Abstract

Since developments in the biotechnological field contribute to the implementation of new enzyme breakers for removing polymer damage, significant improvements in well productivity have been proved mostly in horizontal and multilateral wells. Indeed, a delayed clean-up treatment is often required for removing the filtercake after drilling when the drawdown of the well is not enough. Conventional acid treatments promote a rapid, localized reaction resulting in a partial, non-uniform removal of the mud cake. Additionally more and more restrictive regulations push to find less corrosive and more environmentally safe clean-up options including low toxic oxidizing agents and enzyme breakers. Historically, the enzyme's applications have been limited to low temperature treatments due to their pH, salts and temperature concerns. Recently, new temperature-stable, polymer specific enzymes have been isolated and investigated for degrading the polysaccharides (starches and viscosifying agents) commonly used in the Drill-in Fluids.

This paper describes the laboratory evaluations performed on three new enzymes with the aim to develop a non-corrosive clean-up treatment specifically designed for degrading starches, xanthan gum and scleroglucan at temperatures reaching 100°C. Static break tests showed that these polymer-specific enzymes aggressively attack the polysaccharide backbone resulting in a complete break of the polymer. Nevertheless the treatment fluid do not have an immediate reactivity, which might ensure a uniform distribution of the fluid across long pay sections with minimal leak off during placement. Degradation is very effective and the generated fragments are small, soluble molecules. Besides the temperature and pH dependence, the influence of different heavy brines (CaCl2, CaBr2, etc.) on the enzyme activity and salting-out effect were also investigated. The optimal application concentration range for each brine was identified. By coreflooding tests the effectiveness of these breakers in removing the filtercake has been confirmed and the non-damaging properties of the degraded fragments assessed.

Introduction

Near wellbore damage remained in the well after drilling is one of the main causes of the underperformance of the horizontal and multilateral wells. The uses of drill-in fluids that form a thin and external filtercake favour the removal of the mud cake and accelerate the clean up operations. Nevertheless, only in some cases depending on the characteristics of the well (pressure, temperature, permeability), the filtercake removal is easily cleaned up without the need of any additional and expensive treatment.

Common practice to minimize such damage is the application of acid or strong oxidative breaker systems to dissolve the solids and biopolymers contained in the filtercake. The main issues using acid or oxidizer treatments are that they are extremely reactive and not specific to attack polymer-based filtercake. Because of their high reactivity these chemicals attack any available active sites on a polymer strand and many others materials that they contact in a well. Thus in most cases acid and oxidizers do not react with the polymer backbone or the correct linkage site and leave partially degraded or unreacted polymer strands in place or promote the crosslinking of different part of the polymer producing insoluble products, which have a greater potential to cause damage.

With the development of specific-enzyme breaker that are able to cleave a specific type of linkage on different types of polymer it became possible to target individual polymer groups with individual enzymes for complete degradation.1,2 These enzymes break the critical linkage sites by a catalytic process and produce simple sugar fragments which are soluble in aqueous fluids and result non damaging towards the reservoir.3 However the enzyme breakers are considered to be only mediocre agents for polymer degradation because of their pH, temperature and brine limitations. Consequently, their application was confined mostly to polymer viscosity reduction at low temperature 4 and sometimes to filtercake removal in horizontal wells.5

This paper focuses on the removal of the filtecake using three new specific enzymes stable also at high temperature and able to degrade not only starch but also xanthan gum and scleroglucan, commonly used to viscosify the drill-in fluids. The aim of the laboratory studies was to investigate two aspects: the effectiveness of these enzymes in dissolving the filtercake and the possibility to design a clean up strategy for a time delayed treatment.

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