During a coal seam gas drilling operation in the Surat Basin, the well began to flow when the 9-5/8” casing was being run to approximately 400 m into the Hutton sandstone formation.
The 13-3/8” casing was set above the Hutton sandstone formation at approximately 115 m and cemented to surface. An annulus capacity of approximately 300 m deep between the 13-3/8”x9-5/8” casings was opened across the Hutton formation.
The well bore fluids overpowered the positive mud pump and flowed into the mud tanks. The mud tanks were overfilled and fluids spilled to the cellar area.
The 9-5/8” surface casing was pulled to set inside the 13-3/8” casing shoe and mud weighted up to pump into the hole to dynamically kill the well.
Sampling confirmed that the fluid was water and contained no hydrocarbons.
Key issues
The drilling program did not:
- adequately assess the hazard of pressurised shallow formation in the Surat Basin coal seam gas producing areas
- monitor annulus pressure and possible communication of well bores opened to surface
- install Pressure Control Devices or Diverters after the installation of the surface casing
- install suitable surface gas handling equipment and mud gas separator to circulate the influx from well bore safely in a well control situation.
Recommendations
The petroleum well operator must carry out suitable assessment to:
- determine the risk of pressurised gas-prone shallow formations in Surat Basin coal seam gas producing areas
- install Pressure Control Devices or Diverters when a reasonable potential of uncontrolled flow of well bore fluids to surface is present for all operations after the installation of the surface casing terminating once all hydrocarbon and aquifer zones are isolated
- verify the availability of suitable surface gas handling equipment mud gas separator design to circulate the influx from the well bore safely in a well control situation.
