Reservoir Characterisation Using Permanent Distributed Temperature Sensing in Coal Seam Gas Development: Success and Challenges in the Surat Basin

Abstract

This paper discusses the use of permanent Distributed Temperature Sensing (DTS) using optical fibres for flow zone allocation in Coal Seam Gas (CSG) production wells. In a commingled well the allocation of total well production to the multiple stacked reservoir units is extremely important. Understanding which zones are most productive can influence future development decisions and operational practices. Additionally, the subsurface reservoir model can be calibrated and verified from the data, improving the accuracy of any forecast derived from the model. At QGC, well completion design has focused on simplicity to drive efficiency in the execution of the ca. 2,500 wells drilled so far. These open hole completions have been run with a pre-perforated liner and no external packers to separate formations. This adds uncertainty to the interpretation of results from conventional wireline production logging tools (PLTs) run inside the pre-perforated liner. To perform a conventional PLT, the down hole pump for dewatering must be pulled, requiring a rig and making flow allocation data acquired by PLT very costly. DTS is seen as a viable alternative to PLTs, with the measurement fibre run permanently on the outside of the casing or tubing. It can remain in the well during pump operation and gather data throughout the life of the well. Typically, at QGC, a recording is made every 6 hours and is transmitted to a cloud based database for immediate visualisation. Since a temporary single day DTS installation was trialled in 2014, QGC has run 11 permanent fibres across its Surat CSG development. This has yielded valuable information on production allocation and depletion trends. There have also been a number of challenges encountered. These include interpretation of dual phase flow, and identifying the correct geothermal gradient for interpretation. The advantages and disadvantages of casing conveyed versus tubing conveyed DTS installations are also discussed.