Formation Damage Caused by Emulsions During Drilling With Emulsified Drilling Fluids

Abstract

Summary The potential for macroemulsions to cause formation damage during drilling with emulsified drilling fluids [oil-based mud (OBM)] has been studied for consolidated low-to-medium-permeability sandstone. Two types of emulsified drilling fluids were used, one with mineral-based (MB) oil and one with synthetic-based (SB) oil. The emulsions in tested OBM were found to be stabilized both by emulsifiers and particles (organoclay and drilled solids). At low shear rates, aggregates of water droplets and particles were observed by microscopy. In bulk experiments, the stability of emulsions formed by the mixing of mud filtrates and reservoir fluids was found to depend on the compositions of emulsified drilling fluid and crude oil. These emulsions were found to cause severe formation damage in corefloods at 90°C. The damages were partly removed during aging at higher temperatures (120 and 150°C). At 90°C, the emulsions appeared to be more stable in cores than in bulk. The potential for emulsion invasion will be highest during the spurt period, especially if the emulsions in the muds are not stable. The amount of emulsion invasion will increase with filtration pressure. Creation of emulsion in the oil reservoirs will be easiest at high shear rates (e.g., in the spurt period and in the start of the oil production) and high concentrations of emulsifiers in the mud filtrate and reservoir oil. The risk for emulsion invasion and creation of emulsion during drilling can be reduced by avoiding high overbalanced pressure and minimizing the fluid loss. The potential for the creation of emulsions during start of production can be reduced by starting with low drawdown. Formation damage caused by emulsions can be nonpermanent. Because emulsions are thermodynamically unstable, the potential for permanent damage caused by emulsions will usually be lower at higher temperatures (studied mud systems showed a change between 90 and 120°C). Mechanisms for stabilization and, thereby, also for breaking of emulsions depend on the compositions of the mud filtrate and the reservoir fluids. If the damaged area is swept by the produced oil, removal of emulsions by dilution with the oil can be possible. Use of synthetic oil instead of the crude oil in laboratory experiments can give incorrect estimates of the potential for formation damage caused by emulsions. Short laboratory experiments can also give excessively high estimates of the potential for permanent emulsion-related formation damage.