Characterization of wellbore microannuli

Digital Cement Integrity: A Methodology for 3D Visualization of Cracks and Microannuli in Well Cement

“…Figure 6b shows the resulting debonded area after complete radial cracking of the cement sheath, whereas Figure 6c,d show examples of the debonded area due to mudfilm, cured at ambient pressure and at elevated pressure, respectively. Of these four examples, the microannulus geometry showed in Figure 6a is perhaps the most relevant and interesting and is possibly also the geometry that is most consistent with the experimental findings of Stormont et al [24] and Garcia Fernandez et al [25]. Despite the merits of the simplified approach of assuming uniform microannuli, real microannuli geometries, such as those exemplified in Figure 6, should be taken into account when studying cement integrity and the resulting interfacial fluid flow behavior.…”

“…200 µm, which means that any cracks and debonded areas smaller than this will not be detected. Other studies have shown that microannuli and cracks often are significantly smaller than 200 µm [20,[23][24][25], so it is likely that there are such small defects in our samples as well that cannot be seen due to the detection limit. The CT resolution could potentially be improved by decreasing the sample size further, as CT resolution is dependent upon sample size, or when more advanced CT instruments are developed in future.…”

“…Model simulations have shown that the size of microannuli can be dependent on wellbore characteristics such as formation properties, casing size and cement stiffness [27,67], however, the resulting shapes can be challenging to determine from such simulations. Stormont et al [24] and Garcia Fernandez et al [25] found experimentally that the microannulus aperture can vary considerably around the circumference when they studied gas flow through different microannuli, and concluded that microannuli have complex geometries. For simplicity however, a common approach in many studies is to assume microannuli uniformity, which for example, works well when converting measured fluid flow rates to microannuli apertures by introducing the terms "equivalent" or "effective" microannulus [20,[68][69][70].…”

“…Consequently, several laboratory experiments have been performed to study and understand these cement sheath failure modes and the resulting loss of zonal isolation [15,[18][19][20][21][22]. Furthermore, other authors have focused on studying microannuli formation and the resulting flow through these leakage pathways [23][24][25][26]. All these studies have provided valuable understanding of cement failure and loss of zonal isolation.…”

“…There are several ways to find answers to these questions, both experimentally and numerically. For example, Stormont et al [24] and Garcia Fernandez et al [25] have experimentally studied gas flow through microannuli and found that the microannulus aperture can vary considerably around the circumference. They did, however, not visualize the actual microannulus geometries.…”