Precise Joint Length Determination Using A Multiple Casing Collar Locator Tool
Authors: Ruedrich R.A. , Perkins T.K. , O&apos;Brien D.E. 
Published: December 2007

American Institute of Mining, Metallurgical, and Petroleum Engineers, Inc.
This paper was prepared for the 49th Annual Fall Meeting of the Society of Petroleum Engineers of AIME, to be held in Houston, Texas, Oct. 6–9, 1974. Permission to copy is restricted to an abstract of not more than 300 words. illustrations may not be copied. The abstract should contain conspicuous acknowledgment of where and by whom the paper is presented. Publication elsewhere after publication in the JOURNAL paper is presented. Publication elsewhere after publication in the JOURNAL OF PETROLEUM TECHNOLOGY or the SOCIETY OF PETROLEUM ENGINEERS JOURNAL is usually granted upon request to the Editor of the appropriate journal provided agreement to give proper credit is made. provided agreement to give proper credit is made. Discussion of this paper is invited. Three copies of any discussion should be sent to the Society of Petroleum Engineers office. Such discussions may be presented at the above meeting and, with the paper, may be considered for publication in one of the two SPE magazines.

Abstract
Numerous examples of changes in casing length resulting from fluid withdrawal, subsidence, etc., are reported in the petroleum industry literature. Logging tools have previously been described which measure changes previously been described which measure changes in casing length in situ. This paper describes the design and application of a new tool that provides reproducible measurements with a provides reproducible measurements with a precision of a few hundredths of an inch, precision of a few hundredths of an inch, tenfold better than methods previously reported.
The new tool consists of four magnetic casing collar locators arranged with a particular spacing in the logging tool. laboratory particular spacing in the logging tool. laboratory studies have determined the influence on the collar locator signal of the following factors:stress level in the casing,temperature,logging velocity,changes in radial position of the sonde in the casing,effect position of the sonde in the casing,effect of recorder speed,effect of cable crosstalk,effect of magnetic fields, andchanges in type of collar.
The laboratory studies also gave insight into desirable recording and interpretation techniques. Digital recording permits automatic computer processing of data using crosscorrelation methods.
A field tool was built and tested in 10 joints of 5 1/2-in. casing to verify the tool's capability to detect small changes in joint length that were produced by loading the casing string. The joint length was determined for each joint by analysis of both analog records and digital records. The casing string was stretched by picking up an additional 63,000 lb. and analysis of the logs run on the stressed string shows the expected elongation of the individual joints without a change in the standard deviation of the measured joint length.

Introduction
The precise measurement of casing joint length in situ has been the goal of several research efforts in the petroleum industry. Precise length determination is needed to Precise length determination is needed to evaluate the effects of stress changes that might occur in the vicinity of the well over a long period of time. A loading process of common interest is that produced by pore pressure reduction that increases the stress in the rock or soil matrix. Strains, which result from compaction of the matrix, can be transmitted to the casing and become a factor in casing design.
Several devices employing magnetic casing collar locators have been developed previously to obtain casing strain data. Allen and his associates have described a "collar counting" technique that employed a single magnetic collar locator.