Summary
Cleanup of drilling-fluid filter cake in long horizontal and multilateral
wells is a difficult task. Both mechanical (water jetting) and chemical
means (acids, oxidizers, chelating agents, and enzymes) have been used in the
field. However, these methods have limitations which can adversely affect well
performance. Water jetting may adversely impact well productivity through
sanding and water blockage. Acids and oxidizers are very reactive but
nonspecific species. Enzymes can be used to degrade starch and xanthan
polymers. However, they are not effective in heterogeneous formations with
high-permeability streaks which will require large volumes of treatment
fluids.
This paper examines the use of a combination of enzymes and a new class of
viscoelastic surfactants. In this system, enzymes are used to degrade starch,
whereas the viscoelastic surfactant is used to increase the viscosity and
solids-carrying capacity of the treatment fluids.
Extensive lab studies were conducted to examine the compatibility of
viscoelastic surfactants with enzymes. The apparent viscosity of the
enzyme/viscoelastic surfactant solutions was measured as a function of shear
rate (57 to 1,740 s–1) and temperature (25 to 100°C). Surface
tension was measured at various temperatures up to 140°C. Static fluid-loss
tests were performed in a high-pressure/high-temperature (HP/HT) cell to assess
the effectiveness of the combined system in cleaning filter cakes formed by
water-based drilling mud.
Experimental results indicated that the viscoelastic surfactant is
compatible with the enzyme system. The low shear viscosity of the combined
system is high enough to lift suspended solids during well flowback following
the treatment. The addition of the viscoelastic surfactant to the enzyme
solutions significantly reduced the surface tension of the enzyme solutions,
which will shorten the time required to lift treatment fluids from the
formation. The efficiency of the enzymes in degrading starch did not change
significantly because of the presence of the viscoelastic surfactant.
Introduction
Colloidal starch and its derivatives are widely used in water-based drilling
fluids. This is because of the ability of starch to reduce rapidly the
permeability of mudcake, thereby reducing the invasion of the filtrate
containing damaging water-soluble polymers (such as xanthan gum or
scleroglucan), bridging agents, and drilled solids into the formation.
However, starch and other filter-cake components can cause significant
impairment of well performance. Formation damage treatments need to be
performed to re-establish the native permeability of the critical wellbore
area.
Typical constituents of water-based drilling fluids are sized calcium
carbonate or salt particles, xanthan, starch, biocide, potassium chloride,
potassium hydroxide, sodium sulfite, defoamer, and lubricant. The
drilling-mud filter cake can be removed by acids, oxidizers, chelating agents,
enzymes, or combinations of these chemicals. Lab studies, field applications,
and the advantages and disadvantages of these systems have been discussed in
previous publications. (Moore et al. 1996; Jones and Davies 1996; Beall
et al. 1997; O’Driscoll et al. 1998; Suhy and Harris 1998; Parlar et al.
1998; Stanley et al. 1999; Hembling et al. 2000; Malone et al. 2000; Biezen et
al. 2000; Tibbles et al. 2000; Todd et al. 2004; Al-Otaibi et al. 2004a;
Al-Otaibi et al. 2004b).
An acid wash can dissolve most of the materials present in mudcake.
Fast reaction of the acid with calcium carbonate results in penetration of the
filter cake where the acid first contacts the cake and can result in uneven
removal of the mudcake.Some minerals present in sandstone formations (e.g.,
illites) are sensitive to acids. Acid leakoff into the formation can
cause fines migration and formation damage.
In addition to mineral acids, polymeric material present in the filter cake
can be removed by oxidizers or enzymes. Similarly to acids, oxidizers are very
active chemical species. However, they cannot be used in sour environments
because of potential oxidation of hydrogen sulfides into elemental sulfur.
Enzymes have been used to remove polymeric material for more than 10 years
(Moore et al. 1996; Beall et al. 1997; Stanley et al. 1999; Hembling et al.
2000; Malone et al. 2000; Al-Otaibi et al. 2004a). Enzymes will not dissolve
calcium carbonate particles. These particles can be removed during the natural
flow of the well, especially if the reservoir pressure is high. However, an
acid wash is needed if the reservoir pressure is low and these particles are
causing skin damage.
© 2007. Society of Petroleum Engineers
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History
- Original manuscript received:
2 November 2004
- Revised manuscript received:
24 June 2006
- Manuscript approved:
12 December 2006
- Version of record:
20 March 2007
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