The production of coal seam gas (CSG) commonly requires the extraction of groundwater from the target coals. The regulatory framework in Queensland encourages the highest value beneficial reuse of this water. Origin Energy, on behalf of Australia Pacific LNG, operates the Reedy Creek, Combabula and Ramyard gas fields in the Surat Basin, a sub-basin of the Great Artesian Basin. The gas fields currently comprise 800 CSG wells, extracting a combined volume of 14 megalitres of water per day. A borefield consisting of 11 bores was commissioned in early 2015 to inject treated CSG water into the Precipice Sandstone aquifer, at a depth of approximately 1,400 meters below ground level. To date over 21 gigalitres of treated CSG water has been injected, supporting 460+ terajoule per day of gas production.
Observed performance loss in some injection bores, and ongoing expansion of the fields, necessitated an increase in injection capacity. Under reaming of existing injectors, using an expandable drill bit, was identified as a potentially cost-effective means of increasing capacity. A trial of under reaming was undertaken on two injection bores, with the premise that reducing well losses by (i) increasing the diameter at the sandface and (ii) removal of clogging products in the near wellbore aquifer, would increase bore performance.
The trial was undertaken as follows:
Bore capacities increased by more than 100% from the pre-trial to post-trial performance. A simple analytical model was built and parameterised using trial data that allowed prediction of injection performance improvements at different under reamed diameters. The model was incorporated into a water management options assessment.
Based on the performance-enhancement and cost-effectiveness of the trial, under reaming of injection bores has been identified as a successful means of increasing capacity, ensuring the injection scheme continues to return all of the treated CSG water from the Reedy Creek, Combabula and Ramyard gas fields to the GAB. Furthermore, allowing for multiple workovers and recoveries on each injector will increase the longevity of the existing injection bores and increase capacity each time under reaming is undertaken.