Abstract

The use of water-in-oil emulsions (w/o) to deploy scale inhibitors has been reported in the literature as an alternative to water-based squeeze treatments. The non-aqueous nature of these emulsions has the advantage to prevent water blocking, which adversely affects oil production in aqueous squeeze treatments. Placing the scale inhibitor in a w/o or "invert" emulsion has shown in some cases the additional advantage of enhancing treatment lifetime. However, results from the literature seem contradictory and highlight a poor understanding of this technology.

The present paper aims at providing further insight on emulsified scale inhibitor placement in porous media. Preliminary experiments, using a low molecular-weight biopolymer as scale inhibitor, showed low adsorption/retention in aqueous solution. Re-formulation of the product under invert emulsion was investigated to enhance inhibitor retention.

Results from coreflood experiments, in well-characterized silicon carbide (SiC) packs provided preliminary evidence of aqueous droplet adsorption as the main retention mechanism in porous media. This was expected considering the average droplet size of 0.3 µm. The mother formulation of the w/o emulsion is a concentrate, containing 80% weight of water phase and 8% weight of active scale inhibitor. It can be diluted down to 2% water phase adding the desired oil phase without loosing stability or increasing the droplet size.

These results are a promising first step towards the development of a technically and commercially viable, environment-friendly scale inhibitor technology based on w/o emulsions.

Introduction

The formation of mineral scale in production facilities is a relatively common problem in the oil industry. Most scale forms either by pressure and temperature changes that favor salt precipitation from formation waters, or when two incompatible waters mix during pressure maintainance or waterflood strategies. Scale prevention is achieved by performing "Squeeze" treatments in which chemical scale inhibitors are injected into the producer near wellbore.

Once the scale inhibitor is injected, it is retained in the porous matrix and hopefully it will be returned with produced fluids in slow enough rates to assure long treatment durability and lifetime. Many conventional squeeze treatments, where scale inhibitors are injected in aqueous solution, show lifetimes that are occasionally short (3–6 months). This, together with differed oil production observed after the treatments, adversely affects oilfield economics.[1,2] Differed oil is by far the more detrimental of the two. It is caused by invasion of the aqueous solution in the near wellbore, creating a highly water-saturated region where oil flow is greatly impaired. This effect is commonly known in formation damage literature as " water blocking".[3]

To overcome these problems, one of the many approaches reported in the literature is the injection of oil continous squeeze treatments. These treatments use either oil-soluble scale inhibitors[4,5] or Emulsified Scale Inhibitors (ESI). The latter are the subject of this paper, where scale inhibitor-brine solution is dispersed in a continuous oil phase, forming what is known as a water-in-oil or "invert" emulsion. [6–11]

In this paper we present a brief literature review of previous work published on ESI squeeze treatments, followed by the description of a new invert emulsion formulation, developed and currently under study in our laboratory.

ESI injectivity behavior and droplet retention mecanisms for this new ESI formulation are discussed. The product is developed entirely with biodegradable products, to comply with stricter environmental legislation currently under way for the North Sea region.

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