Abstract

The Perforate, Wash and Cement technique has been widely applied to remediation of annular cement in recent years. Since 2012, extensive experience in the different technologies currently available on the market has been obtained across ten fields in the Danish and UK sectors of the North Sea and the data obtained has been used to attempt to better understand the effective operational window for the technique, and also further enhance reliability and tool robustness.

The run data obtained from several wells has been calibrated against cement bond logging (CBL) responses so that the degree of annular bonding as inferred from the logs may be expressed in terms of the degree to which hydraulic communication or circulation is permitted via perforations and along the annulus. This in turn helps clearer decision criteria to be defined prior to execution that aids in the selection of the most appropriate method for remediating the cement, since Perforate, Wash and Cement may not be the most suitable technique in every case.

Given the criticality of cement remediation to the long-term success of zonal isolation, it is important to demonstrate that effective hydraulic communication with the annulus has been established during the washing phase to ensure that the zonal isolation medium (cement) can be effectively placed and the hydraulic seal re-established, resulting in successful remediation.

Post-execution verification of the effectiveness of remediation is typically performed via Cement Bond Logging. However, this may not always be a definitive verification step since Cement Bond Logging interpretation does have its limitations (both physical and interpretative), and hence annuli have occasionally been known to develop Sustained Casing Pressure even shortly after positive log interpretations following well-executed cementations.

This paper therefore further demonstrates how Sustained Casing Pressure monitoring can be used as a further criterion to verify the integrity of the remediated annulus and hence conclusively demonstrate whether the washing and later cementing of the annulus has been effectively performed. This in turn is used as definitive confirmation that zonal isolation has been performed within the operating limits of the tool and hence qualify Perforate, Wash and Cement as a robust and viable remedial method.

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