Reservoir Engineering in Coal Seams: Part 2-Observations of Gas Movement in Coal Seams
- Ian Gray
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Engineering
- Publication Date
- February 1987
- Document Type
- Journal Paper
- 35 - 40
- 1987. Society of Petroleum Engineers
- 5.1.2 Faults and Fracture Characterisation, 1.10.1 Drill string components and drilling tools (tubulars, jars, subs, stabilisers, reamers, etc), 4.1.5 Processing Equipment, 1.6.9 Coring, Fishing, 1.6 Drilling Operations, 4.1.2 Separation and Treating
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This paper, the second of two concerning the movement of gas in coal seams, covers observations of seam fluid pressures and flows in mines in northern and central Queensland, Australia. Techniques based primarily on underground measurement rather than measurements from surface primarily on underground measurement rather than measurements from surface boreholes were used to gain information on the seams. The techniques used for in-seam studies are described because they differ substantially from conventional oil and gas surface borehole techniques. The paper demonstrates the importance of cleats and joints in the control of fluid movement and records flow increases consistent with increasing permeability with production.
Pressure Measurement. Pressure measurement was Pressure Measurement. Pressure measurement was conducted in seam by two basic techniques. In the first, a hole approximately 50 mm [2 in.] in diameter was drilled in seam and then the drill rods were withdrawn after a predetermined length, typically when 10 to 80 m [33 to predetermined length, typically when 10 to 80 m [33 to 262 ft], had been reached. Following this, a 1- to 2-m [3.3- to 6.6-ft] -long packer was installed as quickly as possible on the end of the conduit string, leaving 2 to 10 possible on the end of the conduit string, leaving 2 to 10 m 16.6 to 33 ft] of open hole ahead of the packer. The packer was inflated with water through a synthetic braid packer was inflated with water through a synthetic braid hydraulic hose. Packer inflation pressure typically was raised to that required to expand the packer to borehole size plus I 1/2 times the fluid pressure being measured. Ocasionally, this packer pressure may have led to some problems because it may have approached or exceeded problems because it may have approached or exceeded total in-seam stress and therefore tended to open the cleats (fractures) within the seam, thus promoting leakage around the packer. The pressure within the zone at the end of the packer was measured through a 4.8-mm [0. 19-in.] -OD nylon tube leading to the borehole collar and connected to either a pressure gauge or a chart recorder. The coal permeability was normally sufficiently low that no significant leakage occurred around the packer into the borehole, and reasonably reliable pressure measurements could be obtained.
By repeating the procedure outlined above, it was possible to gain an understanding of the pressure possible to gain an understanding of the pressure distribution that existed within the coal along the line of a borehole. Up to five tests at 10-m [33-ft] intervals could be conducted in a 7-hour shift with this technique. Attempts were made to extend the use of packers to permanent pressure measurement. A single packer was permanent pressure measurement. A single packer was considered inadequate to measure the overall pressure distribution along a borehole, and therefore multiple packer assemblies were constructed containing pressure measurement ports between the packers as described by Lama et al. These were expensive and difficult to maintain.
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