Long horizontal and multi-lateral wells continue to be used in applications requiring high production rate. However, many of these wells suffer from low productivity due to incomplete filter cake removal. The obstacles encountered are the difficulty to ensure contact of the cleaning fluids with the filter cake throughout the whole interval, as well as controlling the reaction rate between the cleanup fluids and the filter cake to assure uniform dissolution.

The filter cake formed by water-based mud consists of XC-polymer, starch, CaCO3 particles and drilling cuttings. Today several chemical techniques (oxidizers, enzymes and acids) are available to break the polymers and dissolve CaCO3 particles that are present in the filter cake. The limitations of these fluids are fast reaction rates and corrosiveness provided by acids, especially at high temperatures. Also, cleaning fluids are not effective in heterogeneous formations where there are high permeability streaks, which will require using large volumes of the treatment fluids, but with poor performance.

One way to overcome problems associated with the heterogeneous nature of oil and gas reservoirs is to increase the viscosity of the cleaning fluids. This was addressed in the present study by adding a viscoelastic surfactant to the cleaning fluids (mainly enzymes).

Laboratory studies were conducted to examine the effect of viscoelastic surfactants on the performance of specific enzymes (used to break XC-polymer and starch). The apparent viscosity of the combined solution was measured as a function of shear rate (10 to 1,000 s-1) and temperature (77 to 212°F). A modified HPHT fluid loss cell was used to assess the effectiveness of the combined system in cleaning filter cake formed by water-based drilling mud. The effects of temperature, enzyme type and concentration; and surfactant concentration were investigated. Experimental results showed that the surfactant increased the viscosity of the solution. As a result, the rate of polymer degradation by enzymes had decreased. Solutions that contain enzymes and viscoelastic surfactants can give a uniform distribution over the whole interval, which will result in higher production rates. This paper will discuss various interactions of surfactant-enzyme systems and address the advantages and limitations of this system.

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