Hydrocarbon extraction from coal seam gas (CSG) reservoirs relies on horizontal drilling and completions. However, the drilling of horizontal wells usually faces challenges as inappropriate equivalent circulating density (ECD) can induce borehole collapse. Hence, the optimisation of drilling parameters (e.g., well azimuth, ECD) is paramount to successful well delivery. In this paper, an implicit numerical model is developed to obtain the safe drilling pressure for borehole collapse. The model uses Mogi-Coulomb criterion and Shi and Durucan stress model for depleted coals. Case studies are conducted based on samples from Permian Baralaba Coal Measures of the Bowen Basin and Jurassic Walloons Coal Measures of the Surat Basin.

The results first show that the horizontal well is prone to have wellbore collapse issues as the minimum ECD is the highest compared to other cases with smaller inclinations. Then cases with different bulk mechanical properties from Bowen and Surat basins are conducted and analysed. The results suggest that samples from Jurassic Walloons Coal Measures are less sensitive to borehole collapse, which are in good agreement with caliper and ECD data of horizontal drilling observations. The developed numerical model is useful for optimising well drilling in locations with different bulk mechanical properties and can ultimately improve well delivery and economic gas extraction.

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