Experimental and Numerical Determination of Cement Casing Microannulus
Authors: Abid Khizar , Lambrescu Ionut , Teodoriu Catalin 
Published: September 2023

ABSTRACT
The key to successful long term well integrity of cement-casing is to understand and investigate the hydraulic sealing of cementing annulus by detecting an existing or potential fluid movement within the casing-cement or -formation zone after the cement job. The objective of this study is to measure the micro-annulus at the casing-cement interface and produce more accurate identification and prediction by exploring the internal structure under high temperature reservoir. The methodology is using experimental and numerical approaches to estimate the potential gap size with direct applications to geothermal wells since both debonding types are investigated (type I tensile and type II shear). Coexistence of tensile and shear debonding is more likely to appear in Geothermal wells due to casing expansion. The research findings of this approach to investigate and predict cement-casing micro-annulus initiation and failure prediction have shown a higher sensitivity to determine the onset of failure paths. The investigation was able to show a clear gap identified between cement and casing. These findings will have a large influence on the well integrity assessment and prediction.

INTRODUCTION
Well integrity often refers to micro annulus as the main failure mode; thus, its importance has increased over time. The main causes for micro annulus forming are casing-cement debonding and/or pressure penetrations across and existing micro annulus.
Cementing operation is one of the most critical processes that ensure the integrity of the well. In this process, the cement is injected in the annulus space between the casing and the formation and allowed to be set for a specific time. A good cementing job improves the integrity of the well by avoiding the sealing failure of the well bore, restricting the channeling of the formation fluid, isolating the troublesome subsurface formation, and protecting the casing from corrosion and other failures (Xiang, et al. 2021, Xu, et al. 2018, Zhang, et al. 2022). The sealability of the cement can be defined as the capability of the cement to prevent and seal the movement of the fluid through its matrix. Therefore, it is of utmost importance that the cement sheath maintains its structural integrity during the life of the well. However, micro annuls in the cement sheath can be detrimental to the cement matrix, which happens when the cement is placed in the annulus, and the casing is under pressure. When cement sets and the pressure on the casing is released, it might detach itself from the cement and create space between the cement and the casing. The micro annulus can also exist when the pipe is extensively polished, so the cement easily debonds from the casing surface. While the channeling in the cement sheath can also occur when the cement is not placed properly in the well bore and does not completely cover the pipe's outer surface radially, or the cement's bonding with the casing is weak. This channeling will lead to the movement of fluid (oil, gas, water) toward the surface.