Fluid Migration Characterization of Cemented Sections Retrieved from a North Sea Production Well

Abstract

The annulus cement sheath between casings is an important well barrier element for ensuring zonal isolation and preventing cross-flow of formation fluids. Understanding the migration path geometry and how hydrocarbons and treatment materials flow in such paths are important for diagnosing and treating leaking annulus cement. We study the seepage properties of gas and liquid through two cemented annulus sandwich sections recovered from the upper vertical part of a North Sea production well during a permanent abandonment operation. Studying physical and mechanical properties of these sections can provide unique insights about the sealing properties of annulus cement that has been exposed to downhole conditions and more than thirty years of operation. The sandwich sections are each nearly a joint length long and comprised of a 9 5/8-in casing inside a 13 3/8-in casing that was originally cemented by bullheading cement slurry down the annulus. One of the sections contains the top-of-cement while the other is from approximately 140 meter below top-of-cement. A series of seepage experiments have been performed to better understand variations in sealing potential along the assembly, including settled solids above the top of cement. Visual inspection of the annulus cement at the bottom and top ends of the section reveal full cement coverage around the eccentric inner casing. Minor local imperfections in the form of pockets of fluid trapped in the hardened cement are also observed, presumably originating from the original cementing operation. The settled solids above the top-of-cement were found to act as an effective barrier against fluid migration initially but yielded irreversibly once exposed to higher test pressures. Seepage rates through the cemented section are consistent with previous measurements in full-scale annulus test assemblies. The gas and liquid seepage experiments performed on the retrieved section improve our understanding of well cement quality close to the top of cement, including possible sealing potential of the material above the top of cement. Comparing the cement log from the well to seepage measurements helps correlate log response and annulus cement seepage potential which can benefit both remediation design and future abandonment operations.

Fluid Migration Characterization of Full-Scale Annulus Cement Sections Using Pressure-Pulse-Decay Measurements

“…casing. The effective permeabilities are about 2 orders of magnitude greater than the permeability measured from intact cement cores from the bottom of the transition joint [28], supporting the observation that cement-casing interfaces and not bulk cement are likely the dominating annular flow paths, in agreement with previous works such as Ref. [6].…”

“…While this appears to be a reasonable approximation for the PWC Jig, where cement shrinkage during curing is a likely reason for the fairly high permeabilities, the transition joint shows seepage paths along both casing interfaces and through flaws in the cement, as visualized in Refs. [16,28]. In these more complex cases, the effective micro-annulus characterization is not necessarily a true description of the dimensions of the seepage paths.…”

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

“…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].…”