The Effect of Casing-Pipe Roughness on Cement-Plug Integrity

Abstract

Summary In permanently abandoned wells, cement plugs serve as a well-barrier element that is essential for providing long-term zonal isolation. Poorly plugged wells might provide leakage pathways that pose risks to the surrounding environment. It is well-known that the final quality of set cement is influenced by the cement placement, but the influence of downhole-pipe roughness to the hydraulic sealing of a cement plug has not been specifically investigated. This paper presents an experimental study assessing the cement-plug sealing of neat- and silica-cement systems placed in pipes with three different levels of surface roughness. The experimental test uses a small-scale laboratory setup, consisting of a test cell filled with a cement plug, which can simulate realistic wellbore curing and operating conditions. By subjecting the cement plug to gas differential pressure, the setup detects any gas leak through the cement plug and measures the leak rate. As a complement to the experimental study, a computational-fluid-dynamics (CFD) simulation is conducted to study fluid flow around the cement plug. Experimental tests detect an instant gas leak in all samples at low differential pressure (<0.01 bar) through leak sources at the cement/steel interface, presumed to be microannuli. Furthermore, experimental results discover that placing cement plugs in rough-surface pipes could reduce the gas-leak rate, and the most significant reduction was found from samples cured in the roughest pipe. The CFD simulation results, along with pressure-buildup-curve observation, show that the leak path of samples placed in rough-surface pipes tends to be tortuous and less connected, thereby adding resistance for gas to flow.

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.…”

“…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]. 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].…”

“…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].…”