COAL SEAM METHANE
COAL BED METHANE

Coal seam methane reservoir assessment is one of the core businesses of Sigra. Sigra can conduct field and laboratory measurements to assess reservoir properties. Using this information it can predict reservoir behaviour so that the correct production techniques can be used. Sigra also monitors reservoir performance to ensure that maximum production is achieved.

The 1987 SPE Paper "Reservoir Engineering in Coal Seams: Part 1 - The Physical Process of Gas Storage and Movement in Coal Seams", written by Sigra's managing director, was a landmark in presenting an understanding of the unique characteristics of coal seams, especially permeability variations related to effective stress variations. These are brought about by the withdrawal of fluid and coal matrix shrinkage. With refinements this remains the basis for predicting reservoir permeability change. The refinements concern the role of diffusion and fracture permeability.

The technical issues affecting coal seam methane production are summarised in the December 2002 issue of the "Australian Mining Times". The article also outlines some case studies. An Australian perspective on the development of Coal Seam Methane is given in the article "Coal Seam Methane in Australia", from the June 2002 issue of Oil and Gas Australia.

The key to any reservoir assessment is being able to measure reservoir properties and then to use the results of the measurement in some predictive system.

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Field and Laboratory Services

Sigra has developed a series of field and laboratory techniques for coal seam methane reservoir assessment. The field techniques involve measuring and interpreting stress using Sigra's IST tool. Permeability is measured using Sigra's drill stem test tools to perform drill stem tests tests or injection fall-off tests.

Sigra has several forms of DST tool and others can be manufactured to suit specific test programmes. This ability to manufacture equipment to suit requirements is a part of Sigra's capability, which includes the manufacture of packers, flowmeters, and pressure transducers. A free fall DST tool designed to operate through an HQ coring drill string without cable or inflation line is described in the paper "Determining the Gas Production Characteristics of Coal Seams" from the 2000 Bowen Basin Symposium. It is shown in photograph P15. Sigra more usually uses a DST tool that runs with an inflation line and bottom hole pressure transducers that are linked to a surface read-out (P17). This unit comes with a trailer that includes controls, flowmeters, pressure gauges and the read-out. Sigra can also supply a low pressure injection tool to operate on the end of a coiled polyethylene tube.

To monitor reservoirs Sigra installs pressure transducers in- seam. These can be either cemented in place or be set using packers. In either case multiple level pressure transducers can be set in a single hole. Sigra can also supply gas flow meters to monitor individual hole production. Data from these can be gathered and telemetered back to a base using the Sigra field data logger.

Sigra can undertake interference tests(D5,P5) with a central injection or pumping well surrounded by monitoring wells fitted with pressure transducers.

Sigra measures gas content by desorption of core or chips in the field. The desorption rate is used to assess the diffusion coefficient of the coal. When core is obtained field measurements of shrinkage are undertaken during the desorption process. These are achieved by strain gauging the core and automatically monitoring the strain changes at the same time as the core desorption is automatically monitored. The field desorption system is shown in P6.

In the laboratory Sigra can continue the desorption process and can in addition test core permeability. In cooperation with The University of Queensland Sigra have developed a coal permeability test rig. This device enables 61 to 63.5 mm diameter core to be tested with attached strain gauges. The coal can be absorbed with gas, held in controlled axial and confining stress whilst gas is driven through the sample. Testing can be conducted in the axial direction and across the sample. The test rig was built for research purposes and can hold a constant axial load and constant lateral strain by servo control. This loading represents the field behavior of a coal seam reservoir.

The equipment is shown in photograph P7. This rig is available for testing or research.

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Consulting

Sigra provides a consulting service in the field of coal seam methane. This service involves reserve estimation, calculation of reservoir parameters, modelling of reservoir behaviour and comparison of predicted with actual reservoir results. Field information may be derived from Sigra's field data acquisition systems.

The reservoir prediction service naturally leads to a choice of production technology. No single system is universally the most satisfactory as reservoirs are different. Choosing the correct production system is most important. The potential options are simple wells, cavity completion, hydrofracture systems, high energy gas fracturing or surface to in-seam drilling. The paper "Surface to In-Seam Drilling from Surface for Methane Drainage and Exploration" summarises the latter.

Sigra can design well pumping systems and other associated well equipment.

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Equipment

Sigra can provide a range of equipment required for coal seam methane testing and production. With the exception of in-house test equipment this essentially covers three areas.

The first is equipment for field desorption. This includes canisters to hold core during desorption and automatic recording systems for desorbed gas and core strain. These can be seen in photograph P6

The second area is field monitoring equipment which includes Sigra's field data loggers which can be coupled to pressure transducers or flow meters.

The third area is in production equipment. This currently involves packers for hydrofracture, directional drilling and measurement while drilling equipment.