Investigation of the Potential for Pre-Drainage of the Lower Seams at Appin Colliery Utilising Hydra

This report examines the potential for enhancing gas drainage performance from the lower coal seams of the Illawarra coal measures by utilising hydraulic fracturing in cross measure holes to stimulate gas production.  The relative impacts of drainage hole spacing and drainage time are assessed in terms of their interactive influence on drainage performance measured in terms of relative gas content reduction. The results are for a regular pattern of hypothetical vertical holes.

The work is based on extrapolation, using a numerical simulator, of experimental results obtained during a trial of hydraulic fracturing in the Wongawilli Seam at Appin Colliery. In this regard, the Wongawilli Seam has been treated as an analogue of the lower seams in general.

Objectives

The objectives that evolved as the project progressed became:

• To extrapolate experience gained during hydraulic fracture stimulation (in a single hole) of the Wongawilli Seam at Appin Colliery to examine the potential for degassing the Wongawilli utilising hydraulic fracturing in a pattern of interacting holes. 

• To review the results of the above as an indication of the likely effectiveness of hydraulic fracturing from a pattern of cross measure holes drilled from the Bulli Seam workings for degassing the lower seams in general. 
• To make recommendations as to the direction of any further R&D that may be required to facilitate introduction of hydraulic fracturing for this purpose.

Method

The work program comprised a number of discrete steps involving the use of a reservoir simulator:

• History matching of experimental results obtained during a hydraulic fracture trial conducted in the Wongawilli Seam at Appin Colliery, undertaken as a precursor to this project. This process produced a set of input parameters for the simulator, "calibrated" against measured data, that could be used as a basis for extrapolation of the experimental performance to examine potential damage scenarios for the lower seams. 

• Conduct of a series of simulations, based on the above input parameters, investigating the potential for drainage of the Upper Wongawilli Split , the Lower Wongawilli Split , the total Wongawilli Seam

• For each split, an optimum drainage layout was arrived at. In each case the drainage performance for a compromise layout derived for the two splits combined was the prime system investigated. For the total seam, the results for the compromise layout were arrived at by combining the results for both splits on a weighted basis

Results

The study produced graphical results allowing examination of the interaction between hole spacing, drainage time and bottom hole pressure as these impinge on the degree to which the gas content can potentially be reduced in the lower seams. In this context, the Wongawilli Seam was considered as an analogue for the lower seams in general, and the outcome of hydraulic fracture trials in the Wongawilli Seam in a hole at Appin Colliery taken as a benchmark.

The results obtained provide a basis for assessing the potential for employing hydraulic fracturing to enhance gas drainage from the lower seams in the Southern Coal Fields area, based on analogy with measured performance in the Wongawilli Seam. The results in the form presented in this report are not subject to any license or patent arrangements. The simulator used for the work (SIMED II) is a commercial product developed jointly by the University of NSW and CSIRO.

Subject to the results of the current initial study being considered encouraging, it is recommended that more detailed numerical studies be undertaken to investigate the application of hydraulic fracturing from inclined cross measure holes, and/or from surface vertical holes. Ultimately, more field trials will be required to establish confidence.

Implications of the Study

The implications of the study for drainage of the lower seams in general are examined under the following headings:

• Extrapolation of Results

• Impact of Drainage Pattern Optimisation
• Impact of Bottom Hole Pressure
• Impact of Hydraulic Fracture Performance
• Timing of Treatments
• Operational Implications