Manuscript Title: Characterization of Microannuli at the Cement-Casing Interface: Development of Methodology

Abstract

Formation of microannuli at the interface of cement-casing can create well integrity issues. X-ray CT and Optical microscopy are technological trends that may have potential for direct visualization of microannuli. CT has an advantage of providing non-destructive visualization of microannuli, but its resolution suffers with increase in casing thickness. Conversely, Optical microscopy has the potential of providing higher resolution needed to detect smaller sized microannuli; however, information about microannuli is limited to only a few sections where samples have been sliced. The objective of the current article is to describe a methodology to examine the interface of cement-casing. Experimental work was combined with literature review. This includes both direct visualization methods, evaluation of current trends to better understand the characteristics and geometric variation of relevant leakage paths. We generate test specimens consisting of cement plugs, various steel casing thickness and nano-coated aluminium casings. Hydraulic sealability tests were conducted by injecting water at the cement-casing interface. Flow rates are then interpreted in terms of microannuli aperture and direct visualization of the cement plug-casing interface by CT and Optical microscopy was implemented. The experimental findings of this article will form a basis for studying geometry and size of microannuli as well as modelling of fluid migration.

Squeeze cementing of micro-annuli: A visco-plastic invasion flow

“…These experiments are typically conducted on scaled-down sections of wells and aim to recreate downhole conditions such as temperature and pressure cycling. Computer tomography (CT) has been used to determine the size of leakage pathways [10][11][12][13], showing that microannulus geometries are highly variable in nature. A complex network of pathways with non-uniform thickness were observed in their samples with connectivity of pathways in the axial direction.…”

Investigation of the Hydraulic Integrity of Cement Plug: Oilwell Cementitious Materials

“…It should be noted that the microannuli aperture values reported in the study are based on the cubic law, which assumes that the microannuli is constant and uniformly distributed along the cement plugcasing interface. This is a simplification of the true behaviour of microannuli which has been reported to be non-smooth and randomly distributed along the cement-casing interface (Ogienagbon et al, 2021).…”

“…While shear bond strength refers to the minimum force required to initiate shear failure at the interface and displace the cement sheath along the cement-casing and cement-formation interface, hydraulic bond implies the maximum force required to cause debonding and permit the leakage of fluids through the bonded interface. Hydraulic bond strength is typically determined by continuously injecting fluid at the cement-casing or cement-formation interface until tensile failure occurs and fluid breaks through (Khalifeh et al, 2018a;Ogienagbon et al, 2021). It is important to note that shear bond failure does not necessarily lead to loss of hydraulic integrity.…”