G.L. Moake
Halliburton Logging Services, Inc., Houston, Texas
ABSTRACT
The measurement of formation density and lithology in a borehole environment is a fundamentally difficult problem. In recent years, this problem has been compounded by the need for increased accuracy over a greater dynamic range. To accommodate this need, new and better test formations have been constructed, and measurements have been made with a wide variety of muds, mudcakes, and standoffs. These measurements indicate that when mudcake is present, the traditional means of extracting density and lithology from tool measurements is often inadequate. Consequently, a more accurate technique for deriving these values has been determined. This technique utilizes the full energy spectrum of the near detector and the high-energy portion of the far detector to derive compensated density and Pe values.
Continuous, non-linear equations are used to accurately span a density range from 1.00 to 3.05 g/cc. Instead of the traditional, two-dimensional spine-and-ribs approach, density compensation is achieved by solving a four-dimensional system of equations. This technique provides good compensation for mudcakes of any expected composition. It also provides two different density-correction curves, one that responds to the density of the mudcake and one that responds to its photoelectric-absorption properties. The traditional correction is the sum of these two corrections, and it can be zero even in the presence of mudcake if the density effects are completely offset by the photoelectric effects. However, since one or both of the new correction curves will always respond to mudcake, they are more, reliable as quality indicators than the traditional single correction curve.
A new technique for extracting Pe yields values that are accurate from 0 to 5.1 (and probably higher). Using a combination of operator input and information obtained from the density calculation, compensation is applied both for the mud surrounding the tool and for mudcake, with the compensation depending on the weighting agent (hematite, barite, or other). Although compensated Pe values are obtained for all mud types, the absolute accuracy of the measurement degrades as the amount of barite or hematite in the mud is increased.
Calibration procedures are similar to those for the traditional method, although different and more numerous calibration parameters are computed. Log examples illustrate the technique as applied to a new hostile-environment, spectral-density tool. The additional application of temperature compensation, dead-time and pulse-pile-up corrections, and background subtraction yields highly accurate measurements from room temperature to 500 degrees Fahrenheit.