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.
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.