Linear Gas Flow Measurement System For Gas Drainage Boreholes

Requests for this project report will also receive a copy of C17056.

Currently a coal seam gas drainage borehole flow is only measured at the well collar. This does not provide any information on where the gas flow originated and how effective the gas drainage was throughout a large region. Over the life of gas drainage boreholes a number of events and conditions can occur inside the boreholes that can render the well unproductive and result in a costly remediation using work-over rigs. There is currently no method available to detect these conditions. A system of distributed borehole gas monitoring for horizontal boreholes has been conceived that can provide real time gas flow characterisation and event detection for the entire length of the borehole. This information will enable coal miners and coal seam gas producers to take action to pro-actively manage production through early intervention measures, and identify and manage non-producing zones or deploy alternate gas drainage measures.

Gas flow inside well bores produces thermal signatures. Based on this phenomenon, Distributed Temperature Sensing (DTS) is used in the oil and gas industries to identify the locations of reservoirs and measure the total volume flow from each reservoir. The application of DTS for measuring gas flow along the entire length of a lateral in-seam borehole is more complex, requiring very precise measurements to be taken of small temperature variations inside a borehole. Thus, further research and development is required to develop the system for coal mining drainage.

Two ACARP projects C17056 (Phase I, completed Dec 2009) and C19057 (Phase II, the focus of this report) were conducted to assess the technology for monitoring gas flow distribution inside a typical Surface to In Seam (SIS) borehole, and developing prototype software and interpretation tools to bring the technology into the coal mining industry. Phase II was a continuation of Phase I that was required after the borehole that was instrumented with optical fibre in Phase I failed to produce gas.

The objective of this project was to demonstrate the effectiveness of deploying DTS technology in drainage boreholes to identify the changing characteristics of gas flow along the length of the borehole. This involves both the practical aspect of reliable deployment of the fibre in the borehole and developing techniques to identify important gas flow characteristics from the DTS data.

This report details the deployment of the DTS technology at two mine sites. One installation was in a SIS borehole at Carborough Downs, which was monitored over a period of three months. A second installation was conducted inside a vertical well drilled into the Bulgo Sandstone at Appin Mine. This well was monitored over a six week period to monitor gas release as strata dilation occurred after mining.

The results demonstrated the ability to identify "signature" changes in temperature inside the borehole that could be directly associated with events such as, gas injection through cleats and fractures, changes in flow across sections of the borehole, and the presence and source of water inside the borehole. The results and methods used to conduct the analysis are presented.

A challenge in the project is that the data that has been collected provided a limited number of scenarios; thus, there is no practical means of observing and validating all of the possible scenarios. Some approaches to address these challenges are also presented.

THE FINAL REPORT IS AVAILABLE FROM THE ACARP WEB SITE