Acoustic Waveform Modelling to Improve Confidence in Wellbore Integrity Assessments Across the Utsira Formation

Abstract

Wellbore integrity is recognised as one of the key factors in maintaining the integrity of geological CO2 storage sites. Analogue cement bond logs (CBLs) from 4 wells near the CO2 plume at the Sleipner CO2 storage site were examined to determine how effective they are at assessing the quality of the cement bond between the steel casing and rock formation in low-velocity reservoirs and their associated caprocks. It is important to note that the CO2 plume at Sleipner does not intersect any wells at present and predictions indicate that it most probably will not reach any currently existing wells in the future (Statoil, pers. comm.).The Utsira Sand CO2 storage reservoir has a very low velocity and large hole size in the wells examined, resulting in difficulties in interpreting the efficacy of the cement-formation bond from the acoustic waveforms recorded by the CBL. It was found that acoustic waveform modelling helped to constrain arrival times and amplitudes of wave-fronts from the formation-cement interface. This helped to differentiate these from other arrivals and lent confidence to the wellbore integrity estimates.