Linear Gas Flow Measurement System for Gas Drainage Boreholes

Requests for this project report will also receive a copy of C19057.

The coal mining and coal seam gas industry routinely uses long in-seam boreholes (up to 2000m) for extraction of gas from coal.  The monitoring of gas flow from these holes is usually carried out at the borehole collar using a pressure differential method (orifice plate), or not monitored at all.  Whether the gas flow in the borehole emanates from a single source or from a zone is not determined, but rather it is assumed that gas extraction is distributed evenly along the entire length of hole, an unlikely proposition.

The following are some of the benefits that could be realized by having better knowledge of the gas inflows along a borehole:

· For pre-mining gas drainage applications, improved confidence that the whole zone covered by the borehole is drained, thus ensuring a safer mining environment;

· For commercial gas production, a valuable tool to assist operators in managing production (blockages, hole closure, re-drill requirements, etc.);

· Data that is invaluable to validate and improve existing gas reservoir and drainage models. A spin-off will be the ability to determine from these results important reservoir parameters such as relative permeability and geological structure; and

· By reducing underground mining hazards associated with improper gas drainage. In particular gas outbursts.

To this end, a project to develop equipment that can measure the increase or decrease of gas flows along the length of a borehole was conceived.

An optical instrument, originally developed by British Power to optimize the carrying capacity of electrical transmission lines was procured. It has typically been used to monitor temperatures during concrete curing in large structures, for leak detection in pipelines etc. Today's devices are capable of measuring  temperature variations to better than 0.05°C at 0.5 meter intervals along a conventional optical fibre for up to 25km. This is achieved by measuring the Stokes and Anti-Stokes components of the glass fibre's Raman Backscatter.

This project has shown that it is possible to determine gas flows. More importantly, changes in gas flows over time at different positions within a borehole can be determined by measuring the temperature along the borehole. The Joule Thomson effect or Joule Kelvin effect mathematically describes the temperature change of a gas or liquid when it is forced through a constriction or expansion in its flow path.  This is the principal concept used to derive gas flow rate.