The annular casing cement is an important part of the well barrier throughout the life cycle of a well. With the increasing number of subsea plug and abandonment (P&A) operations, increased attention is now given to annular cement evaluation and the ability to prove adequate zonal isolation. Today, cement evaluation by logging is performed almost exclusively using acoustic logging tools. One of the concerns when it comes to cement integrity is the frequently occurring micro-annulus at the casing-cement interface. Yet, a typical cement evaluation tool may lack of accuracy on its evaluation. Hence, a novel concept for the evaluation of casing-cement micro-annulus has been proposed.
This paper describes a mechanical-based approach for cement evaluation – the Annulus Verification Tool (AVT). The AVT applies a radial force on the casing inner wall that yields an ovalization of the cross-section, while recording the radial displacement of the casing. A prototype of the AVT has been constructed along with an experimental setup to allow for initial testing of the tool. This comprises the construction of full-scale diameter samples representing a typical 9 5/8-in. production casing cement job, with the possibility to generate a uniform micro-annulus of a known size at the casing-cement interface.
The tests performed have shown that the AVT is able to differentiate a casing supported by an annular cement sheath from a free pipe, due to the stiffness contrast. By measuring the casing radial displacement with high resolution, the results have shown that a microannulus can be detected and its size quantified with good accuracy. Experimental tests performed with tool eccentricity and tilting has shown that the AVT should be kept centralized to achieve accurate quantification of the microannulus size.
The AVT module is meant to complement the acoustic tool sting used today, and to improve evaluation of the cement sheath's sealing capability, especially in cases where a micro-annulus is detected or suspected. If an existing microannulus is suspected, the AVT logging response may confirm its occurrence, quantify its size and aid the planning of remedial operations to restore the annular barrier.