A trial was conducted by Origin Energy to study the feasibility of re-injecting produced coal seam gas (CSG) also known as coal bed methane (CBM) back into depleted coal formations. The trial specifically set out to identify any practical limitations of re-injecting gas as a means of keeping wells online during downstream process interruptions. This included identifying any changes to coal permeability, evidence of gas re-adsorption to the coal and operational learnings with the injection system. The trial met its objectives in that analysis indicated a high likelihood of injected gas re-adsorbing to the coal matrix, and there were no significant operational issues.
The trial was conducted using two injection wells converted from production wells in a depleted, high permeability area of an Australia Pacific LNG gas field operated by Origin in Queensland, Australia. Injection periods were conducted at different rates, with data recorded from injection wells and offset wells. Analysis included pressure transient analysis (pressure build-up and injection fall-off tests), numerical simulation modelling, nodal analysis, and material balance modelling.
Gas injected into Well 1 appeared to migrate away from the well, with an increase in production rate observed at an offset production well. Interpretation of down-hole pressure data indicated likelihood that Well 2 was injecting into a closed reservoir compartment. Numerical simulation modelling indicated a high likelihood that injected gas was re-adsorbing to the coal in this area. Numerical simulation, nodal analysis and material balance modelling were used to estimate maximum injection rates and storage volumes for further injection. Groundwater monitoring bores were also monitored for changes in pressure and water chemistry, to validate containment of injected gas within the coal reservoir.