Visualization of Fluid Flow Through Cracks and Microannuli in Cement Sheaths

Abstract

Summary Cement-sheath integrity is important for maintaining zonal isolation in the well. The annular-cement sheath is considered to be one of the most-important well-barrier elements, both during production and after well abandonment. It is well-known, however, that cement sheaths degrade over time (e.g., from repeated temperature and pressure variations during production), but the link between leak rate and the cause of cement-sheath degradation has not yet been established. In this paper, we have studied fluid flow through degraded cement sheaths. The degree of degradation of the cement sheaths varied from systematically connected cracks to real microannuli. The leak paths, created by thermal-cycling experiments, were imported into a computational-fluid-dynamics (CFD) simulation software. The pressure drop over the cement sheath was used as a boundary condition, and the resulting pressure-driven flow was studied using methane gas as the model fluid. The Forchheimer equation was used to estimate the effective permeability of the cement sheaths with defects. Our results show that the pressure-driven flow is complex and greatly affected by the geometry of the flow paths. A nonlinear pressure-buildup curve was observed for all experimental cases, indicating that Darcy's law was not validated. For homogeneous microannuli, the pressure-buildup curve was linear. The estimated effective permeability for all cases was observed to be orders of magnitude larger than that of a good cement sheath.

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

“…The specific volumes of the cracks and microannuli were extracted using Avizo and imported into StarCCM+ [61] for computational fluid flow (CFD) simulations as described in references [45,[50][51][52]. The surface was manually repaired to maintain the structural features of the 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]. A limitation of this approach is that the estimated microannuli sizes are not really correct and give no accurate information about the actual microannuli geometry [9,50]. Figure 6 shows four examples of experimentally created cement-casing microannuli [9,41] and it is seen that they are not homogeneous and uniform.…”

“…This assumption is incorrect, and for example, increased pressure differences may open up microannuli, and thus, increase the available flow path size [27,74]. Figure 7 shows three examples where experimentally created cracks and microannuli have been used as leak path geometries in CFD flow simulations [50][51][52]. Methane was used as fluid in these simulations.…”

“…Both the non-uniformity of the overall geometry and local heterogeneities such as surface roughness and flow path bottlenecks contribute to these complex flow patterns. Furthermore, it is observed that the resulting flow rates are not linearly dependent upon pressure difference [50][51][52], as could be expected from Darcy's law. It is seen that the less uniform the leak path geometry is, the less linear the relationship between flow rate and pressure difference is, whereas a completely uniform and homogeneous microannulus shows predicable and linear flow behavior.…”

“…In our group, we have recently developed an experimental characterization procedure where we use CT for 3D digital visualization of well cement in realistic annular geometries. With our dedicated and tailor-built cement sheath integrity experimental set-up [41,42], we have visualized microannuli and cracks in cement formed during thermal cycling [41,[43][44][45][46] and pressure cycling [42,[47][48][49], and we have also used the digital cracks and microannuli obtained from CT scans as imported leak path geometries in computational fluid flow simulations [45,[50][51][52]. Furthermore, we have also used CT to study cement-formation bonding [53][54][55][56], cement durability [57,58], and cement-mud interactions [59].…”