Disposable Fibre Optic System: Tiny Downhole Behind Casing Leak Path Detection in a Semi-Urban Environment

A suspended coal seam gas well in NSW, Australia was exhibiting sustained casing pressure which built to 20PSI over 8 weeks. To diagnose the pressure source and communication path fibre optic sensing was chosen as the complete section of interest could be surveyed simultaneously while surface pressure manipulations increased leak activity. Due to proximity of a train line and highway to the well, an overnight DAS well survey was preferred to reduce ambient noise. Consequently, a disposable fibre optic system was selected which could be deployed and logged to comply with overnight well pad noise restrictions.

The application identified a consistent band of acoustic energy from a potential pressure source within a sandstone formation. An acoustic response was observed at the same depth during surface casing annulus pressure manipulation, indicating behind pipe communication from the formation to surface. In addition, inside pipe fluid movement was observed originating from a newly set cement plug. The case study demonstrates the utility of DAS surveys for land well abandonment activities, using a disposable bare fibre optic system to reduce mobilisation footprint, cost and environmental impact.