Cementing and evaluation of the cement sheath bond to casing and formation of depleted reservoirs drilled with highly inclined well trajectories poses numerous challenges. Starting from the risk of cement slurry not reaching the desired depth and insufficient circumferential coverage, to potential induction of microannuli due to several changes in drilling and completion mud weight to counter narrow fracture gradient. This paper reviews the best cementing and bonding evaluation practices under such circumstances applied in a well in offshore Sabah, Malaysia.
The narrow window between pore pressure and fracture gradient, with an uncalibrated depletion coefficient, made it extremely challenging to design a suitable cement job. Multiple simulated scenarios, followed by a carefully designed borehole strengthening static squeeze provided the framework to design the 14.5 ppg fit for purpose slurry with an expanding additive to eliminate shrinkage during cement hydration. Extensive cement bond evaluation was conducted to ensure the bond quality was understood accurately. As subsea wellhead ruled out the possibility of a pressure pass to detect any possible liquid-filled microannulus, a new shear and flexural wave cement evaluation tool was used to achieve the objective without a pressure pass.
The optimized pumping and displacement rates based on the model ensured the cementing ECD remained below the fracture gradient. Additionally, base oil was pumped ahead of the spacer to manage ECDs and to condition the mud prior to cementing. To achieve the desired casing centralization and effective mud removal in the near horizontal section, the casing was run with 2 centralizers per joint. Finally, the cement bond evaluation was conducted after 99 days of the cement job to allow the cement to cure. The cement sheath bond data was acquired using a new wireline pad-type cement bond tool utilizing electro-magnetic acoustic transducers (EMAT) to induce shear and flexural waves in the casing. These data were analyzed to determine cement bond and liquid-filled microannuli together in a single pass. Unlike compressional waves (used by standard CBL tools), shear waves only attenuate in the presence of solids behind the casing enabling a clear determination of cement/solids bonding to the casing. This technology enables solids to be detected even in the presence of liquid-filled microannuli by analyzing the shear and the flexural wave simultaneously. This integrated strategy helped to establish the cement design, execution and provided confidence on zonal isolation with comprehensive evaluation.
This solution showed importance of integrated approach of cementing and bond evaluation at all stages of planning and operation under such challenging condition. This paper also emphasizes the versatility of the EMAT based technology to address different cement sheath evaluation related problems faced by operators in the industry.