A Magnetic Device To Detect Tension Failures in Oilfield Casing
Authors: Patterson M.M. , Murphey C.E. , Sheffield B.C. 
Published: December 2007

Down-hole television inspection of separated oilfield casings indicated the presence of a magnetic field at the separations. A sensing device, the Magnetic Casing Logging Tool, was built capable of locating these yields. The tool has been used successfully to determine the integrity of casing in steam injection wells.

Introduction

A down-hole television system was used to inspect the casing of several steam injection wells. During these inspections, an intense magnetic field caused severe distortion in the surface television picture every time the down-hole television camera passed a tension failure in the casing. (Fig. 1 shows a typical tension failure in casing, together with a schematic of the same failure. Fig. 2 shows how the television surface monitor was distorted when the down-hole camera passed through a magnetic field.) Subsequently, a tool was built that consisted of a simple search coil capable of detecting a magnetic field caused by this type of casing tension failure. Since the magnetic field was great enough for the search coil to detect it from inside oilwell tubing as well as from inside casing, a new tool was designed, built, and tested that was capable of logging casing tension failures through the tubing. The device, now used routinely by Shell Oil Co., is called the Magnetic Casing Logging Tool (MCLT).

Field Tests Prototype Tests Prototype Tests Earlier down-hole television inspections of steam injection wells had shown that most casing failures were of the tension type. Accompanying each tension failure was a strong magnetic field that caused severe distortion on the televisions system's surface monitor. Passing a search coil (requiring only two conductors Passing a search coil (requiring only two conductors leading to a surface voltmeter) through the magnetic field at some reasonable velocity would produce a voltage that could be easily detected at the surface. (The physical principles involved in the creation and detection of such a magnetic field are discussed in the Appendix.) A prototype system was assembled and adapted to fit the television logging cable so that a magnetic survey might be run along with the television survey. This operation would provide a measure of reliability for the new magnetic device. The prototype was designed to operate in open casing only. prototype was designed to operate in open casing only. The first well inspected had 17 tension-type failures in 1,500 ft. Eleven of these were viewed with a down-hole television system. The remaining six failures were in an area of opaque well fluid but still caused severe magnetic distortion of the TV surface monitor. After the TV survey was completed, the prototype MCLT was run into the well. Nine separate prototype MCLT was run into the well. Nine separate logging runs were made to check the recorder performance and to ascertain the speed at which to run performance and to ascertain the speed at which to run the logging tool. Surveys were made at speeds from 60 to 100 ft/min. All tension failures located by TV inspection were recorded by the MCLT. A Mosley autograph X-Y recorder was used to record the voltage output of the tool. Another well nearby was inspected by down-hole television. Tension failures were evident at 180, 591, and 758 ft. Below 800 ft, the well fluid was opaque, but magnetic distortion on the surface TV monitor revealed eight more tension failures. In this well two runs were made with MCLT. The first was at 60 ft/min while going to a depth of 1,200 ft.

JPT

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