This paper presents a technology for water-based drilling fluid that is weighted with calcium carbonate. This technology has both functions of drilling and completion fluids and can reach to the total depth of MRC wells. It creates a filter cake that prevents fluid leak off and formation damage during the drilling process, but readily cleans up the filter cake over time. This technology is based on the use of an ester, in a solid form, which hydrolyzes and produces an organic acid. The latter will dissolve calcium carbonate particles that are used as a weighting material in water-based drilling fluids. Polylactic acid (PLA) particles were used in the present study to generate lactic acid.
Experiments were conducted to find an optimum particle size of the solid ester and calcium carbonate particles that should be selected as a compromise between those that are optimal for fluid loss control or filter cake formation and those that are optimal for self-destruction at the desired time and rate. Other set of experiments were conducted to determine the temperature and soaking time required for the solid ester to hydrolyze and for the resulting acid to dissolve calcium carbonate particles.
All experiments showed that in a controllable-condition of temperature, acid concentration, and particle size the clean up efficiency exceeded 70%. The optimum particle size was found to be 50 µm calcium carbonate and 150 µm PLA. The optimum temperature was 230°F for running experiments due to high filter cake removal efficiency, low filtrate volume and not having returned mud high viscosity problems. Enough soaking time is needed for PLA to be hydrolyzed and for the acid to remove the filter cake (20 hours).