During offshore oil and gas well decommissioning, well casing cement bond verification is frequently a critical initial step of well plug and abandonment (P&A) operations. This is done to determine the isolation between geological intervals. To acquire the essential data, conventional cement bond logs (CBLs) and ultrasonic imaging logs require unimpeded access to the casing annulus. Consequently, the production tubing needs to be removed from the wells. These operations are costly as they require securing the well and removing the tubing with an offshore rig, in some cases a mobile offshore drilling unit. If cement integrity or isolation could be determined without removing the production tubing, operational time and costs would be significantly reduced. Hence, multi-string cement bond evaluation for isolation verification without pulling the tubing has been desired by the oil and gas industry for many years.

The innovation described in this paper, consisting of a novel methodology and an associated logging apparatus, leads to a new technique that can evaluate cement bond or isolation around the casing through the production tubing. The acoustic energy emitted and received by a logging tool inside the production tubing travels through tubing and the surrounding annulus to determine the attenuation caused by the material behind the outer casing. The smaller bonding signal, challenged by the dominating signal from the tubing wall, is then successfully identified through unique frequency domain signal processing.

The accuracy of a multi-string logging technology has been verified by extensive offshore field tests in Southern North Sea. Side-by-side comparisons with conventional CBL and ultrasonic logs after the tubing is pulled out have been conducted. The field tests also demonstrate the multi-string logging technology is applicable to all typical casing and tubing weight or size combinations and provides quantitative assessments on isolation conditions of wells through the production tubing.