Achieving Zonal Isolation Using Three-Stage Cementing Systems (An Australian CSG First)

Abstract Regulatory requirements for Coal Seam Gas (CSG) wells in Queensland, Australia dictate that all surface casing strings must be cemented from shoe to surface and all aquifers must be isolated by casing and cement to ensure zonal isolation. A CSG operator drilled a well at an altitude of approximately 500 meters above sea level (m ASL). This altitude is considered high for the area and led to a hydrostatic overbalance across the exposed, permeable Precipice aquifer. The overbalance caused severe to total losses of drilling fluids during the drilling phase. A wide range of conventional lost circulation materials (LCM), light-weight cement formulations and two-stage cement operations were used to mitigate the lost circulation during cementing with limited success. A three-stage cement job was designed for the 9 5/8 in. (244.5 mm) casing inside 12 1/4 in. (311.1 mm) drilled hole to bring cement around the shoe up to surface across the permeable formations. The casing program utilised 9 5/8 in. (244.5 mm) inflatable casing annulus packers, compatible hydraulic and mechanical stage tools and standard float equipment to create a cement base and cementing flow paths to perform the cementing operations. This paper will present actual case histories where a three-stage cementing system has been deployed in CSG applications. Proper pre-job planning, detailed procedures and careful attention during on-site execution were key to the success of the multi-stage operation. By providing a custom solution, the objectives of the operator, the regulatory agencies and the well contractors were met, where other conventional methods had not been successful. We understand this was the first time this application has been applied to Australian CSG wells.