The role of spatial variability in coal seam parameters on gas outburst behaviour during coal mining

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Abstract

Gas outburst is recognized as a potentially fatal hazard to be managed during the mining of gassy coal seams. Gas outbursts in Australian coal mines have been associated with the presence of geological structures in the coal and surrounding rocks, having a range of spatial scales from millimetres to metres. The contributing mechanisms are influenced by coal gas reservoir variables including gas composition, fluid pressure, desorption pressure and rate, porosity, intrinsic permeability and relative permeability. They are also influenced by geomechanical variables such as strength, in situ stress and mining-induced stress.

The work described here concerns the influence of variability of permeability and strength on outburst behaviour. Key coal properties are characterised for a field site in an operational underground coal mine and then used in a series of hypothetical simulations of outburst initiation.

At the field site an array of horizontal holes was cored in-seam. Well tests for permeability and stress were performed in the holes and recovered core was tested in the laboratory for permeability, strength and sorption properties. For permeability and strength sufficient numbers of measurements were obtained to describe these properties statistically assuming that the measurements are uncorrelated spatially. Analysis of the cumulative probability distributions of the field and laboratory permeability data shows a strong size effect that cannot be linearly up-scaled.

Using Monte Carlo techniques, these statistical descriptions are used to generate realisations of property values across the model grid used in the simulation analyses. The stochastic model approach demonstrates that the variability of the permeability and strength fields can lead to both outburst and non-outburst outcomes from the same measured input data, depending on the corresponding spatial distribution of permeability and strength at the face. These results suggest the potential application of this approach as a tool for outburst risk analysis and assessment.

Introduction

Gas outburst is recognized worldwide as a potentially fatal hazard to be managed during the mining of gassy coal seams. Outbursts are dynamic, energetic events which may result in the projection of fragmented coal and rapid release of gases from the working face. In Australian experience the gaseous components are primarily CH4 or CO2 or mixtures of both. Coal masses greater than 400 t have been displaced and gas volumes released have been estimated at up to 18,000 m3 at atmospheric pressure (Hargraves, 1958, Hargraves, 1983, Lama and Bodziony, 1996). Outburst cavities can vary from small cone shaped cavities of typical dimension 1 m, with well defined boundaries, to large rubble filled zones of the order of 20 m across.

Outbursts are driven mainly by gas pressure (Gray, 1980, Paterson, 1986, Paterson, 1990). For small events, initial gas pressure may violently accelerate and project fractured coal from the face apparently instantaneously. For large events the rate and duration of gas release may be sufficient to entrain and transport fractured coal over distances of 20–40 m, over a time of 10–20 s.

A majority of the gas outbursts that have occurred in Australian underground coal mines have been associated with the presence of geological structures in the coal and surrounding rocks, having a range of spatial scales from millimetres to metres. In the Bulli seam of the Southern Coalfields, NSW, the vast majority of outbursts events have been associated with geological structures within 2.5 m of workings (Lama, 1995) — primarily faults, shear zones and igneous intrusions, with characteristic dimensions greater at least than the prevailing seam thickness. Reverse and strike slip faults have historically been outburst prone in both the Southern Coalfields and the Bowen Basin, Queensland (Shepherd and Rixon, 1983). In the Gemini seam, Bowen Basin, coal cleat structures and mining-induced cleavage fractures from a length scale of seam thickness down to 3 mm were associated with outburst events (Gray, 1980, Hanes and Shepherd, 1981).

The mechanisms that contribute to outburst risk are influenced by coal gas reservoir variables which include gas composition, fluid pressure, desorption pressure and rate, porosity, intrinsic permeability and relative permeability. They are also influenced by geomechanical variables such as strength, in situ stress and mining-induced stress concentrations. Recent advances in computer modelling now enable some quantitative simulations of outburst events, including the contributions of geological structures to the physical mechanisms (Wold and Choi, 1999, Wold and Choi, 2001, Choi and Wold, 2001a, Choi and Wold, 2001b, Choi and Wold, 2002, Choi and Wold, 2003). The approach adopted initially was to couple a geomechanical model and a CBM reservoir model to simulate conditions of outburst initiation. A schematic of this modelling approach is given in Fig. 1. Subsequently this was extended by coupling with damage mechanics and particle fluid dynamics models, focussing on the likely volume of coal that can be expelled and the volume of gas that can be emitted, taking into account the influence of gas composition (schematic, Fig. 2).

