Successful Zonal Isolation of Liner and Casing in Severely Depleted Formations by Utilizing Hollow Light Beads Cement Formulation
Authors: Anggarawinata Rama , Faizol Muhammad , Loh Mavis 
Published: October 2024

In its recent infill drilling campaign, an operator offshore Malaysia was facing challenges to isolate the shallowest hydrocarbon zone during the Production Liner and Casing cementing operations. This challenge is mainly caused by the severely depleted reservoir section resulting from existing wells production and a naturally weak formation. Cementing across this formation requires cement slurry optimization particularly in terms of Density and Rheology. The primary goal of the cement job is to ensure that the Top of Cement (TOC) reaches the minimum required height and provides a sustainable barrier for zonal isolation downhole. This can only be achieved by managing the Equivalent Circulating Density (ECD) effect to be below the fracture gradient which indirectly avoids losses during cementing operations.

Based on the predicted fracture gradient and pore pressure, optimization on mud weight against borehole stability requirement, casing design, casing shoe depth and hole size was made and based on the engineering simulation and sensitivity analysis, it is important to consider the overall properties, pumping procedure, and effect of mud-spacer-cement to mitigate the risk of losses during cementing operations and achieve the desired TOC. From the desktop engineering simulation and lab analysis of few formulations’ options of normal G-cement, proprietary blend and spacers, the team has opted to include a loss-circulation spacer pill to be pumped ahead followed by a tailored high strength low density cement design. Throughout this article, the hollow light beads will be referred as high strength low density beads. This approach reduces the overall fluid (spacer & cement) downhole ECD, maintains the rheological hierarchy compliance among all fluids, improving mud displacement efficiency, encourages good cement bond with the formation, and helps to achieve the required minimum cement compressive strength for future well perforations scope. Post-cement job condition testing was also conducted in the laboratory including sedimentation test and survivability (%) between crushed (popped) density of cement slurry with high strength low density beads against increasing pressure exposure test to ensure no segregation of particles takes place after the cement sample hardens, by measuring the static stability density in several specimens, with multiple pressure exposures versus density performance conducted which aim to validate the survivability of high strength low density beads cement particles, simulating downhole conditions upon cement placement.

The need to introduce high strength low density beads particles into the cement formulation option is driven purely due to lower Specific Gravity (SG) than that of fresh water. The beads type, size and usage are also designed specifically based on its pressure rating (ability to withstand up to the rated pressure before crushed and later causing the slurry density to be higher than the design intent). High strength low density beads bulk blend cement transfer requires extra handling precautions due to component size and material weight. Excessive pneumatic pressure during transfer operations may cause overproduction of dust through vent lines creating an inhomogeneous blend that could potentially impact transfer consequently leading to partial or full blockage of transfer lines. During cementing operations, when the cement slurry is mixed and pumped downhole and circulated up the annulus for the final slurry displacement, the high strength low density beads particles will exhibit higher rheology readings (more viscous) due to the particles inside the slurry. It is vital that the high strength low density beads’ crush point be verified against the maximum pressure exposure to keep the surface and downhole densities constant as well as hydrostatic pressure and ECD profile residing within the formation fracture gradient limit.

The two 7" production liners and one 9-5/8" production casing cement jobs utilized 10.5ppg loss circulation spacer pills and 11.0 ppg gas tight properties of high strength low density beads cement design successfully and stably mixed and pumped ‘on the fly’ across severely depleted formation(s) and achieved good zonal isolation(s) above the shallowest hydrocarbon zone as verified with a Cement Bond Log run within 10 days after slurry placement. This success story has instilled the confidence for the operator to adopt similar technical concepts, basis of design, planning and execution for future wells having similar challenges of cementing across depleted formations.