Curing Losses in Vuggy Carbonate Formations

Abstract Despite the preventive wellbore strengthening (WBS) approach, lost circulation (LC) followed by well control events have been encountered where thousands of barrels of mud were lost to a formation resulting in long periods of shutdowns. The aim of this paper is to identify the LC mechanism in this formation and propose conventional loss circulation materials (LCM) treatments that stand a higher chance of curing the losses based on experimental investigation of irregular shaped openings to mimic vuggy porosity. A review was conducted to identify the depositional and diagenetic processes for the "A" formation. The study concluded that touching vugs type of porosity, created through massive dissolution, is the main LC mechanism. To propose a treatment, four stainless steel disks with different irregular opening shapes and sizes, to mimic the reported vugs in the "A" formation, have been developed. As vugs reportedly have wider openings than what anticipated in fractures, an equivalent vug diameter was defined as the maximum width along the longest vug axis. As an initial screening stage, these disks were used in a low-pressure LCM apparatus (LPA) with different LCM additives to identify if the concentration and particle size distribution (PSD) are able to form a seal. The integrity of the seal was then investigated using a high-pressure fluid loss apparatus. Synthetic rubbers and calcium carbonates were specifically sized to meet the needed PSD and effectively initiated a seal. Other commercially available products, such as synthetic & acid soluble fibers and reticulated foam, were successful when mixed with other LCM to meet the need PSD. Several combinations of the LCM additives have been successfully identified to form a seal at high pressure differential reaching up to 3000 psi. Such a seal integrity would make a very good potential candidate treatment for "A" formation.