Civil Engineering ETDs

Publication Date

Summer 8-1-2023


In wellbore systems, the microannulus between the steel and the cement has been shown to be a critical leakage pathway. This microannulus offers unique roughness, aperture distribution, and wetting conditions, which are critical in understanding and explaining the potential sealing efficiency of a repair material. An experimental configuration was developed to better understand how a repair material would displace an existing material in the microannulus, visually evaluate potential instabilities on the flow patterns developed, and how efficient the potential repair would be. For that purpose, the experimental configuration consisted of a planar, rough cement surface and a transparent acrylic plate on top of it to serve as a surrogate for steel while maintaining comparable wetting conditions with a wellbore microannulus. Three repair cases have been studied in this work: a polymer-based material repairing a wellbore microannulus in (1) horizontal configuration, (2) vertical configuration, and (3) a microfine cement repairing a horizontal microannulus with the injection of polymer-based material after four attempts of repair with the microfine cement.

The importance of the injection parameters and the existing fluids in a wellbore microannulus has been shown in this work. This highlights the importance of the injection operations in the potential effectiveness of a repair work. In addition, it has been shown experimentally in a representative microannulus that the resin-based repair material has the ability to flow into smaller apertures than cementitious materials, and the results suggest that it is an option to mitigate unsuccessful repair attempts with cementitious repair materials.

For case one, the polymer-based repair material was injected into a microannulus with a hydraulic aperture of ~50 µm under different injection velocities (modifying the capillary number) and displacing water and air (different mobility ratios). Compact displacement was found only with resin displacing air at the highest velocity, and an apparent saturation at breakthrough of ~90%. Compact displacement is a relatively uniform front that displaces most of the fluid in the microannulus (residing fluid), and it is desired for a repair since it fills most of the microannulus and leaves small amounts of residing fluid behind. Under these experimental conditions, a transition zone and capillary fingering were found under different capillary numbers and mobility ratios. This allowed the generation of a partial phase diagram for a repair material invading a wellbore microannulus. When a vertical microannulus was repaired, different injection velocities (capillary numbers) were affecting the repair of a microannulus with a hydraulic aperture of ~150 µm. The highest apparent saturation at breakthrough was 67.8%, and the lowest was 4.4% (lowest capillary number). It was observed in this experiment that was under conditionally stable conditions, that the preliminary estimation of a critical velocity was at least two orders of magnitude off when used for a wellbore microannulus due to the effect of the variability in the aperture distribution. In addition, while viscous forces dominated the displacement for the first portion of the sample with the larger capillary numbers, the displacement was dominated by the buoyant forces at the top of the sample for all cases, yielding a relatively poor apparent saturation at the top of the sample at breakthrough for all cases. To repair a horizontal microannulus with microfine cement, the microfine was injected in the microannulus, and the hydraulic aperture was measured after 24 hr to allow the microfine cement to set. The two samples used had a hydraulic aperture of 158 µm and 85 µm before injection. After four injections of microfine cement, the hydraulic aperture decreased to values below 20 µm, but the microannulus was not completely sealed due to bleeding and filtration. A resin-based repair material was injected after the microfine cement, and it further decreased the hydraulic aperture of the microannulus to <5 >µm.


Wellbore microannulus repair, remedial methods, wellbore integrity, repair efficiency, flow regimes, flow patterns

Document Type




Level of Degree


Department Name

Civil Engineering

First Committee Member (Chair)

John Stormont

Second Committee Member

Mahmoud Reda Taha

Third Committee Member

Ryan Webb

Fourth Committee Member

Hans Joakim Skadsem