An Evaluation of the Cement Sheath Quality of Casing Sections Recovered During a Well Abandonment Operation

Abstract

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.

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

“…The surface re-logging confirmed the log results acquired in the well prior to cutting and retrieving the sections, namely, the existence of poor-to-moderate cement bond to the inner casing and indications of gas on the ultrasonic cement log [15,16]. An initial comparison between the cement logs and seepage records showed good, qualitative agreement [15,16]. In this paper, I expand on the above by measuring wellbore permeability, or the equivalent micro-annulus gap size, in one of the two sandwich sections mentioned above, and in another, yardcemented full-scale annulus test section, using two different techniques, namely, the transient pressure-pulse-decay method and steady-state seepage measurements.…”

“…The well was permanently abandoned in 2018, and two of the retrieved sections were saved for surface re-logging and characterization. The surface re-logging confirmed the log results acquired in the well prior to cutting and retrieving the sections, namely, the existence of poor-to-moderate cement bond to the inner casing and indications of gas on the ultrasonic cement log [15,16]. An initial comparison between the cement logs and seepage records showed good, qualitative agreement [15,16].…”

“…As per the PWC Jig, also the transition joint was tested from bottom to top and in the reverse direction from top to bottom in order to identify and confirm spatial variations in the permeability. From previous surface re-logging of the section [15,16], the transition between annulus cement and the material above the cement has been found to occur between approximately 4.5 m and 5.8 m measured from the bottom of the test section. Previous pressure measurements have indicated that the material above the cement does not provide any significant resistance to flow, whereas the cement in the bottom half covers the entire eccentric cross section and provides a better seal.…”

“…To test whether an internal casing pressure will affect the effective permeability of the transition joint, pressure-pulse-decay and steady-state seepage measurements have been performed also with 30 bar internal casing pressure. While the surface re-logging did not identify a distinct change in acoustic properties of the annulus material when logged with inner casing pressure [15,16], the permeability is seen to decrease slightly with increasing borehole pressure, as indicated by the arrows in Fig. 15.…”

“…annulus top-of-cement about mid-way along the length of the section. Previous analyses of the test section include surface re-logging [15,16] which indicated the transition from cement to mud from 4.5 m to 5.8 m measured from the bottom of the section. This part of the well was cemented by reverse circulating cement slurry down the annulus as the second stage of the primary cementing operation of the 9 5/8-in.…”