Smart Drill-In Fluid and Breaker Design for a Limestone Reservoir for Persian Gulf Offshore Wells

Offshore reservoirs in the Southwest Persian Gulf are commonly oil-wet limestone with an average permeability of 10 md. High production of hydrogen sulfide and carbon dioxide is often encountered in the oil producer wells. The tight reservoirs are commonly drilled with water-based reservoir drill-in fluid (DIF) with high concentrations of lubricants. DIFs based on sodium chloride or calcium chloride brines with corresponding optimal breakers to remove the filter cakes were formulated and evaluated to optimize production in newly drilled wells. Fluid displacement by return permeability (RP) testing was used to evaluate the fluid/limestone rock interaction. This paper discusses the compatibility of a sodium chloride-based and a calcium chloride-based DIF with limestone formation and the necessity of introducing an optimal breaker to maximize the opportunity to achieve high production rates. RP tests are widely used to determine the potential damage caused by the DIF and production enhancement after removing the DIF filter cake with a breaker. Desired results for RP tests performed with the brine-based DIF in limestone cores were a minimum of 75% regain permeability to oil production. The cores used for the RP tests were from an analogue limestone outcrop from a Mississippian formation with permeability between 9-16 md and 14-18% porosity. DIF properties were determined following API RP-13I recommended practices. Emulsion tendency for the fluids was determined by using emulsion tendency testing with a high-speed mixer to mimic shear at the pore throat. A 10.0 lb/gal sodium chloride water-based DIF with a high content of ester-based lubricant was designed for drilling a limestone formation. A high pH close to 10 was necessary to control H2S and CO2 corrosion. The return permeability of the 10.0 lb/gal fluid was 44% using LVT-200 oil as an analogue for the native hydrocarbon permeating fluid. The low return permeability was likely caused by emulsion blockages generated by the saponification of the ester-based lubricant used in the sodium chloride-based DIF. Emulsion tendency was observed between the DIF filtrate and permeating fluid in a fluid/fluid compatibility evaluation. Therefore, a breaker system was formulated and customized to enhance RP from 44% to a minimum of 75%. In contrast, a 11.0 lb/gal calcium chloride-based DIF with pH of 9.0 and same ester-based lubricant content was evaluated using a comparable limestone analogue core and demonstrated a high return permeability (>80%). Filtrate of the calcium chloride-based DIF did not form emulsions during fluid displacement in the RP test. Compatibility evaluation (return permeability) between drill-in fluids and reservoir rock is essential for oil producer wells in order to determine and avoid potential problems caused by interactions between them.