Developing Potential Coal Seam Gas Reserves

Do you have a potential coal seam gas reserve and want to develop it?

There have been successes and failures in the coal seam gas (CBM) and coal mine gas (CMM) businesses, however many of the failures would have been avoidable with suitable attention to detail. Indeed, the successes have generally followed a diligent approach.  Let us look at what really matters in determining whether success will be achieved.

A loose and imprecise—albeit useful—equation to illustrate what constitutes a gas reserve is in terms of the gas bearing strata:

Gas reserve = area x thickness x gas content x permeability

Where any of the terms tend to zero, so does the reserve. Therefore, it is necessary to be certain of all four. There are of course more factors to take into account, such as the gas value, local production economics and detailed behaviour of the reservoir. While this equation does not meet any reserve definition it is still quite illustrative of what needs to be determined.

A detailed knowledge of the reservoir by correct exploration and testing techniques relieves a huge level of uncertainty, and ultimately gives the basis for determining whether a production operation is likely to succeed.

It is useful to examine important factors that have contributed to gas reserve development disasters and successes.

Gas Composition

There have been many instances where the gas type changes within a kilometre or so. The common gas types are methane, carbon dioxide and nitrogen; as the latter two have no commercial value, determining the gas composition is very important. Igneous intrusions are indicators of gas types other than methane. However, there are numerous cases where the variation cannot be associated with these, but is more a function of the groundwater chemistry.

Irregular Geology 

Extracting gas from lensing seams and from around faults can be challenging. Such geological variations reduce the probability of success and will, at the very least, require more drilling cost. Faulting is associated with increased or decreased permeability, depending on its form.

However, chasing sweet spots around faults is seldom worthwhile.

Presence of Aquifers 

Aquifers always pose a potential complication in gas extraction because recharge of the coal seam must be prevented or it will limit production. Aquifers may also lead to problems because of the emotive political nature of any discussions relating to gas production in proximity to them.

Variable Permeability (spatially)

There have been numerous instances where inadequate exploration has led to a failure to detect changing permeabilities of coals. It is quite possible to have a change in permeability of two orders of magnitude in 10 km. In several cases these changes have actually been detected in exploration but have been ignored as anomalous test values.

Anisotropic Permeability

Coal frequently exhibits very highly directional permeability, sometimes up to a 100 fold difference between the major and minor values. If this is the case then the production method will need to be by drilling perpendicular to the direction of major permeability.

The key here is to measure stress and its direction, to look at structure, and ultimately to test for directional permeability in the field.

Varying Permeability With Production

Coals are extremely sensitive to changing stresses.

Some of them drop an order of magnitude in permeability with about 3 MPa (435 psi) change in effective stress. The effective stress is the difference between the total stress in the coal and some fraction of the fluid pressure. The major factors that lead to changes in total stress are the degree to which the coal shrinks as it gives up gas and its stiffness. Examples exist where the coal permeability has dropped by more than an order of magnitude with production, while in others the permeability increased by two orders of magnitude.

Determining the variation in stress with production, before investing in a production operation, is extremely important. This requires determining the stress path. This is the variation in effective stress during the production cycle.

Completion Method 

Coals are sensitive entities, and stressing them by using the wrong completion method is not the way to entice them to yield their gas.

The completion method is critical, as near well bore damage brought about by using the wrong drilling mud or frac fluid can destroy production from a good reservoir.

Sigra’s Role

Despite the current dominance of major players in the development of gas reserves in coal, there is still room for the smaller operator. The technology base is not held within these groups, and if the local economics are favourable, successful projects can be achieved.  The key here is the term ‘local’, as there are many uses for gas. Frequently gas is needed where there is no pipeline.

Sigra is a company that has nearly a quarter of a century of experience in exploration and production of coal seam gas (CBM), mine methane (CMM) and other shallow gas reserves. We do this work for gas producers and for mines.  If you think you may have a resource and want to find out what to do to develop it, a discussion with Sigra should be your next move, to improve the prospects for success and to avoid the potential for failure.

Most failures in gas production have been caused by inadequate exploration. A premature desire to conduct production trials is seldom a wise economic choice. The wish to conduct these is frequently driven by a need to access further funding or to sell the lease. Too early a production trial is frequently an indicator either that the lease owner wants to make use of a potential sweet spot to raise financial interest in the lease. It may also be an indication of ignorance or desperation.

If you have the use for the gas then Sigra can determine whether you have a resource, and if so, whether it can be converted into an economic reserve. The secret here is to explore carefully, measuring what you need to.

What We Can Do

Sigra has built up a suite of tools and techniques to do this efficiently. In any project our geological team would assess the local geology and any information that might come from mines, water bores, gas seeps or other pre-existing exploration. We take this information and, if it appears promising, work out a drilling programme. This will vary from site to site, but the key elements will be to confirm the area, thickness, gas content and permeability of the target seams.

Our tools enable the determination of all of these, and additional important features such as the directional permeability and likely permeability variation related to the stress path.  Our techniques include field measurement and laboratory measurement of gas and rock properties. It also includes simulators to enable us to predict production. We can then design and cost a suitable production system.

Sigra provides a one stop shop to getting gas out of the ground. We provide geological review, exploration designs, field testing, laboratory testing, analysis, reservoir simulation, production trials, production design, and supervision of its implementation.

Author: Ian Gray
Ian initially worked in Australia supervising the installation of the first gas drainage system at Central Colliery and then went on to work as Senior Geotechnical engineer building a mining wing to the consulting company DJ Douglas and Partners. In 1990 he became Principal Engineer, Mining Research with the Safety In Mines Testing and Resarch Station of the Department of Resource Industries of the Queensland Government. In this role he worked on frictional ignitions, mine explosions, windblasts, gas drainage and directional drilling including the first surface to in-seam operation in the country in 1991. In 1994 he started Sigra as a one man business. Since then the company has grown under his guidance to span mining, gas, civil, geotechnical work and a number of mechanical and electronic product developments.

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