Flexible Cement Extends Wellbore Life with an Integrated Approach to Zonal Isolation

Abstract

Sustained casing pressure (SCP) is a major risk encountered in most wells in India, particularly in developmental oil and gas fields situated in the northwest area of the country. The inability of conventional cement systems to withstand cumulative stresses from completion and production phases might be one of the primary reasons for the failure of expected barriers during zonal isolation, which can lead to SCP. This case study reviews a flexible cement system used in an oil and gas field located in northwestern India, including extensive laboratory evaluation, operational design, execution, and well evaluation. The field requirement was to design a gas-tight, lightweight slurry for a production zone to treat losses and address issues of SCP. A finite element analysis (FEA) structural simulation was performed, which modeled the lifetime operation of the well. Including elastomeric and tensile strength enhancement materials in the cement system was recommended based on results from structural simulation to reduce risks of SCP. Incorporating these materials can enhance a cement system's mechanical properties (i.e., increase Poisson's ratio and tensile strength and decrease Young's modulus). The slurry was designed with cement, hollow spheres, lost circulation material (LCM), a tensile strength enhancer, and elastomeric material; the cement slurry was batch mixed to ensure homogeneity. The case study examines the impact of the fluid rheological hierarchy, pump rates, and multiple bottom plugs on cement slurry displacement efficiency using computational fluid dynamics (CFD). A finite-difference, three-dimensional (3D) displacement simulator was used to predict the flow behavior of fluids, both inside and outside the casing, throughout the operation using actual operational parameters. Based on two-dimensional (2D) hydraulics analysis, various factors, including rheology, pumping rates, and fluid density, were adjusted to maintain equivalent circulating densities (ECDs) below the fracture pressure to help avoid losses. A tailored cement design, which created a stable gas-tight resilient cement system, helped minimize risks to production by achieving effective zonal isolation. This was confirmed through a post-operation review. The cement bond log, including a microseismogram and circumferential visualization, indicated that the cement was adequately placed around the casing. The increase in the cement system tensile strength helped ensure wellbore integrity, even in high-stress/strain load cycle production environments. Cementing operation success was confirmed by excellent cement bond logs and zero SCP observed between the production and surface well casing annulus. The life-of-the-well tool is an engineered, proactive, and interventionless zonal isolation solution to help extend well economic life, thus preserving production while reducing or even eliminating costly remediation (Ravi et al. 2002).