Summary In past years, the industry has focused on ensuring that cement is efficiently placed in the wellbore and that it does not become mechanically damaged during the life of the well. However, few efforts have been made to determine how cement mechanical integrity (CMI) relates to cement hydraulic integrity (CHI) (i.e., evaluating the flow rate that could occur through the cement barrier), even though CHI is one of the main objectives of placing a cement plug in a wellbore. The analysis of hydraulic integrity requires that a CMI model be used to compute the state of stress and pore pressure in the cement and to estimate which type of mechanical failure might occur during the life of the well. It also requires that a CHI model be integrated with the CMI model to estimate the rate of fluid that might flow through a cement barrier, should it mechanically fail. This provides the engineer with insight into the long-term integrity of a cement plug. This paper describes the work conducted on CMI/CHI models for cement plugs, and it presents a sensitivity analysis that demonstrates the value of an integrated CMI/CHI model. The study indicates that (1) well geometry, cement properties, reservoir pressures, cement heat of hydration, and fluid properties are required inputs for proper analysis; (2) the changes of stresses and pore pressure over time need to be computed along the length of the cement plug, with sensitivity analysis to consider the existing uncertainties; (3) a cement plug might preserve its sealing capability, even if the CMI model shows the existence of a microannulus (e.g., when the fluid viscosity is very high); and (4) a cement plug might lose its sealing capacity, even if the CMI model shows no induced defect (e.g., when a microannulus is propagated as a hydraulic fracture). These last two observations are important because they show that what a CMI model cannot predict, a CHI model can.

Summary Important functions of well cement are to provide zonal isolation behind casing strings and to mechanically support and protect the casing. Experience suggests that many wells develop integrity problems related to fluid migration or loss of zonal isolation, which often manifest themselves in sustained casing pressure (SCP) or surface casing vent flows. Because the characteristic sizes of realistic migration paths are typically only on the order of tens of micrometers, detecting, diagnosing, and eventually treating migration paths remain challenging problems for the industry. As part of the recent abandonment operation of an offshore production well, sandwich joints comprising production casing, annulus cement, and intermediate casing were cut and retrieved to surface. Two of these joints were subjected to an extensive test campaign, including surface relogging, chemical analyses, and seepage testing, to better understand the ultrasonic-log response and its potential connection to rates of fluid migration. One of the joints contained an apparently well-defined top of cement (TOC) with settled barite on top. Although the settled material initially provided a complete seal against gas flow, the sealing capability was irreversibly lost as part of subsequent testing. The two joints have effective microannuli sizes in the range of tens of micrometers, in agreement with previous reports on SCP buildup in wells. On a local scale, however, we observed significant variations in cement quality from both the log results and the seepage testing. Further, we found qualitatively very good correlations between seepage-test results and the log results for the bond between cement and casings. The best bonded cement was found directly above a production casing collar, where a short segment of well-bonded cement prevented measurable steady-state seepage of nitrogen. Additional tests involving internal pressurization of the production casing suggested that certain annular-seepage characteristics are well-described by an effective microannulus at the cement/casing interfaces. We consider the two sandwich joints to be highly representative and relevant for similar mature wells that are to be abandoned.

Cemented casing sections were recovered from the upper part of a Norwegian North Sea production well during a permanent abandonment operation. The barrier quality of the cement sheath, which has been sandwiched between two casing strings for more than 30 years since the well was constructed, has been investigated. Measurements recorded using acoustic logs, fluid leakage testing, and core plug analysis are evaluated, and the results are presented. Two sections were selected from those recovered during well abandonment, the deepest from approximately 260 m depth and a second from the interval covering the top of cement (TOC). The leakage properties of these sections were measured using water and nitrogen. Core plugs were recovered from the top and bottom of each section and petrophysical, chemical, and mechanical properties were measured. The casing ends were sealed with pressure-tight bulkheads enabling acoustic logging to be performed under pressure. The sections were logged at their initial condition: "dry", as delivered onshore and thereafter "wet" after attempting to saturate them with water. Initial leakage testing with nitrogen enabled the distribution of the fluid leakage paths through the ends of each section to be visualized. The leakage path properties were observed to vary axially along each section and also for different positions around the azimuth of the outer casing. The acoustic logs were analyzed and provided a detailed map of the axial and azimuthal variations of the cement bonded to the inner casing of each well section. The variation of the log response recorded under dry and wet conditions was correlated with the leakage property variations measured along and around the casing sections. The log response was found to be consistent with the physical observations and leakage test results. The presence of microdebonding between the casing and cement and the transition to mud solids and fluid pockets above the top of the cement were readily observed. A unique and comprehensive data set has been acquired comprising acoustic logs and laboratory measurements of water and gas leakage rates and cement properties, recorded on casing sections retrieved after more than 30 years of operational exposure downhole. The measurements enabled the cement log analysis to be compared directly to physical measurements of the well barrier quality.

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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.