Abstract
One of the greatest challenges with respect to coal seam gas (CSG) wells in Queensland, Australia is the many loss zones encountered during drilling and cementing operations. Typical directional wells in the area are up to 2000 m measured depth (MD) and 1200 m true vertical depth (TVD). In some instances production enhancement by fracture stimulation is required and therefore high-strength cement is necessary while maintaining the lowest possible pressure on the formation and natural fractures.
A trial was setup on a directional well pad where various methods of returning cement to surface were attempted. All three wells had similar well design, expected losses, and drilling times. The first well was unsuccessful and no cement was returned back inside the previous surface casing shoe. With the addition of a reactive spacer and more excess volume, cement inside the previous casing shoe was achieved on the second well. Both wells used standard 12-lbm/gal slurries. The third and final well on the pad required a step-change in the cement job design to achieve cement to surface.
The most successful cementing job was achieved on the third well by redesigning the slurry to a lower density of 11 lbm/gal, without compromising set times, thixotropic properties, and high compressive strength. To improve equivalent circulating densities (ECD), the new slurry was designed with lower rheology. The slurry's low solids-to-water ratio (SWR) and large slurry volume, which precluded batch mixing, required the job to be mixed and pumped on-the-fly using an automated volumetric mixing system rather than using density mixing.
This paper discusses the job preparation and technical details involving how the 11-lbm/gal cement slurry achieved a successful production casing cement job with returns to surface. With similar successes demonstrated on subsequent wells, this case history set a new standard in the field.
