Methodology to Use the Cement Bond Quality Classification Matrix as Input for Leakage Rates Assessment

Abstract

AbstractVerification of the quality of the cement barrier behind casing is often performed through acoustic or ultrasonic cement logs that probe the presence of cement and its bonding to the casing. In risk-based approaches to permanent well abandonment design, the probable conditions of the existing barriers in the well should be reflected in terms of reliability, or conversely, the risk of leakage through the barrier over time. Connecting the log response to a predictive model of leakage potential behind casings remains a key challenge for the industry.We build on ultrasonic cement logs acquired in two sandwich joints recently retrieved from a North Sea production well as part of the permanent abandonment operation. The joints consist of 30 years old annulus cement sandwiched between production and intermediate casing joints that have been subjected to a comprehensive test regimen involving pressure and seepage testing. Combined with the log response, these measurements offer an unprecedented opportunity to compare log response and physical measurements of leakage potential. We have developed a methodology for converting a log response to a likely, local microannulus size that in turn reflects the seepage potential of the annular barrier.Physical testing of the two sandwich joints used as basis for the study revealed significant local variations in seepage potential ranging from effective microannuli of tens of microns down to a very low permeable region with practically impermeable features over the timescale of measurement. Our methodology is based on a Monte Carlo approach for propagating uncertainty from log interpretation to annular seepage potential. We discretize the log response azimuthally and axially into volume elements of locally constant permeability and compare numerical log realizations to the physical measurements. We find generally good qualitative agreement with the cement log and demonstrate conceptually the possibility of utilizing a cement log response for a quantitative assessment of the barrier quality.Improved interpretation of barrier quality is a key step toward new rig-less abandonment operations and more efficient risk-based abandonment design. Our study represents a first conceptual study that attempts to connect physical measurements of the cement quality behind production casing to its log response. More efforts are required to fully utilize the information contained in cement logs for quantitative comparison of different P&A designs.