Traditionally, the Barite (Specific Gravity of about 4.2-4.7) has been using as the weighting agent in conventional drilling fluid. But, the drilling of oil and gas wells from surface to the pay-zone with conventional drilling fluids may damage the producing interval. The formation plugging by drilling fluid’s compositional solids, drilled cuttings and polymers’ particles;hydration of clay envelop around pay zone particles by filtrate; formation of scales due to chemical reaction between formation fluid and mud filtrate are the most common damage mechanisms attributed to drilling fluid. So,to counter the damage, an ideal mud should not use of non-degradable compositional fine solids; must have minimum drilled fine solids; should reduce the filtration loss; and should generate inhibitive filtrate.Effective field development needs high quality drilling fluid to minimize formation damage and maximize productivity. This paper discusses the development of least damaging drilling fluid basically for development wells in the Upper Assam Basin of India. The idea behind the development of drilling fluid was to avoid fines and polymer plugging by optimizing the Particle Size Distribution (PSD) of fine and medium sized particles of calcium carbonate. The laboratory results have shown the effectiveness of sized particles of CaCo3 in Non Damaging Drilling Fluid (NDDF) to bridge the pore throat on the formation surface to build an external filter cake which is much easier to be removed than an internal filter cake based on the Particle Size Distribution of CaCo3 and Pore-throat diameter study of the sandstone reservoirs of some oilfields of Upper Assam Basin. CaCo3(Specific Gravity of about 2.7-2.8)is also used as the weighting agent in NDDF which is acid soluble and can be removed easily later on. The optimum composition of CaCo3also designed based on the interpretation of the mud properties of laboratory formulated NDDF and required mud properties for successful drilling and completion in the study areas.
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