Abstract

Cleanup of drilling fluid filter-cake in long horizontal and multilateralwells is a difficult task. Both mechanical (water jetting) and chemical means(acids, oxidizers, and enzymes) have been used in the field. However, thesemethods have limitations, which can adversely affect well performance. Waterjetting introduces large amounts of water into the formation, and can causeloss of well productivity through sanding and water blockage. Acids andoxidizers are very reactive and non-specific species. Enzymes can be used todegrade starch and xathan polymers. However, they are not effective inheterogeneous formations where there are high permeability streaks, which willrequire using large volumes of the treatment fluids, but with poorperformance.

This paper examines using a combination of enzymes and a new class ofviscoelastic surfactants. In this system, enzymes are used to degrade starch, whereas the visocelastic surfactant is used to increase the solutionviscosity.

Extensive lab studies were conducted to examine compatibility ofvisocelastic surfactants with enzymes. The apparent viscosity of theenzyme/viscoleastic surfactant solutions was measured as a function of shearrate (57 to 1,740 s−1) and temperature (77 to 212°F). Surfacetension was measured at various temperatures up to 284°F. Static (a HPHT fluidloss cell) tests were performed to assess the effectiveness of the combinedsystem in cleaning filter cake formed by water-based drilling mud.

Experimental results indicated that the viscoelastic surfactant iscompatible with the enzyme system. The low shear viscosity of the combinedsystem is high enough to lift suspended solution during well flowback followingthe treatment. The addition of the viscoelastic surfactant to the enzymesolutions significantly reduced the surface tension of the enzyme solutions.This will reduce the time required to lift treatment fluids from the formation.The efficiency of the enzymes in degrading the polymers did not significantlychange due to the presence of the surfactant.

Introduction

Colloidal starch and its derivatives are generally used in water-basedreservoir drilling fluids. This is due to the ability of starch to rapidlyreduce the mud cake permeability, thereby reducing the invasion of the filtratecontaining damaging water-soluble polymers (such as xanthan gum orscleroglucan), bridging agents and drilled solids into the reservoir. However, starch and other filter-cake components can cause significant impairment ofwell productivity. Formation damage treatments need to be performed toreestablish the native permeability of the near wellbore area.

Typical constituents of water-based drilling fluids are sized calciumcarbonate, XC-polymer, starch, biocide, potassium chloride, potassiumhydroxide, sodium sulfite, defoamer, and lubricant. The drilling mud filtercake can be removed by acids, oxidizers or enzymes. Lab studies, fieldapplications, advantages and disadvantages of these systems were discussed inprevious publications.1–14

An acid wash can dissolve most of the materials present in mud cake. Fastreaction of the acid with calcium carbonate results in the penetration thefilter cake where the acid first contacted and can result in uneven removal ofthe mud cake. Some minerals present in sandstone formations (e.g., illites) aresensitive to acids. Acid leak-off into the formation can cause fines migrationand formation damage.

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