Oil-based mud is commonly used in drilling of the pay zones in sandstone formations as a less/non damaging fluid. Oil-based mud typically contain emulsifier, viscosifer and other additives including polymer blend and calcium carbonate to serve different functions. Presence of emulsifier may increase emulsion tendency upon interaction with downhole environment. The resulting emulsion might be tight to an extent that a thick sludge is formed which can impair well productivity. Identification of the sludge material will help in development of an effective chemical treatment to remove formation damage and restore well productivity.

In this study, an extensive laboratory work was conducted to explore potential interactions of an oil-based mud with different contaminants encountered in downhole environment. A typical sludge sample was characterized using different analytical techniques including solvent extraction, XRD, TGA, ICP and viscosity. The results showed that the sludge sample contained calcium carbonate, dolomite quartz as the main components in the inorganic phase while the organic phase include polymers and oil. The source of calcium carbonate might the drilling additives while the source of quartz is the formation. Analysis of supernatants generated from solubility tests conducted for the sludge sample revealed in addition to the high amount of calcium presence of iron in considerable amount (nearly 1,000 mg/L). Interaction of ferric chloride, quartz with an invert-emulsion mud was investigated. A significant increase in viscosity was observed upon incorporation of these contaminates with the mud sample. This paper presents in detail the results of interactions of iron ions, quartz, and pipe dope. It also examined several chemical formulations for removal of sludge formed and filtercake.

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