4-D coal permeability under true triaxial stress and constant volume conditions

Emission of methane from coal seams has been known for centuries. Up until recently, underground coal mines have vented the methane with large volumes of dilution air, blown in to keep the methane concentration at the coal face below the explosion limit. The ability to capture this gas in high concentrations for economic gain has been developed only in the last decade or so. Safe and efficient engineering design of methane extraction processes from coal seams requires in-depth understanding of bulk coal's in-situ permeability, the primary petrophysical property controlling methane flow rates. Advancing this understanding and technology is central to unlocking vast resources of coalbed methane (CBM) around the world: in Queensland alone, the resource base is very large, with some 5,000 billion m of inplace gas. Even a modest 10% recovery factor would add $30 billion of clean fuel to industries (Massarotto, 1998). The USA CBM in-place volume has been reported at some 11,000 billion m3. Huge potential exists in Russia, China and Canada. Coal permeability is also a key design parameter for underground coal gasification schemes and for natural gas storage by pipelines. A thorough understanding of the permeability of coal leads to increased coal mine safety and to reduced greenhouse gas methane emissions and CO2 sequestration.This research has addressed a long-held need to investigate the in-situ behaviour of coal permeability, under realistic pressure and stress conditions and with large enough samples representative of bulk coal. A major portion of the research was done utilising large coal samples (80mm cubes and 80x80x160mm prisms) mounted in a world-first True Triaxial Stress Coal Permeability (TTSCP). This work was supplemented with smaller 40mm cubes. The pioneering use of this geometrical shape allows realistic testing of the three mutually orthogonal stresses acting on in situ coal, and measuring the permeability in each of the three orientations that define its 3-D anisotropic character. The research investigated:► the effect of test fluid on permeability measurements;► the effect of sample size on measured coal permeability, towards deriving a scale-up correlation to allow working with smaller cores in the future;► correlations of permeability with cleat spacing and direction to confirm the permeability anisotropy ratio and confirm the use of cubic samples as practical and representative testing methodology;► the effects of three separate, mutually-orthogonal directional stresses on the directional permeability of coal;► the effects of methane desorption on coal permeability, under a novel approach of constant volume conditions as compared to the more traditional constant external stress conditions, allowing full 4-D characterization of coal's dynamic permeability.This work has confirmed several postulates and answered long-held questions about the behaviour of coal permeability, essentially proving that it is a very dynamic property with four-dimensional character. First, because permeability is not directly measured but rather derived from the flow rate and pressure drop of a probing fluid, and because almost any fluid save helium gets appreciably adsorbed unto coal's internal micro pores, it is important to specify which fluid is used during measurements. For example, this work indicates that the permeability to helium, the absolute permeability, is higher than that to nitrogen or methane, which have distinct effective permeabilities.Secondly, the bulk in-situ horizontal permeability of coal is seen to be very anisotropic, with a measured permeability anisotropy ratio of 19:1 in this research, resulting from face cleat fractures being much more continuous than the orthogonallypositioned, butt cleat fractures. Vertical permeability can be higher or lower than the horizontal, being principally a function of any impermeable layers in the seam. These facts define the 3-D anisotropic character of coal permeability.Additionally, external compressive (sometimes tensional) stresses act on coal (vertical lithostatic, horizontal tectonic maximum and horizontal tectonic minimum); they are also directional and generally mutually orthogonal, further affecting the 3-D character of coal permeability. Offsetting the external compressive stress is the internal pore pressure exerted by methane and other reservoir fluids. Because of the coal matrix's high compressibility, the permeability loss at high net stresses (external compressive stress less internal pressure) can be one to two orders of magnitude. However, the author postulated that this high permeability loss occurs principally when the horizontal tectonic maximum stress vector is orthogonal to the face cleats or when these stresses are isotropic. When the maximum stress is parallel to the face cleats, an increase in permeability can develop as the net stress increases. The author has termed this the correspondence principle and has shown that a permeability increase of some 100% occurs when the maximum stress and face cleat azimuths are aligned.Thirdly, very recent and limited research, not widely accepted in CBM international technical circles, has shown that pressure depletion, with associated desorption of methane, can actually increase permeability due to matrix shrinkage of coal. This research has corroborated two previous findings, with indicated 100% and 400% increases in methane effective permeability for two specimens of the same general rank, using the novel constant volume approach. The measured increase in permeability has been corroborated by strain gauge readings. This dynamic behaviour of coal permeability over time, from the combined but opposing effects of external net stresses and internal matrix shrinkage, defines its 4-D character.There is research yet to be done to fully address the scale-up objective, as appreciable time was required to design, fabricate and commission all the components of this world-first TTSCP. Time was also devoted to develop the new high-pressure sealing system for the 40mm and 200mm cube test cells, which have now been designed and fabricated. This Dissertation was finalized before any tests with the 200mm cube test cell could be started. As well, too few 40mm cube tests could be performed to develop a sturdy lab-based scale-up correlation. However, a theoretical set has been derived. Future investigations need to be pursued with the large 200mm cube test cell to complete the scale-up analysis. Finally, there is important work to be done on the effective permeability to CO2 and on the relative permeabilities of CO2-CH4 mixtures.

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