Study of Ultrasonic Logs and Seepage Potential on Sandwich Sections Retrieved from a North Sea Production Well

Abstract

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.

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].…”

“…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.…”

“…The bottom cross section to the right shows full cement coverage around the annulus and also significant contamination in the form of local heterogeneities. Previous observations of gas migration through the bottom cemented cross section indicated communication along both cement-casing interfaces, that is both along the outer wall of the inner casing and along the inner wall of the outer casing [16]. The ends of the sections have since been dressed with bulkheads to allow testing under pressure and to avoid further drying of the cement.…”

“…A more detailed description of the transition joint, including an initial examination of the cement, can be found in Ref. [16].…”