Caliper Behind Casing: Using Nuclear Logging Tools to Replicate Openhole Caliper Measurements in Cased Holes

Abstract

There are approximately ten thousand coal bed methane (known locally as coal seam gas or CSG) wells in the Surat and Bowen Basins of Australia. It is estimated that a third of these wells experience significant levels of solids production. This is a notable problem for CSG operators, which requires some form of corrective action. Believed to be caused by deterioration of interburden clays, a measurement of how the well bore outside of the casing has changed since openhole logging, such as caliper, would enable a more intelligent approach to any remedial workover. Research into such a ‘caliper behind casing’ measurement is relatively unexplored. This paper introduces and investigates the use of wireline nuclear logging tools for this unique problem.

Using nuclear simulation software, the relationship between borehole diameter and detector count rate was characterised for standard completions found in Queensland CSG wells. A range of typical formation properties were then simulated to develop a correction algorithm which was able to compensate for the variations in formations found in these environments, the input to be provided by pre-casing openhole logging. These variations are normally the variables of interest in conventional logging, but for a caliper behind casing measurement, they represent sources of error. Testing of this measurement was done with controlled lab logging of well characterised blocks in addition to field-based testing on mature and recently drilled wells.

In this paper we show how nuclear logging tools can provide a caliper behind casing measurement of up to eighteen inches when logged through casing. Therefore, estimations of hole volume are calculated more accurately by taking into account the unknown rugosity. Identification of potentially problematic features such as bridges is made possible. Excellent verification of the modelling provided by the lab-based benchmarking is seen. Finally, example logs from case studies show measurement repeatability and improve understanding of the fundamental problem of formation deterioration in the CSG wells which were included in this study.

The application of this new solution provides significant potential for improvements in operational efficiency in Queensland CSG.