Oilfield Glossary
Oilfield Glossary
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compensated-density log

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1. n. [Formation Evaluation]
A density log that has been corrected for the effect of mud and mudcake by using two or more detectors at different spacings from the source. The shorter the spacing, the shallower the depth of investigation and the larger the effect of the mudcake. Thus, a short spaced detector, which is very sensitive to the mudcake, can be used to correct a long-spaced detector, which is only slightly sensitive to it. In a typical two-detector compensation scheme, the density measured by the longest spacing detector is corrected by an amount, delta rho, which is a function of the difference between long- and short-spacing densities. The correction is found to depend on the difference between formation and mudcake density multiplied by mudcake thickness. Although there are three unknowns, simple functions are reliable for moderate corrections. Experimental results are often presented in the form of a spine and ribs plot. There are other schemes using, for example, more detectors. Dual detector density logs were introduced in the mid 1960s.