Butt cleat

A system of joint planes is often observed in coal formations, and these joint planes (cleats) are usually perpendicular to the bedding planes. Thus, cleat joints are usually vertical. The main system of joints is more commonly called the face cleat, whereas a cross-system of jointing is called the butt cleat. Furthermore, the cleat system in coal has a pronounced effect on the properties of a coal deposit. For example, holes drilled into coal perpendicular to the face cleat are said to yield from 2.5 to 10 times the amount of methane gas from the formation as holes drilled perpendicular to the butt cleat. Also, the cleat system of fracture and the frequency of cleats may determine the size of run-of-mine coal. In general, a pair of cleats will be oriented at about 90° to each other, and the orientation of the cutting elements influences the output of coal-mining machines (Figure 8.4). 

Endogenetic cleats are normal to the bedding plane of coal and generally occur in pairs. There are at least two sets of near perpendicular fractures that intersect the coal to form an interconnected network throughout a coal bed. These two fracture systans are known as face and butt cleats. The shorter butt cleat normally terminates at a face cleat, which is the prominent type of cleat. 

Bituminous coal seams often exhibit numerous fractures (or cleats) that may be relatively closely spaced (from a fraction of an inch to a few inches apart). There are usually two preferred directions to the cleat which are at approximately 90° to each other. The more prominent cleat is termed the face cleat whereas the less prominent (or less well-developed) cleat is known as the butt cleat. The natural trend of the cleat may facilitate the breaking of the coal during the mining operations and may, therefore, also determine the direction of the mining operation. 

A conventional reservoir is assumed to be approximately (if not fully) homogeneous and gas will tend to flow to a wellbore in a pore-to-pore Darcy flow. Coal had minimal porosity and, therefore, limited communication from one micro-cleat to the next. The best flow communication through the cleat system is along/ace cleats, which typically run vertically and tend to align themselves from cell to cell along the axis of maximum external stress on the coal. Butt cleats, which intersect the face cleats at 90° in the direction of least external stresses, do not tend to align, and they typically have a minimal contribution to gas production. 

A system of joint planes is often observed in coal formations and these joint planes (cleats) are usually perpendicular to the bedding planes (Chapters 1 and 7) thus, cleat joints are usually vertical. The main system of joints is more commonly called the face cleat whereas a cross system of jointing is called the butt cleat. 

Butt cleat A short, poorly defined vertical cleavage plane in a coal seam, usually at right angles to the long face cleat. 

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