Numerical Computer Modelling and an Innovative Spacer-Cement Train Results in Successful Zonal Isolation in Critical Well Offshore Australia with Minimal Casing Centralisation

Abstract

Challenging wellbore conditions in an offshore well in the Northern Territory, Australia led an Operator and Cementing Service Company to pursue a sophisticated approach to achieve zonal isolation on the critical 9–5/8-in. casing cement job. Some of the specific well challenges prior to and during the cementing operations included high bottomhole temperatures of 135°C at 3,724 m, lost circulation, low fracture gradients, and over-pressured source rock (gas). In particular wellbore instability due to tectonic stresses made it difficult to run casings to bottom, so centralizers were used only on the shoe track. The potentially increased casing eccentricity was a major concern during the cement job, risking channeling and resulting in insufficient zonal isolation. Consequently, a comprehensive study focused on enhanced mud removal and cement placement to compensate the lack of proper pipe centralization and pipe movement during the job. Mud displacement and slurry placement efficiency were investigated as functions of important spacer and slurry design features as well as the given wellbore parameters (such as fluid density and friction hierarchies, gel strength, pump rates, fluid expansion and buoyancy, eccentricity, and temperatures) using computational modelling. The simulations and findings enabled optimization of fluid designs with the corresponding job execution increasing the success for zonal isolation. Proper pre-job planning, cementing program simulations, fluid lab tests, and reliable job execution were performed to achieve zonal isolation for the overall integrity of sidetracked wellbore section. Post-job analyses of the cement jobs, lab test results, pressure tests, and azimuthal ultrasonic cement bond log results were evaluated concluding sufficient cement coverage along the desired annuli, meeting the operator's requirements to demonstrate zonal isolation. This paper reviews the job performance and lessons-learned from the 9–5/8-in. casing cement job on this well. The findings contain useful cementing guidelines for critical well scenarios where optimum pipe centralization and movement are not feasible.