Enhanced Statistical Variation Method For Determining Zone Isolation In Complex Cements

Abstract

Complex, or non-standard, cement slurries create special, difficult cement evaluation problems which are magnified when high density drilling fluids are used. These complex slurries include lightweight cements (water-extended, ceramic bead, and nitrified cements), gas-channeled cements, multistage density cements, as well as intervals of cement that are contaminated with drilling fluid. Current cement evaluation methods, via conventional logging techniques, are based on signal amplitude, acoustic impedance, and compressive strength relationships. These properties are derived from the materials behind the pipe and interpretation methods rely on how these properties differ in fluids versus standard cement. This desired contrast is not always evident in complex cements; however, statistical variance techniques have been developed that permit accurate interpretations of zone isolation in low-density cements. This new approach to determining zone isolation in lightweight and complex cements with ultrasonic cement evaluation tools is called Statistical Variation Plot (SVP) and is based on the following two factors:Distinguishing solid materials from fluids in the casing-borehole annul usAssuming that solid material cannot be displaced during the remedial cementing operation whereas fluids may be displaced1. It will be shown that this technique is superior to conventional evaluation techniques when the acoustic properties of the cement are similar to the acoustic properties of the annular fluid.