Experience with the initial outburst model assuming homogeneous uniform coal showed that the likelihood of outburst is sensitive to the values of the permeability and strength under certain critical reservoir and geomechanical conditions. Permeability plays a key role in the formation of the pressure gradients within the coal that can lead to failure, while coal strength acts to resist this failure process. Under in situ conditions, these key properties can vary significantly over short distances, undergoing step changes often associated with presence of features such as bright and dull bands, cleat and fractures at various length scales, and mineralisation in the fractures. While this variation is complex, and can operate over a range of spatial scales, for modelling purposes the average reservoir properties must be estimated over the size of a grid block. A frequently encountered question is the relationship of core measurements to the typically much larger grid blocks. Well testing to determine permeability does provide estimates of this property at the larger scales required for reservoir simulation purposes. However these are difficult to perform, particularly in mining environments and core scale measurements may represent the only data available.

The work described here was undertaken to investigate the influence of variability of permeability and strength on outburst behaviour, by providing measurements as input to the outburst computational model (Wold et al., 2006). This involved intensive characterisation of key coal properties at a field site in an operational underground coal mine and then the use of this information in a series of hypothetical simulations investigating outburst behaviour for various case studies. At the field site an array of horizontal holes was cored in-seam. Well tests for permeability and stress were performed in the holes and recovered core was tested in the laboratory for permeability, strength and sorption properties. For permeability and strength sufficient numbers of measurements were obtained to describe these properties statistically assuming that the measurements were uncorrelated spatially. This statistical description was then used to generate realisations of property values across the model grid used in the simulation analyses. The combined measurements of permeability through core samples and well testing allows the relationship between these two scales of measurement to be considered.

Section snippets

Well tests for permeability and stress

Permeability in coal is sensitive to the effective stress, decreasing as the effective stress increases. Therefore in situ permeability characterisation also involves characterization of the in situ stress. Well testing for permeability and stress is widely used in the petroleum industry for reservoir evaluation and fluid production forecasting, generally in surface-drilled vertical wells (Matthews and Russell, 1967). These methods were adapted for well testing from an underground coal mine

Permeability

Seven well tests for permeability were completed. Four of these were analysed as single well tests, where no pressure response was observed in the monitoring wells within the allowed test duration. For three tests a significant pressure response was observed in the monitoring wells, in addition to that in the injection interval. For these tests the monitoring well pressure behaviour was used with that observed in the injection interval to determine the formation hydraulic properties.

Because of

Core permeability tests

In comparison with field measurements, values from laboratory tests on coal core are generally subject to the influences of core size, recovery and orientation. The need to use intact core for the laboratory tests tends to bias the test outcomes towards lower permeabilities and higher strengths compared to larger scale field tests, because major structures and discontinuities such as fractures, shear zones, changes in lithology, etc., are generally not represented in the intact core.

Statistical analysis of test results

Within the coal seam permeability and strength could be expected to be highly heterogeneous but spatially correlated. In addition, since coal seams have a highly layered structure, the vertical nature of this spatial correlation could be complex and it is likely that the correlation will be non-stationary. The relatively small number of permeability measurements means that it was not possible to realistically interpret this complex spatial correlation behaviour at the field site. The approach

Mechanistic-stochastic outburst modelling

For the purpose of the numerical modelling presented here, an outburst is considered to comprise three distinct stages: pre-initiation, initiation, and post-initiation (or outburst evolution). During the pre-initiation stage, deformation of the yielded or failed material occurs in a quasi-static manner. This can occur under many mining conditions when the level of stress exceeds the strength of the coal, and is not unique to outbursts. Initiation refers to the moment in time when the

Discussion and conclusions

Coal can be considered to be a naturally fractured reservoir where the permeability is governed by the fluid conductivity of the cleat, and of other structures present on a broader length scale. The potential for use of coal strength and permeability data in gas outburst risk assessment is demonstrated using an integrated field, laboratory and numerical modelling approach. The natural variability of strength and permeability was targeted for measurement and/or assessment at both field and

Acknowledgements

The authors gratefully acknowledge ACARP, CSIRO Petroleum, and BHP Billiton Illawarra Coal for their financial and logistical support of this work.

They are indebted to their colleagues, Messrs. Tim Ferguson, Michael Camilleri, Leo Connelly and Greg Lupton, for their expert assistance with equipment development and physical measurements.

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    Present Address: 14 Foam St., Parkdale, Victoria, 3195 Australia.

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