Estimating production in coal accurately is crucial for promoting the process of safe, efficient and green coal mining. It has been gradually recognized that horizontal wells with multiple fractures are employed to develop the coal reservoir, which signifies that the linear flow regime will dominate for a rather long time. However, the traditional analysis approaches of transient linear flow regime may yield the overestimation of coal reservoir property. In this work, a new analytical model was proposed to estimate the rate-transient of wells with multi-fractures in coal reservoir that produce at a constant flowing-pressure, which takes multiple flow mechanisms into consideration. Especially, the matrix shrinkage effect caused by water extraction from microscopic pores was incorporated, which has never been investigated by current production analysis models. In comparison with the conventional reservoir, the advanced pseudo-pressure and pseudo-time equations incorporating above critical mechanisms were established, including the four effects of gas slippage, effective stress, and matrix shrinkage caused by gas desorption/water extraction. In addition, the excellent agreement between the predicted rate by the proposed model and field data was achieved to validate the reliability of proposed models. Furthermore, the sensitivity analysis was carried out to clarify the influence of a series of factors on the seepage mechanism and productivity curve. Results demonstrated that the matrix shrinkage effect caused by water extraction may increase the well production rate in coal reservoir. Selecting one field case as an example, the production rate predicted by the red curve is obviously higher than that by the green curve, the average discrepancy yields around 39.5%. The relative humidity in coal matrix will present a positive impact on well production performance. Taking a field case as an instance, when the relative humidity varies from 8% to 14%, the well production sharply increases by about 11.6%.
Discontinuous Long Fibre (DLF) composites, composed of randomly-oriented strands of chopped unidirectional pre-impregnated tape, have been used in the aerospace industry to produce intricate, net-shape parts with complex features – replacing complicated metallic brackets with single, lightweight parts. Carbon/PEEK DLF composites suffer from warpage problems driven by several factors including their high processing temperatures and semi-crystalline matrix shrinkage. This work aims to characterize warpage of thin-gauge parts and pursue mitigation. Results showed that the magnitude of warpage reduces with decreasing strand size and increasing thickness. At thicknesses greater than 2 mm, warpage appeared relatively stable. The introduction of ribbed features was explored as a mean of mitigating warpage by increasing part stiffness. No significant impact on the magnitude of warpage was observed within parts. However, the addition of ribs helped to control the warped shape of the part.
An alternative geomechanical reservoir boundary condition is proposed for ultra-deep coal seams of the Cooper Basin in central Australia. This new concept is embodied within the author’s ‘Expanding Reservoir Boundary (ERB) Theory’, which calls for a paradigm shift in gas extraction technology, diametrically opposed to current practices. As with shale, full-cycle, standalone commercial gas production from Cooper Basin ultra-deep coal seams requires a large stimulated reservoir volume (SRV) having high fracture surface area for gas desorption. This goal has not yet been achieved after 13 years of trials because, owing to the bipolar combination of shale-like reservoir properties and coal-like geomechanical properties, these poorly cleated, inertinitic coal seams exhibit ‘hybrid’ characteristics. Stimulation techniques adopted from other play types are incompatible with the highly unfavourable combination of nanoDarcy-scale permeability, ‘ductility’ and high stress. Nevertheless, gas flow potential counterintuitively increases with depth, contingent upon the creation of an effective SRV. Optimum reservoir conditions occur at depths beyond 9000 feet (2740 m), driven by dehydration, high gas content, gas oversaturation, overpressure and a rigid host rock framework. The physical response of ultra-deep coal seams and the surrounding host rock to pressure drawdown is inadequately characterised. It remains to be established how artificial fracture and coal fabric aperture width change due to the competition between desorption-induced coal matrix shrinkage and compaction caused by increasing effective stress. Studies by the author suggest that pressure arching may ultimately control gas extraction efficiency. Harnessing this geomechanical phenomenon could resolve the technical impasse that currently inhibits commercialisation. Pressure arching neutralises SRV compaction by deflecting stress to adjacent strata of greater integrity. These strata then function as an abutment for accommodating increased stress outside the SRV. This shielding effect allows producing ultra-deep coal seams to progressively de-stress and ‘self-fracture’ naturally, in an overall state of shrinkage-induced tensile failure. An ‘expanding reservoir boundary and decreasing confining stress’ condition is generated by the combined, mutually sustaining actions of coal matrix shrinkage and sympathetic pressure arch evolution. This causes the SRV to steadily increase in size and permeability. Cooper Basin ultra-deep coal seams may be effectively stimulated by harnessing this self-perpetuating, depth-resistant mechanism for creating permeability and surface area. The ultra-deep coal seams may be induced to pervasively ‘shatter’ or ‘self-fracture’ naturally during production, independent of ‘brittleness’, analogous to the manner in which shrinkage crack networks slowly form, in a state of intrinsic tension, within desiccating clay-rich surface sediment.
Gas flow mechanisms and apparent permeability are important factors for predicating gas production in shale reservoirs. In this study, an apparent permeability model for describing gas multiple flow mechanisms in nanopores is developed and incorporated into the COMSOL solver. In addition, a dynamic permeability equation is proposed to analyze the effects of matrix shrinkage and stress sensitivity. The results indicate that pore size enlargement increases gas seepage capacity of a shale reservoir. Compared to conventional reservoirs, the ratio of apparent permeability to Darcy permeability is higher by about 1–2 orders of magnitude in small pores (1–10 nm) and at low pressures (0–5 MPa) due to multiple flow mechanisms. Flow mechanisms mainly include surface diffusion, Knudsen diffusion, and skip flow. Its weight is affected by pore size, reservoir pressure, and temperature, especially pore size ranging from 1 nm to 5 nm and reservoir pressures below 5 MPa. The combined effects of matrix shrinkage and stress sensitivity induce nanopores closure. Therefore, permeability declines about 1 order of magnitude compare to initial apparent permeability. The results also show that permeability should be adjusted during gas production to ensure a better accuracy.