Biotechnological Treatment Removes Xanthan-Based Skin Damage
Authors: TjonJoePin Robert M. , Brannon Harold D. , Beall Brian B. 
Published: December 2007

Abstract
Conventional reservoir drilling fluids typically consist of xanthan-based polymers to optimize recovery of drill cuttings and reduce friction or torque during the drilling operations. Previously conducted laboratory studies have indicated that the damage produced by these types of xanthan-based drilling fluids can be easily removed. Field data, on the other hand, has indicated that substantial amounts of polymeric damage remain in the formation. Even though these psuedoplastic, non-thixotropic, xanthan-based drilling fluids exhibit low shear rheologies during the operations, they can contribute to tremendous amounts of skin damage. This insufficient degradation of filter cake may impede flow and dramatically reduce formation permeability hence well productivity.
Recent extensive research was conducted to develop a treatment which could effectively reduce this type of polymeric damage. A novel biotechnological treatment has been developed, which effectively removes xanthan-based filter cakes or skin damage at temperatures from 70 F to well over 250 F.
Core flow evaluations have shown that multi-fold improvements are achieved with this optimized biotechnological removal treatment. A detailed study of the laboratory and field case histories are presented. Average incremental production rates of the treated wells were significantly improved, clearly demonstrating the effectiveness of this removal system.

Introduction
The practice of drilling wells in horizontal or highly deviated configurations, as well as multilateral completions, has developed rapidly in recent times. The purpose of this activity is to contact more hydrocarbon-bearing pay zone area within a single well in order to maximize productivity. Such wellbores often penetrate thousands of feet of productive zone as opposed to the tens to hundreds of feet contacted in vertical well configurations.
The fluids historically utilized in drilling applications for lubrication and cuttings transport typically contain high concentrations of clays such as bentonite. These are known to cause damage to the permeability of the near wellbore area due to leakoff and mudcake deposition on the face of the production zone. Thus, the formation damage can be related to both the filter cake and the filtrate that invades the productive zone. It is often necessary to apply stimulation treatments to bypass the drilling fluid damage in such intervals.
Insufficient degradation of the filter cakes resulting from these drilling fluids can significantly impede flow capacity at the wellbore wall. This reduced flow capacity can result in significant reduction of the well productivity or injectivity. Formation damage from drilling fluid leaking off into the formation, as well as filter-cake impairment, must be eliminated to realize the full potential of horizontal completions.
A common approach to minimizing such damage is the application of acids or strong oxidative breaker systems to dissolve filter-cake solids and polymers. The typical wellbore treatment to remove damage consists of either hydrochloric acid solutions, solutions of lithium or sodium hypochlorite or highly concentrated solutions of conventional oxidizers like anmonium persulfate or perborate. Acids and oxidative solution washes appear to perform reasonably well in the laboratory environment where contact of the filter-cake damage with the reactive solution is easily achieved. However, previous testing has indicated that application of acids or oxidative solutions may not be effective for removing the damage in horizontal intervals. Field experience has demonstrated that acids and oxidative solutions used to remove drilling mud filter cake have proven relatively ineffective based upon well performance. The problem is particularly evident when such treatments are applied in extended length openhole intervals. The primary problem is thought to be contact of the reactive solution with the drilling mud damage. Studies by Burnett indicated that the polymer-coated carbonate particles used for weighting and fluid loss control can be resistant to acid attack and prevent complete removal of the filter cake. Additional concerns regarding the use of acidic or oxidative cleanup treatments include the reactivity with tubulars which result in elevated iron concentrations injected into reservoir which can promote sludging problems. A new technique was recently disclosed to address the filter-cake damage produced by drill-in fluids in openhole horizontal wells. The technique involves the use of viscosified, brine-based, drill-in fluids with acid or water-soluble weighting and loss circulation control agents. P. 425^