Q3 or Residual Gas Volume Changes Over Time

The objective of this project was to explore the relationship between residual gas (Q3) and lost and desorbable gas (Q1 and Q2) as a component of total desorbable gas (Qm), determine the Method Detection Limit (MDL), Practical Quantitation Limit (PQL) and uncertainty for Q3 and evaluate the change in Q3 value after an extended period of time. The role that Q3 or residual gas plays as part of the total gas content has been discussed in numerous publications. Generalised percentages of the total gas content range from 10 to 80% (Diamond et. al., Black et. al., McLennan et. al.).

A review of existing gas content data from across Eastern Australian coal seam gas and fugitive emissions

testing showed no distinct trend between Q3 as a percentage of total gas content or other sample properties found in the dataset available. There may be trends with other rock properties not available for this study. Samples from the coal seam gas data showed most of the gas content (~80%) in the Q2 portion and the rest in Q1 and Q3. Samples selected using the study requirements from the fugitive emissions data show that most of the gas content is in Q3, a small amount in Q2 and the least in Q1. This study found that a broad application of a default value for Q3 cannot be accurately applied across an area.

The report is published as two stages:

· Stage 1: Phase 1

· Stage 2: Phases 2 and 3

Phase 1 of the study showed that 5 samples retained gas contents above the Q3 detection limit even after extended periods of time (>500 days). There is a possibility that if enough pores are 'locked' and contain gas from the coalification process that they will never desorb that gas until crushed to a certain grain size to liberate it, therefore resulting in higher than detection limit gas content over time.

Phase 2 of the project was conducted to evaluate the rate at which Q3 gas content is emitted into the atmosphere.

Phase 3 of the project tested Q3 samples exposed to atmosphere for an extended period of time and compared the results to the values obtained when the first Q3 sample was tested at the end of the Q2 period. Most samples had a reduced Q3 value over time.