Solids Laden Divalent-Based Drill-In Fluid Development for Pre-Salt Reservoirs Off-Shore Brazil

Abstract Drilling offshore pre-salt reservoirs offshore Brazil with water-based drill-in fluids (WB DIF) represents a challenge. The system must be reservoir friendly, lubricious, and capable of withstanding high differential-pressures in heterogeneous carbonate sections. WB DIF are desirable for offshore operations due to fewer environmental restrictions, lower cost, and carbon footprint. A CaCl2-based DIF was successfully designed and used offshore Brazil. Expanded density range was accomplished by using high concentrations of calcium carbonate avoiding environmental restrictions around CaBr2 brine. A unique chemistry package is used to suspend the high concentration of solids while providing optimum rheology. Fluid loss control and rheological properties are tailored to deliver efficient drilling performance. The system was designed for densities up to 13.5 lbm/gal using up to164 lbm/bbl of sized-CaCO3. Testing followed the API 13I recommended practices incorporating testing for fluid rheology, sagging, fluid loss and filtration control, and return permeability (RP) using pre-salt reservoir cores. The sized-CaCO3 bridging package was engineered to efficiently seal the targeted reservoir formation, resulting in minimum filtrate invasion, seepage loss and preventing reservoir formation damage. Moreover, the bridging package provided additional wellbore strengthening, preventing lost circulation problems and loss of well integrity. Fluid additives controlling viscosity were fine-tuned to avoid gelation issues at high pressure and temperature variation for the deepwater operation in off-shore Brazil. Measured fluid filtration loss volume was lower than 5 mL for disk sizes of 20, 55, and 120 microns with filter cake thickness less than 2 mm. Return permeability testing showed excellent results with returns higher than 70% without the use of breakers, and filter cake lift-off pressures around 4 psi. The WB DIF was successfully and easily introduced and scaled to field operations including mixing large fluid volumes at the liquid mud plant to offshore operations on the rig for fluid additions, maintenance, and storage. Minimal changes were observed when blending the product in the liquid mud plant. The low-end rheologies were slightly lower but did not affect the application in the field. The fluid was capable to stand over 250 hours on static conditions while logging with no issues with sagging, separation, or density decreased in the wellbore. The divalent brine-based fluid with a high load of solids has an immense potential for being used to drill a wide range of pre-salt high-pressure wells while maintaining the reservoir integrity and minimizing damage. WB DIF are desirable for offshore operations since they allow discharge, minimizing environmental constraints, and lowering overall cost and carbon footprint.