Case Study: Barium Sulfate Scale Removal by Dynamic Underbalance

Abstract

Abstract Petroleum wells producing water are likely to develop deposits of inorganic scales that may form near the wellbore and may plug perforations, coat casing, production tubulars, valves, deteriorate pump performance, and affect downhole completion equipment. Scales form and precipitate because the solution equilibrium of water is disturbed by pressure and temperature changes, dissolved gases or incompatibility between mixing waters. If scale formation and precipitation are allowed to proceed, scaling will limit production, eventually requiring abandonment of the well. In order to remove the effects of scale on production after a well undergoes sharp or early decline in production, it is essential to first determine which scales are forming and where they are forming. Some of this information can be reliably inferred from computer simulation procedures or by running calipers down the wellbore and measure decreases in the tubing inner diameter so that the scale can be physically detected. Gamma ray log interpretation may also be used to detect barium sulfate scale because naturally radioactive radium precipitates as an insoluble sulfate with this scale. Scale remediation techniques must be quick and nondamaging to the wellbore, tubing, and the reservoir. If the scale is in the wellbore, it can be removed mechanically or dissolved chemically. Selecting the best scale-removal technique for a particular well depends on knowing the type and quantity of scale, its physical composition, and its texture. Mechanical methods such as Dynamic Underbalance Pressure (DUP) technique are among the promising methods of scale removal in tubulars and across perforations. The purpose of this work is to present a case study of removing barium sulfate (BaSO4) scales from perforation tunnels utilizing dynamic underbalance technique. Wells from a North African oil field were selected for designed and optimized dynamic underbalance treatments to remove barium sulfate scales that precipitated in the perforation tunnels, preventing hydrocarbons flow from the formation to the production tubing. Gamma ray log and production logging tool were used before the treatment to detect and evaluate the type of scale and the intervals affected. Then the same tools were used after the treatment to assess stimulation taking place in the wells. Data obtained from the treatment was used to develop a model for predicting productivity index/inflow performance relationships. The dynamic underbalance technique successfully removed scale from all targeted wells, leading to an increase in oil production, without killing them (i.e. while still in production). Some wells achieved increase in oil production after the treatment of up to 65%. A predictive model was developed in order to estimate the performance of an underbalance scale removal treatment.