Density Logging in Slim Holes With a Novel Array Density Tool
Authors: Stoller C. , Darling H.L. , DasGupta T. , Jorion B.M. , Perchonok R.A. , Philip O.G. , Wijeyesekera N.I. 
Published: December 2007

A novel 2.5-in. diameter, three-detector density tool addresses the needs of the slim drilling and reentry market. It overcomes limitations of older slim density tools and offers a measurement of bulk density and photoelectric factor with the same precision and accuracy as larger tools. The use of advanced technologies in detector design and electronics yields a compact tool with increased reliability, better quality control and improved wellsite efficiency.

Introduction

Interest in drilling slim holes has been increasing steadily over the last few years. Slim holes are not only used more in exploration drilling but also in the development of mature fields through sidetracks. The small hole diameter as well as the usually small turning radius in sidetrack drilling requires a new set of wireline tools that are suited for this environment while providing the same quality answer as traditional larger tools. The tools must have a smaller diameter and must be short to improve the turning radius.

The new slim litho-density tool (SLDT) has been developed to meet this need as part of a slim wireline quad-combo, which consists of gamma ray, thermal neutron, lithodensity, array induction and monopole sonic. Innovative approaches were required to make the tools simultaneously slimmer and shorter while matching the performance of larger tools. The SLDT is a 2.5 in. diameter, three-detector mandrel tool with a titanium housing. An array of three gamma ray detectors is used to improve precision, accuracy and quality control of the formation density and formation photoelectric factor (PEF) measurements. Two of the three detectors use the same spacings as the traditional pad density tool's long-spacing (LS) and short-spacing (SS) detectors. Therefore, the new tool provides an answer that is similar in vertical resolution and depth of investigation to the pad tools. A new high-density scintillation material is used for the SS detector to improve the shielding, collimation, count rate capability and compactness of the tool. The other two detectors use Nal scintillators, which can meet the tool's requirements at the longer spacings. All three detectors are used with compact high-speed electronics to make optimal use of the high counting rates.

The density answer is derived from all three detectors using an extended spine-and-ribs algorithm. The PEF is measured by the middle detector and offers enhanced precision and accuracy. The accuracy of the density measurement depends critically on good tool calibration.

The calibration for this tool has been improved to provide more checks to ensure accurate calibration and proper tool operation. The data from the present and past calibrations is stored in the tool's nonvolatile memory and can be retrieved by the surface acquisition system. The downhole microprocessor allows the tool to perform a quick, yet extensive, self-test. The tool's density and PEF answers are based on more than a thousand data points obtained in a series of calibration measurements in well-characterized density calibration blocks. The accuracy of the answer was confirmed in more than 30 field tests, in many of which the tool was run in combination with a larger pad-type density tool. Adapters allow the slim tools to run in combination with most other wireline tools. The tool has been designed for enhanced reliability by incorporating design features from logging-while-drilling tools. The modular, compact design facilitates troubleshooting in the rare case in which it may be necessary.

The following sections give an overview of the tool, describe the algorithms for density and PEF and show examples of logs in various logging situation. In most cases, a density log from a larger pad tool was available for comparison.

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