Annular cement sheath characterization and hydraulic aperture assessment through single- and two-phase seepage tests

Abstract

The integrity of well barriers is critical throughout the life cycle of oil and gas wells, with annular cement as a key component in maintaining the structural integrity of the barrier system. In this study, an annular cemented section with a well-defined microannulus was systematically constructed and characterized, and both single- and two-phase seepage tests were conducted to gain a deeper understanding of how relative permeability impacts the estimation of hydraulic aperture in potential leakage pathways. The test cell design allows modification of the microannulus gap size by changing the pressure applied inside the inner casing. This enables studying different microannulus sizes throughout the conducted tests. The single-phase seepage tests showed consistent measurements of hydraulic aperture for both gas and water, despite variations in fluid type and applied pressure, with surface roughness likely affecting the non-linear behavior of the hydraulic aperture under stress. The two-phase seepage tests demonstrated a clear relationship between breakthrough pressure and microannulus size, with the van Genuchten model providing a better fit for relative permeability and capillary pressure data. Breakthrough time experiments revealed that initial permeability estimations were significantly lower than those obtained from single-phase tests; however, once relative permeability at breakthrough was accounted for, the estimated absolute permeability values aligned closely with the single-phase experimental data. These findings offer valuable insights into the complex interactions between cement sheath integrity and gas migration, potentially contributing to developing new well integrity assessment technologies, including tracer gas below the well barrier element, as well as insights relevant for both gas migration and sustained casing pressure analysis.