Rheological Modifiers in Drilling Fluids

Abstract One of the big issues in the oil industry is lost circulation. It occurs when the drilling fluid invades the formation (reservoir), and can happen in oil well drilling operations in a formation with fractures (natural or artificial), large pores, vugs and/or leaky faults. Lost circulation can cause well instability, since reduction of the fluid level in the annular space between the column and formation leads to a decrease in the hydrostatic pressure inside the well, possibilitating an undesired influx of formation fluids (kick). These fluids can travel from the formation to the surface (blowout), causing the collapse of the oil well. The goal of the present work is to study effect of additives as modifiers of the rheology of the drilling fluid, in order to reduce lost circulation. When a viscoelastic drilling fluid reaches a region of the formation with high porosity, the fluid elasticity is expected to increase the resistance to lost circulation. Aiming to analyze a drilling fluid with viscoelastic properties, two polymers (sodium carboxymethylcellulose and polyacrylamide) and one mineral (Laponite® - magnesium, lithium and sodium silicate) – commonly used in the oil and gas industry – were tested as rheological modifiers for a water-based drilling fluid. We performed steady shear, oscillatory shear and extensional rheological characterizations, and analyzed experimentally the entrance flow of the modified drilling fluids in a slit due to a pressure differential, to assess the flow rate reduction effect of the modifiers. We observed dramatic flow rate reductions, proving the effectiveness of the modifiers to control lost circulation.