SO YOU HAVE A COAL SEAM METHANE LEASE - WILL IT PRODUCE?

Ian Gray - Sigra Pty Ltd *

Published in the Australian Mining Times, December 2002

Coal seams are complex and behave substantially differently from conventional gas reservoirs. In this short article the important features of coal seam methane reservoirs are described with case examples. These cases demonstrate a wide range of variability.

The major difference between conventional gas reservoirs and coal seams is that the gas is mostly stored in the coal by a process of sorption. In a few cases where sorption pressure is equal to the reservoir pressure free gas exists in the coal. More usually free gas only comes out of the coal into the cleat space when the water pressure drops below the sorption pressure.

The process of gas flow from the solid coal to the cleats (small fractures) is one of diffusion. Usually the cleats are filled with water and the desorption of gas within the cleats leads to the two phases existing within the cleats. If a secondary major fracture system exists flow may then take place from the cleats to the major fractures. Both the cleats and major fractures exhibit their own phase dependent permeabilities.

Most coals show a significant relationship between effective stress (total stress - fluid pressure) and permeability. A reduction in permeability is caused by the cleats being reduced in width by increasing effective stress. If fluid pressure is high it tends to open the cleats and as it drops off with fluid withdrawal the cleats close. The reduction in permeability may be of an order of magnitude for anything between two and ten MPa of increasing effective stress. The softer the coal the more pronounced is this effect.

As water and gas are produced from the seam the effective stress increases leading generally to a reduction in permeability. However many coal seams exhibit an increase in permeability with production. This is caused by an effect that tends to de-stress the seam. This de-stressing is a result of the fact that most coals shrink with the desorption of gas. This shrinkage reduces the lateral stress in the seam and shifts that stress into the surrounding rocks. The opposing effects on effective stress mean that permeability of the seam may either increase or decrease with the removal of gas and water from the seam.

The critical question for the would-be producer is which effect will dominate? Will the reservoir permeability increase or decrease with production? Frequently the answer is both. Initially permeability will decrease with a drop in reservoir pressure around the production well followed by an increase as significant desorption induced shrinkage occurs within the coal.

To illustrate the variability of coal seam methane reservoirs I have chosen an assortment of cases which have crossed my path.

Case 1: Very high permeability (>1000 mD) coal seam with a low sorption pressure. The reservoir never produced satisfactorily because the water pressure could not be lowered adequately to release the gas. It was basically a big permeable aquifer with lots of recharge and a little bit of gas.

Case 2: Very gassy reservoir with a gas cap so that reservoir pressure was sorption pressure. The reservoir was deep, the coal soft and in a high stress environment. The coal displayed little shrinkage behaviour. Initial gas production was low and fell off to virtually nothing within 6 weeks. This was a classic case of a self sealing coal.

Case 3: Middle depth seam with sorption pressure near reservoir pressure. Initial permeability estimates 0.1 mD. As expected initial production was very low but this increased and kept on increasing for three months before levelling off. Permeability estimates in this stage indicated an effective permeability of about 100 mD. This reservoir showed the best example of shrinkage related permeability enhancement.

Case 4: Sloping series of seams with initial permeability estimates of around 30 mD. Sorption pressure was near reservoir pressure. Seams showed initial water production with increasing gas. They then produced reliably for an extended period. Everyone would like one of these reservoirs.

Case 5: Shallow reservoir with sorption pressure about half reservoir pressure. Surface permeability tests showed low permeabilities of the order of a few millidarcies. In seam drilling showed a wide range of good to negligible production. This reservoir produced down major fractures. If the borehole did not intersect one of these then it did not produce.

Case 6: Medium depth reservoir with thick seams and sorption pressure about 2/3 of reservoir pressure. This reservoir generally had a low permeability that locally changed to absolutely zero. The reason for the low permeability was cleat infilling with carbonates carried in hydrothermal fluids. Locally the cleat and joint infill became complete and only diffusion could occur. Thus gas production was negligible.

Case 7: Moderately deep reservoir with sorption pressure near reservoir pressure. A low permeability was measured from surface and hydrofracture completion produced low flows. In seam drilling from underground produced substantial flows if the hole was drilled in the correct orientation but virtually nothing otherwise. This reservoir showed huge cleat and stress controlled directional permeability with a directional permeability ratio of at least 100:1.

Case 8: Large sequence of seams with sorption pressure about 2/3 of hydrostatic and exhibiting high permeability. Production from these seams significantly exceeds indications of gas in place based on core desorption. The additional gas is apparently supplied from deeper shales through fractures.

Some of the issues that need to be addressed in assessing a coal seam methane reservoir are:
a) Do we have a market for gas?
b) Do we have coal?
c) Does it have gas?
d) Does it have water? and if so what do we do with it?
e) What is the sorption pressure?
f) What is the reservoir pressure?
g) What is the permeability?
h) Is the permeability directional?
i) Will the permeability change and if so will it increase or decrease?
j) Can we keep a hole open in the seam?

Based on the above information it is possible to design drilling and completion programme and work out the economics of developing the reservoir.