Gas Wettability of Coal and Implications for Gas Desorption and Drainage

A key parameter affecting the flow of gas in coal is the wetting potential of gas, in comparision to water, to spread over the wall of coal micropores and microfissures. Wettability is quantified in terms of the contact angle of the fluid interface with the solid surface. A fluid with a small angle of contact would spread over the pore walls and eventually displace the non-wetting fluid. Depending on the nature of the coal, gas type and environmental conditions in coal reservoirs, either water or the gas phase could wet coal more strongly. Furthermore, in mixed gas conditions, one gas may be more strongly attached to coal than the other gases. In water-saturated coal, gas desorption in small pores -where most adsorbed gas is stored - can be totally inhibited by water if it is a strong wetting phase. Reducing the hydraulic head (drawdown to achieve the gas desorption pressure) should allow desorption of gas in larger fractures, whereas in small pores, gas desorption could be inhibited by capillary pressure due to the effect of interfacial tension and gas-wetting properties of coal. In this study, we built a new system to quantify the wettability of coal by gas. The contact angle of the water-gas interface with the coal surface inside the gas phase was measured using a captive gas bubble technique. The contact angles of CH4 and CO2 bubbles in water with a coal from the Sydney Basin were measured at different gas-water pressures of up to 15 MPa for CH4 and 6.1 MPa for CO2. The results show that as gas bubbles dissolve in water, the contact angle of the bubble with the coal surface reduces. The contact angle values were smaller for CO2 gas than CH4, and in general, the contact angle value decreases as gas–water pressure increases

Developing a New Method to Identify the Source of Gas Emissions into Longwall and Goaf from Surrounding Strata

During coal mining, strata is fractured and gas trapped in the roof and floor of coal seams travels into the workings. Depending on the extent and shape of fractured zones suitable gas drainage patterns are required to maximise the gas capture from strata but also to minimise the cost of operations. In this paper a new method to identify gas emitting zones/seams in the embedding strata and gas migration pathways is presented. The developed method was used in a coal mine in the Southern Coalfield of the Sydney Basin. Geochemical properties of gas trapped in coal seams above and below the mining horizon were analysed and compared with similar properties of gas collected from goaf areas. This study shows that using this method it is possible to identify the source of gas in goaf areas and thus determine the extent of fracturing in the strata around the mined seam

Parameters affecting coal seam gas escape through floor and roof strata

Coal seams are compact gas reservoirs and can contain large volumes of methane (CH4) and carbon dioxide (CO2) which are the main constituents of coal seam gas (CSG). CSG is present in various volumes and concentrations across the mining regions in the coalfields of the Sydney and Bowen basins. The variations in actual gas volumes and relative concentration of these gases in coal could be due to different gas generation/accumulation rates and different adsorption capacity of the coals, but also because of the difference in the sealing capacity of the non-coal sediments enclosing the coal seams. It is postulated that the sealing capacity of the main roof and floor rocks at a coal seam could have a major effect on the volume of gas in place (gas content). This paper reports some results of an ongoing investigation on the gas flow parameters which affect the sealing capacity and retention of gas in coal reservoirs. The results discussed here concern, in particular, the matrix permeability (or micro permeability) and the diffusivity of the non-coal horizons in the roof and floor of the coal seams. These properties could be limiting factors on the rate of gas escape from a coal formation to the surrounding strata

On the applicability of connectivity metrics to rough fractures under normal stress

Rough rock fractures have complex geometries which result in highly heterogeneous aperture fields. To accurately estimate the permeability of such fractures, heterogeneity of the aperture fields must be quantified. In this study heterogeneity of single rough rock fractures is for the first time parametrized by connectivity metrics, which quantify how connected the bounds of a heterogeneous field are. We use 3000 individual realizations of synthetic aperture fields with different statistical parameters and compute three connectivity metrics based on percolation theory for each realization. The sensitivity of the connectivity metrics with respect to the determining parameter, i.e the cutoff threshold, is studied and the correlation between permeability of the fractures and the computed connectivity metrics is presented. The results show that the Θ connectivity metric predicts the permeability with higher accuracy. All three studied connectivity metrics provide better permeability estimations when a larger aperture value is chosen as the cutoff threshold. Overall, this study elucidates that using connectivity metrics provides a less expensive alternative to fluid flow simulations when an estimation of fracture permeability is desired.ISSN:0309-1708ISSN:1872-965

Flow-through Drying during CO Injection into Brine-filled Natural Fractures: A Tale of Effective Normal Stress

Injecting supercritical CO2 (scCO2) into brine-filled fracture-dominated reservoirs causes brine displacement and possibly evaporite precipitations that alter the fracture space. Here, we report on isothermal laboratory experiments on scCO2-induced flow-through drying in a naturally fractured granodiorite specimen under effective normal stresses of 5-10 MPa, where two drying regimes are identified. A novel approach is developed to delineate the evolution of brine saturation and relative permeability from fluid production and differential pressure measurements. Under higher compressive stresses, the derived relative permeability curves indicate lower mobility of brine and higher mobility of the scCO2 phase. The derived fractional flow curves also suggest an increase in channelling and a decrease in brine sweep efficiencies under higher compressive stresses. Finally, lowering compressive stresses seems to promote less water evaporation. Our experimental results assist in understanding the injectivity of single fractures and eventually of fracture networks during subsurface applications that involve scCO2 injection into saline formations.ISSN:1750-5836ISSN:1878-014

Customer Interaction in Software Development: A Comparison of Software Methodologies Deployed in Namibian Software Firms

Software methodologies provide guidelines for the development of software applications. Studies reveal that customer interaction in the software development process improves the chances that software applications will meet customers' needs. Despite a number of software methodologies introduced and a comparison of these methodologies, there is a dearth of studies that empirically investigate customer interaction between these software methodologies within the Namibian context. The purpose of this study was to examine the differences in customer interaction between software methodologies deployed in Namibian software firms. The study adopted a qualitative, case study approach. Data was collected through standardized, open-ended interviews. The findings show that the methodologies deployed in Namibian software firms include the waterfall model, Scrum, iterative model, eXtreme Programming (XP), and rapid application development (RAD). The findings also reveal that although there was in-depth customer interaction in Scrum, the iterative model, XP and RAD, customer interaction in the software development process could also be challenging. The findings provide useful insights in software methodologies deployed in Namibian software firms and the experience within the Namibian context. An implication for software project managers and software developers is that customer interaction should be properly managed to ensure that the software methodologies for improving software development processes are effectively deployed