ABSTRACT:

Failure at the cement-casing microannulus is thought to be one of the most significant mechanisms that compromises wellbore integrity and can result in leakage of CO2 and other fluids. In this work, we developed an experimental system that enables the investigation of the initiation of a microannulus and fluid flow behavior along the interfacial pathways under in-situ conditions. The cement was cured inside a casing for seven days under downhole conditions. Without depressurization or cooling of the coupled cement-casing system, the initiation of the microannulus was studied by injecting fluids in one end, and the permeability was measured under different injection pressures. Strain gauges were mounted on the outer surface of the casing, which could provide key information for microannulus opening and closure. Results show that compared with water, liquid CO2 more easily initiates the microannulus, but the permeability of the microannulus is around 0.2 mD, which is much smaller than that created by water of 10 mD. An exponential function was used to fit the relationship between fluid pressure and microannlus permeability. Finally, the data has been implemented into a finite element simulator, and we didn't find any leakge of CO2 in 20 years using 0.2 mD microannulus permeability. Overall, cement hydration under downhole conditions generates a cement that is closer to field settings, and our experimental systems allows measurement of fluid flow in the casing-cement microannulus that more realistically reflects the field.

1. Introduction

A wellbore is a channel that is mainly used for the extraction of fossil energy, and it is also used as a pathway to inject carbon dioxide into a subsurface storage site. Well designs usually involve multi-size casing and cement annulus sections that overlap at certain depths and provide multiple barriers to avoid the carbon dioxide leaking to the surface atmosphere. However, for operating and abandoned wells, it is possible that the carbon dioxide could penetrate through the casing-cement interface and leak into the atmosphere.

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