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.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.
© 2009. Society of Petroleum Engineers
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History
- Original manuscript received:
21 February 2006
- Meeting paper published:
28 February 2007
- Revised manuscript received:
5 December 2008
- Manuscript approved:
9 December 2008
- Published online:
1 June 2009
- Version of record:
1 June 2009
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