Density tools provide valuable information required to locate gas and evaluate formations. Although 200 Tcf of gas resources are estimated to exist behind pipe in the U.S. alone, present-generation density tools are unable to deal with the magnitude of cement thickness that is typically encountered in a cased borehole. Thus, use of these tools is generally restricted to open boreholes.
This paper describes the design study, partially funded by Gas Research Institute, to be used in developing a density tool capable of providing consistent, reliable measurements in a cased-hole environment without prior knowledge of the casing or cement properties. A special laboratory tool was constructed to acquire data used to develop the design.
In studying the variables relevant to a cased-hole density measurement, it was noted that the density and composition of steel casings are very uniform and thus do not need to be addressed. Also, measurements with normal and hematite-loaded cement showed that the composition of the cement is not significant. Similarly, the elemental composition (lithology) of the formation is expected to be unimportant. Consequently, the tool response is determined primarily by four variables: formation density, casing thickness, cement thickness, and cement density.
Analysis of measured gamma-ray spectra indicated that the spectral shape contains little information about the variables that is not contained in the total count rate. Therefore, four detectors are required to evaluate the four variables when the variables are independent. When cement thickness is small, the four primary variables are not independent, and only three detectors are required to determine the formation density. However, analysis of an optimized three-detector tool indicated that such a tool would not be accurate for cement thicknesses much greater than an inch. On the other hand, analysis of an optimized four-detector tool indicated that the design would make quality measurements through up to two inches of cement. A cobalt-60 logging source, which is more penetrating than cesium- 137, is used to improve precision and depth of investigation.
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