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Application of Gas Content Test Data to the Evaluation of Gas Emission and Hazards During Longwall Development

and Extraction

R. J. Williams GeoGAS Systems Pty. Ltd.

Abstract

This paper compares aspects of fast and slow desorption methods and attempts to provide guidelines for their application and associated sampling strategies.

The importance of determining Total Desorbable Gas Content (Q1+Q2+Q3) as opposed to Desorbable Gas Content (Q1+Q2) is highlighted. The former involves partial destruction of bore core, but provides considerably more accurate data. It is also more amenable for use in gas reservoir simulation modelling.

Fast desorption techniques are shown to have considerable advantages over slow desorption techniques. These are, greater accuracy in gas composition assessment, the potential to rationalise sampling, fast turnaround involving fewer resources, finalization of geological logs and early application of the data. Q3 can still be differentiated. A more sensitive indicator of gas desorption rate is incorporated in GeoGAS’s fast desorption method (GeoGAS Desorption Rate Index).

Gas reservoir size determination is facilitated by a sampling strategy where relationships between gas content and gas composition with depth and mineral matter are defined. This enables indirect assignment of gas content to those gas bearing strata not directly tested. The importance of testing/assigning gas contents to inferior coaly horizons is indicated. Gas content test results are basic input into any mining application. An overview of its application to modelling gas emission is given. Statistical analysis of the data enables modelling inputs and outputs to be expressed as means and probability distributions. Introduction

Gas content testing has become a routine part of most surface borehole exploration programs for underground coal mining. The gas content test data are basic input into costed approaches to gas, ventilation and spontaneous combustion control.

安全工程专业外语翻译

With the development over the past three years of fast desorption methods of gas content testing, the exploration geologist is confronted with additional choice of method and sampling strategy.

This paper compares aspects of fast and slow desorption methods and attempts to provide guidelines for their application and associated sampling strategies.

The most important of determinant of methodology is how the data will be used. Application to longwall development and extraction is broadly covered.

Gas Content Testing

Terminology

The following terminology is used in this paper. It should generally be accepted across the industry.

Total Desorbable Gas Content (TDGC) - the sum of Lost Gas (Q1), Desorbed Gas (Q2) and Residual Gas (Q3)

Lost Gas (Q1) - Gas lost from the sample between coring and sealing in a gas canister. Desorbed Gas - The gas desorbed (per unit mass) from an uncrushed coal sample in the time between lost gas testing and crushing of the coal. The term is applied to fast desorption testing and is not the same as Q2.

Q2 - The gas desorbed (per unit mass) from an uncrushed coal sample held within a seam gas atmosphere, to the point in time where the partial pressure of the gases in the gas bomb is in equilibrium with the remaining gas in the core. The ambient pressure is approximately 1 atmosphere.

GeoGAS’s fast desorption method determines Q2 by difference by subtracting the separately determined Q3 value from the initial desorbed gas, plus the gas on crushing.

Q2 = (“desorbed gas” + “gas on crushing”) - Q3

Gas on Crushing - that gas released during crushing of the coal sample, at ambient pressure. Applies to fast desorption testing. The definition is not the same as Q3.

Residual Gas (Q3) - The volume of gas per unit mass desorbed at atmospheric pressure from the crushed coal sample after it has been allowed to desorb to its equilibrium gas content level in a seam gas atmosphere.

Desorbable Gas Content - the sum of Q1 and Q2.

GeoGAS DRI - A measure of the rate of gas desorption during crushing of the coal sample, corrected to the TDGC of the sample. (The gas volume generated after 30 seconds of crushing a 200 g sample corrected from the “Gas on Crushing” value to the TDGC value).

安全工程专业外语翻译

While AS 3980-1991 specifies reporting results to STP (0°C, 101.3 KPa), it is common practice in the industry to report results to 20°C and 101.3 KPa.

A Comparison of Fast and Slow Desorption Methods

Australian standard AS 3980-1991, Guide to the determination of the desorbable gas content of coal seams was developed to address the need to adopt a more uniform approach to gas content testing. Since then (1991), a number of limitations and deficiencies have been identified, to the point where a new standard, incorporating fast desorption techniques, is currently being developed.

Traditionally, gas content testing has been undertaken by the slow desorption method (USBM), with testing being mainly confined to Desorbable Gas Content Q1+Q2 determinations. With the emergence of the fast desorption methods and associated review of these and slow desorption methods (as in Working Group MN/1/5/3 - Standards Australia), limitations of these methods have been more clearly recognized.

The comments and comparisons that follow refer only to GeoGAS’s method of fast desorption testing and experiences in its application. For a more complete view, the interested reader should contact other organizations who have developed fast desorption methods (ACIRL, BHP, CSIRO, KCC, Lunagas).

The fast and slow desorption methods primarily differ in a number of ways, among the most important being destruction of the bore core during the test, the time taken to …… 此处隐藏：28900字，全部文档内容请下载后查看。喜欢就下载吧 ……