Abstract
In some Middle Eastern wells, the serious problem of shallow casing leaks results from an insufficient cement seal across a corrosive water formation containing hydrogen sulfide (H2S). Factors contributing to this poor seal include multiple weak adjacent zones and thin lenticular washouts, which complicate conventional cementing practices by preventing slurries from sealing off water-bearing formations and protecting the well casings. Historically, multistage cementing has only marginally improved zonal isolation in this region. High-quality foamed slurries [slurries with high volumetric concentrations of nitrogen (N2)] can enable coverage of the weak zone, but these slurries are too permeable to provide long-term casing protection.
A joint study conducted to address casing-leak problems in a Middle Eastern field has yielded custom cement blends that mitigate the combined detrimental effects of (1) water containing H2S, (2) mud or whole-cement losses to lost-circulation zones (zones with very low fracture gradients), and (3) muddisplacement deficiencies primarily caused by multiple washed out sections. Successful slurries that appear (based on laboratory results) to combat these problems are foamed with nitrogen and feature a combination of Portland and pozzolan cements plus hollow pozzolan spheres. This paper discusses large-scale tests and the blends designed for these tests. Researchers conducted these tests in an attempt to prove the effectiveness of certain cement blends and to help optimize those blends. The tests show that foamed, lightweight (8 to 10 lbm/gal) slurries containing hollow pozzolan spheres with pozzolan cement can enhance the sealing of harsh-water zones. This enhancement is achieved by the combined effects of two events: (1) improving displacement of drilling mud and cuttings by optimizing foam quality, and (2) cementing past lost-circulation zones. Details of blends, test setups, and test results are discussed.
