed for porosity using the continuous device, Comparison with conventional laboratory porosities has been excellent.

Although the device was designed for density and porosity evaluation of sedimentary rocks obtained in oil exploration, other applications of this system are possible. They include porosity measurements of unconsolidated sediment samples and Teflon plugs, determining oil content of oil shale samples, saturation determination and component identification of flood fluids in observation wells and, in the mining industry, as a means of measuring the ore content of drill core or rock samples.