Two-Phase Relative Permeability in Wellbore Microannulus and its Significance in Long-Term Risk Assessment

Abstract

Summary In plug and abandonment (P&A) wells, the interface between the steel casing and cement in a typical wellbore may debond and establish leakage pathways called microannulus. This study aims to understand the behavior of two-phase flow of water and gas in the microannulus and evaluate the relative importance of absolute and effective permeabilities on long-term leakage potential. In this study, we conducted experiments to quantitatively determine the relative permeabilities of water and gas within the microannulus established at the interface between a 9 5/8-inch cemented casing and the enclosed cement matrix. To do this, we saturated a cell with water and introduced nitrogen from the bottom at incrementally increasing pressure, aiming to identify the gas breakthrough pressure within water-filled leakage pathways. Two-phase relative permeabilities were computed using the Brooks-Corey and van Genuchten models, which establish relationships among capillary pressure, saturation, and relative permeability at each pressure step in this unsteady-state approach. These tests were carried out in the short term to verify repeatability and in the long term to assess how cement and casing alterations affect two-phase relative permeabilities. Furthermore, we conducted a simulation sensitivity study to express the relative significance of absolute and effective permeabilities in terms of long-term leakage potential. This study reveals that the conventional X-curve relative permeability inadequately captures the two-phase flow behavior in leaky wellbores. Furthermore, it illustrates that even with alteration in cement and casing as well as variations in microannulus size over time, relative permeability remains quite stable. These results imply that in the studied P&A cases where two phases flow within the microannulus, comprehending this complex two-phase flow behavior in the microannulus and applying an accurate representative relative permeability model are critical for effectively assessing the long-term leakage risks. This research contributes significantly to the understanding of multiphase flow dynamics within the microannulus and underscores the critical significance of utilizing representative relative permeability models, as opposed to the commonly used X-curve relative permeability, in the analysis of fluid flow behavior and assessing associated risks.