Many horizontal wells are drilled using the clean "drill-in fluids" introduced in recent years. The drill-in fluids are typically comprised of either starch or cellulose polymers, xanthan polymer, and sized carbonate or salt particulates. They were introduced to minimize the mud damage to the wellbore relative to that typically observed with conventional drilling muds. However, testing and experience have shown that insufficient degradation of the filter cakes resulting from even these "clean" drill-in fluids can significantly impede capacity at the wellbore wall. This reduced flow capacity can result in reduced well productivity or injectivity. Consequently, wellbore acid or oxidizer treatments are typically applied in attempts to remove or "bypass" the filter cake. These treatments are often only marginally successful, particularly when applied in extended length intervals. Formation damage, as well as filter-cake impairment, must be eliminated to realize the full potential of horizontal completions.
Previous laboratory studies have demonstrated that drill-in fluid filter cake can be effectively removed through the application of a newly developed technique incorporating an enzyme-based polymer degradation system.1 This treatment can be designed to degrade xanthan-based, starch-based or cellulose-based drill-in fluid. Following degradation of the specific polymer, the weighting and/or fluid loss control material is easily removed using smaller, less costly acid treatments.
Case histories of wells treated with the new enzyme technology compared to direct offsets treated with conventional acid or oxidative breaker treatments will be presented. Field experience has shown that through utilization of this new technology, smaller, less costly treatments can be used to treat openhole intervals to zero-skin potential with dramatically improved treatment efficiency. Production results have indicated that three-fold increases are possible when applying a two-stage enzyme/acid treatment to acid or oxidative treatments alone. Post-treatment production logs have indicated not only increased flow, but also flow throughout the entire openhole interval.