Experimental Investigation of Well Cement Integrity in Abandoned Oil and Gas Wells
Creators: Abdurrahman Thaika, Muhammad
Publisher: name: University of Alberta Library
Year: 2024

<p>When a well reaches the end of its life cycle, it needs to be abandoned. Leakage issues are being reported in legacy wells and the literature blames cement shrinkage as the main culprit for these leakage issue. Instead of blaming the cement, the isolation capabilities of the entire system need to be evaluated as there is a lack of cement integrity testing at relevant wellbore conditions. In this study, we evaluated the integrity of the cement plug under relevant wellbore conditions. We used 4 methods to investigate the integrity of cement plugs used in abandonment operations, including: 1) Testing the isolation capabilities of small-scale samples; 2) testing the isolation capabilities of<br>
optimally placed cement using the wellbore simulator; 3) investigating the effects of cement contamination; and 4) the remediation capabilities of various cement blends.<br>
In the first test, the permeability of small-scale cement samples was tested. From the testing results, we determined that the permeability of the bulk cement was very low, but with the addition of an interface around the cement plug (metal casing), the permeability of the system increased. In the second test, the permeability of two different cement blends was tested using a physical wellbore simulator, which allowed cement curing and subsequent permeability testing under elevated pressure and temperature conditions. The two blends were Neat G cement without any additives and a Class G cement with additives that had been engineered to enhance its sealing performance. The wellbore simulator experiments showed that the Class G cement blend with additives outperformed Neat G cement blend in every testing scenario. The class G cement with additives also resisted the formation of a micro annulus better than the Neat G cement. These results formed the baseline guidance towards how to ideally place cement plugs during abandonment operations.<br>
In the third test, we tested the effect of contaminants on the integrity of the cement plugs. Water contamination was first investigated. For the neat G cement, it was observed that as the water contamination increased, the zonal isolation capabilities of the cement decreased. The dominant fluid pathways were located at cement-casing interface. For Class G cement with additives, contaminated with water, the isolation capabilities of the cements were reduced drastically. The dominant fluid pathways were through the center of the cement matrix. The fluid loss additives used in this slurry prevented the cement from expelling excess water out of the slurry. This excess water would have been trapped in the cement matrix, forming larger pores; increasing the permeability. The effect of oil contamination was also investigated. Neat G cement was contaminated with 2.5% light crude oil. This had little to no effect on the isolation capabilities of the cement as long as the entire sealing system was left undisturbed (i.e., without stress changes).<br>
Changes to the effective stresses acting on the cement caused the sealing capabilities of the cement plug to reduce drastically (i.e., water permeability increased). Isolation capabilities against<br>
nitrogen remained almost unchanged when the effective stress changed. Class G cement with additives was then contaminated with 2.5% light cured oil; a similar trend was observed. As long<br>
as the effective stresses acting on the cement were unchanged, the isolation capabilities against the flow of water was unchanged. Changes to the effective stress reduced the isolation capabilities against water drastically. The isolation capabilities against nitrogen, however, was reduced greatly. Gas breakthrough was immediately observed for the contaminated sample.<br>
Lastly, since the micro annulus was the dominant flow pathway for the majority of the experiments, remediation experiments were carried out in the fourth test in an attempt to improve cement’s sealing performance. Three cements with varying grain sizes were used for the remediation experiments: the standard class G cement with the largest grain size, Portland limestone cement (PLC) with the medium grain size, and microfine cement with the smallest grainsize. It was observed that the microfine cement could penetrate fracture widths as low as 35 microns and<br>
reduce the fracture conductivity by 5 orders of magnitude. The PLC could penetrate fractures as small as 65 microns, but bridging was observed at fracture widths of 50 microns. Class G cement bridged at a fracture width of 200 microns.</p>