Abstract

Evolution of coiled tubing services requires greater down-hole depth accuracy. Depth critical jobs include coiled tubing conveyed perforating, zonal isolation and plug setting, among others.

Coiled tubing depth measurements are performed at surface, using devices to measure the length of coil tubing being deployed in the well. Measurement methods and their procedures have varied over time and have been continuously developed and improved on in an attempt to achieve higher degrees of accuracy and reliability in the length measurement. By principle, these methods provide a measurement of the length of the pipe deployed into the well but may considerably differ from the true depth of the coil end due to down-hole deformation of the coil tubing.

Various factors influence the down-hole deformation of the coil tubing; among these factors are temperature and pressure effects on the pipe. Elastic deformation of the pipe due to its own weight, sinusoidal and helical buckling effects occur and can substantially affect true depth. In view of this complexity, down-hole depth correlation is executed in depth critical jobs using various techniques, such as; tubing end locator, wireline logging tools, memory runs and reference depth tagging technique.

This paper discusses the different methods of surface pipe length measurement, their historical developments and the errors in true depth from measured depth due to down-hole deformation of the coil tubing. We present the results of a newly developed tool, DepthLOG, that provides a reliable and accurate down-hole depth correlation without the drawbacks and expense of other currently used down-hole depth correlation runs.

Introduction

As Coiled Tubing Services have evolved into a complete reservoir remedy solution, services have been developed which require particular accuracy in job execution. This accuracy, for some jobs, is required in correlated depth - that is, the requirement that a service be performed at the proper location in the formation, as mapped and targeted on a previous log of the subject well.

Reaching that target depth has required improvements in coiled tubing control at the surface, as well as procedures to correlate the position of the bottom hole assembly (BHA), the tool string at the end of the coiled tubing, with the location of the target depth in the well.

In SPE 38422, JL Pessin and BW Boyle1 discuss the requirements for depth accuracy in Coiled Tubing Services, along with improvements made in tube-length measuring technology found at the surface. With these developments, the CTS provider and client gained confidence in their BHA position relative to the surface (and subsequently, relative to previous surface measurements such as logs).

It is not enough to be highly accurate at surface, however, especially as depth critical services require even greater accuracy and confidence. Two goals must be attained in order to reach utmost (correlated) depth accuracy:

  • Errors in tubing measurement must be minimized.

  • Discrepancies in target depth must be eliminated.

It is the goal of the improved surface measuring equipment to achieve the first goal. It is necessary to use down-hole depth correlation equipment to achieve the second goal.

Sources of depth error

Several sources of error, causing a discrepancy between the target location and the placement of the tool, can be identified:

  • Pipe length measurement inaccuracy

  • Tubing deformation, both elastic and plastic

  • Errors in references (well references, hardware location) or procedure.

  • Errors in depth on baseline log

Sources of depth error

Several sources of error, causing a discrepancy between the target location and the placement of the tool, can be identified:

  • Pipe length measurement inaccuracy

  • Tubing deformation, both elastic and plastic

  • Errors in references (well references, hardware location) or procedure.

  • Errors in depth on baseline log

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