An effective fluid formulation to remove drilling-fluid mudcake in horizontal and multilateral wells

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 degrees C). Surface tension was measured at various temperatures up to 140 degrees 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.