Impact of water-based drill-in fluids on solids invasion and damage characteristics

Drilling the U-formation (shale and sandstone) in Saudi Arabia requires high mud density (± 95 pcf) to mechanically stabilize the shale and balance the reservoir pressure. Two fluids have been used to drill this formation: KC1/BaSO₄/CaCO₃ and potassium formate/CaCO ₃. Barite is added in the first fluid with CaCO₃ to reduce the amount of solids needed to formulate the drill-in fluid. However, BaSO ₄ is insoluble in acids and requires chelating agents (DTPA) to remove it from the formation. Formate drill-in fluids with low solid content can be used, but they are expensive and corrosive at pH values less than 7-8. A third drilling fluid was recently developed by Al-Yami et al (2007) to overcome some of the problems associated with these two systems. The objective of this study is to determine solids invasion and damage characteristic for the three fluids. Core flood tests were conducted using reservoir cores at bottom hole conditions. Then the cores were examined by ESEM analysis to determine solids invasion, and characteristics of damage. The three drilling fluids examined did damage the core plugs either through external, internal filter cake, or both. The ESEM images showed that the least damage and minimum solids invasion occurred when Mn₃O₄ water-based drill-in fluid use tested. The highest damage was observed when barite/CaCO₃ mud was examined. This was followed by the potassium formate drill-in fluids. Kaolinite booklets were not attacked by the filtrate of the three drill-in-fluids examined. The damage noted with potassium formate mud was due to incompatibility of mud filtrate with the formation brine where potassium chloride crystals was noted in the core.