Abstract

Cleanup of drilling fluid filter cake in long horizontal and multilateral wells is a difficult task. Both mechanical and chemical means (acids, oxidizers, chelating agents, and enzymes) have been used in the field. However, these methods have serious limitations and can adversely affect well performance. Acids and oxidizers are very reactive, but are nonspecific. Enzymes can be used to degrade starch and xanthan polymers. The major limitation of enzymes is their inability to stay active at temperatures above 200ºF.

The activity of enzymes at 207ºF is less than 10% of their activity at 200ºF. Previous studies to increase the stability enzymes were aimed at:

  1. changing salinity,

  2. changing the source of the enzymes,

  3. using additives that alter the conformation of enzymes, or

  4. chemically modifying the enzymes.

These techniques did not maintain the activity of the enzymes above 200ºF. For mudcake removal, enzymes have been used globally at higher temperatures, though their activity is known to be much less than 2%.

This paper examines the use of chemicals that work as a stabilizer for enzymes up to 250ºF. Optimized concentrations of this stabilizer not only inhibit the coagulation of enzymes at these high temperatures, but also maintain the activity of enzymes at the high temperatures that are encountered in the oilfield. Laboratory studies on the stability, compatibility, and mudcake damage reversal were conducted with the stabilized enzyme formulation.

Experimental results indicated that the new proprietary enzyme stabilizer is very effective in eliminating the deactivation tendency of enzymes at high temperatures. Conventionally, enzymes coagulate above 140ºF and char when exposed to temperatures above 200ºF, but in the presence of the stabilizer, this precipitation and burning were not observed. When using an enzyme at high temperatures without the stabilizer, the burned residue accumulated on the surface resulting in a thin film that reduced the permeability of the cake even further. Core flood studies using the enzyme stabilizer showed that this film was not formed, which resulted in a cake with much higher retained permeability.

Introduction

Leak off of drilling fluids to the formation can be very damaging, and, to minimize this, drilling fluids are often intentionally modified to form a coating on the wellbore called filter cake or mudcake. The major components typically found in filter cake are polymers such as starch and xanthan gum and solids such as weighting materials, drilling cuttings, and simple inorganic salts. Once drilling is completed and production is desired, this filter cake must be removed. If the filter cake damage is not removed prior to or during completion of the well, completion equipment failures and impairment in well productivity can occur. Conventional treatments for removing filter cake include: aqueous solutions of oxidizers (such as persulfates), hydrochloric acid solutions, organic (acetic, formic) acids, combinations of acid and oxidizers, and aqueous solutions containing enzymes. Enzymes are used to remove the biopolymers, xanthan and starches.

Hydraulic fracturing and gravel packing require the use of viscosified fluids to suspend and transport the gravel or proppant. However, whenever polymeric viscosifiers are used, some degree of formation damage is created which requires removal to optimize oil and gas production and recovery. Therefore, breakers, such as enzymes, are frequently employed to reduce or remove the effects of formation damage.

Benefits potentially associated with enzymes include polymer specificity; autocatalysis, which means just small amounts can be effective; and a better health, safety and environmental (HSE) profile than with chemical catalysts and oxidizers. Enzymes can be higher in molecular weight than oxidative breakers, so they tend not to leak off into the surrounding formation and can also be less susceptible to dramatic changes in activity by trace contaminants. The major limitation of enzymes is their instability at high temperatures. In this paper, the development of a new enzyme stabilizer for oilfield application is presented.

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