Pressure transient testing for assessment of wellbore integrity in the IEAGHG Weyburn–Midale CO2 Monitoring and Storage Project

Abstract

This paper presents an overview of a field program undertaken to assess wellbore integrity in a 54-year old vertical well, conducted as a component of the IEAGHG Weyburn–Midale CO2 Monitoring and Storage Project. The well investigated had been drilled to a depth of 1471m (4824ft), cased with 139.7mm (5.5in.) production casing and cemented with a 1-to-1 blend of Portland cement and pozzolan. Cased-hole logging indicated that the top of the cemented annulus was located approximately 500m (1640ft) above total depth of the well. A built-for-purpose tool was used to conduct pressure transient (vertical interference) testing to assess the hydraulic properties of the cemented annulus within the Watrous Formation, which serves as the regional seal in the vicinity of the Weyburn and Midale fields. A modified version of an existing tool was used to mill through casing then drill small cement core samples. This paper presents an overview of the entire testing program, and a detailed account of the analysis methodology used to estimate the equivalent permeability of the cemented annulus from a vertical interference test. The results of this analysis suggest that vertical permeability of the cemented annulus is very low, falling somewhere in the 7–80 microdarcy range. This result is consistent with cased-hole logging results; more specifically, the combined interpretation of sonic and ultrasonic logs, which suggest effective cement emplacement, and the paucity of casing defects identified by an electromagnetic log, which suggests the cement sheath has effectively shielded the casing from exposure to native formation fluids. The results presented here demonstrate that a well constructed using standard practices and materials of the 1950s can serve as an effective barrier against CO2 migration, although similar work should be undertaken in more wells to confirm that this holds across a broad spectrum of conditions.