Formation Damages Caused by Emulsions During Drilling With Emulsified Drilling Fluids

Abstract The potential for macroemulsions to cause formation damages during drilling with emulsified drilling fluids (OBM) have been studied for consolidated low-medium permeable sandstone. Two types of emulsified drilling fluids were used, one with mineral base oil and one with synthetic base 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. The potential for emulsion invasion will be highest during the spurt period, especially if the emulsions in the muds are not stable. The risk for emulsion invasion will increase with filtration pressure. In bulk experiments, the stability of emulsions formed by 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 damages in core-floods at 90°C. The damages were partly removed during aging at higher temperatures (120°C and 150°C). At 90°C the emulsions appeared to be more stable in cores than in bulk. The potential for creation of emulsion in the oil reservoirs will be highest at high shear rates, e.g. in the spurt period and in the start of the oil production. The risk will increase with the concentrations of emulsifiers in the mud filtrate and reservoir oil. Formation damages caused by emulsions can be non-permanent. Since emulsions are thermodynamically unstable, the potential for permanent damage caused by emulsions will usually be lower at higher temperatures. Mechanisms for stabilisation and thereby also 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 in stead of the crude oil in laboratory experiments, can give incorrect estimates of the potentials for formation damages caused by emulsions. Short laboratory experiments can also give too high estimates of the potential for permanent emulsion related formation damages.