The Basics of UCG

UCG is a method of converting un-worked coal,while still in the ground,into a combustible gas which can be used for industrial heating, power generation or the manufacture of hydrogen, synthetic natural gas or diesel fuel. The gas can be processed to remove its CO2 content, thereby providing a source of clean energy with minimal greenhouse gas emissions. Studies have shown that significant commercial quantities of coal can be obtained using UCG.

The basic UCG process has two wells drilled into the coal, one for injection of the oxidants, another to bring the product gas to surface. Coal has considerable variation in its resistance to flow, depending on its age, composition and geological history, so simply relying on the natural permeability of the coal to transport the gas is generally not satisfactory. High pressure break-up of the coal with water (hydrofraccing), electric-linkage and reverse combustion have all been used with varying degrees of success in both pilot and commercial scale operations. 

Different Methods

Two different methods of UCG have now evolved, both are commercially available.

The first, based on technology from the former Soviet Union, uses vertical wells and a method like reverse combustion to open up the internal pathways in the coal. The process has been tested (1999-2003) in Chinchilla, Australia using air and water as the injected gases.

The second, tested in European and American coal seams, creates dedicated inseam boreholes, using drilling and completion technology adapted from oil and gas production. It has a moveable injection point known as CRIP (controlled retraction injection point) and generally uses oxygen or enriched air for gasification.

Inseam Drilling

In seam drilling was identified at an early stage as a way forward, but steerable drilling in coal only started to become available in the latter stages of the US programme of UCG (1975-1990). The breakthrough came when directional inseam drilling was combined with a system of retractable injection known as CRIP as illustrated in the above animated figure. This arrangement provided an unobstructed path for the departing gases.

Site Selection

Site selection is paramount  to a successful UCG project. The characteristics of the coal seam, the permeability and fault structure of the local strata and the geology and hydrogeology of the area which surrounds the target coal seam must be fully understood. This requires the drilling of pilot bore holes to coal seam depth for coring and seam characterisation, and a good quality seismic survey (preferably 3D) of the whole area. Modelling of the hydrogeology will be required to meet most countries ground water requirements. This detailed exploration work is one of the major upfront costs of UCG, the same is true for conventional mining.