Abstract

Since the middle 1980s, seawater breakthrough has been observed for a number of wells in a major offshore Field operated by Statoil. This Field has also several tie-ins. Both sulphate and carbonate scale deposition has been identified in the near well bore area and tubing. Inhibitor squeeze treatments were regularly carried out to prevent wells from damages due to scale precipitation. The conventional phosphonate scale inhibitor (DETA-phosphonate) was often used. Some of the sandstone formations in the oil field are relatively "clean" with very little amount of clay materials i.e. Etive and Tarbert formation. As a result, a relatively short inhibitor squeeze life was seen in these formations after the squeeze treatments with a phosphonate scale inhibitor. Other reservoirs had acceptable squeeze life, but due to environmental requirements a wide set of inhibitors was studied.

In order to reduce the well intervention frequency and extend an inhibitor squeeze life, a method involving a polymer interaction between a polymer scale inhibitor and polymer additive has been developed. In addition to the enhanced adsorption; attributed by the surface charge modification by the adsorption of the positively charged polymer additive on the sandstone surface, the interactions between polymer inhibitor and additive further increase the inhibitor retention in the formation. This polymer interaction approach is different from a conventional precipitation squeeze where calcium chloride was used. Also, unlike the conventional precipitation squeeze, the polymer additive can be pre-injected and rapidly adsorbed onto a rock surface. The subsequently injected inhibitor will react with the additive on the rock surfaces, resulting in a precipitation on a rock surface. This avoids the bulk precipitations where a permeability reduction was often caused by a conventional precipitation squeeze. During the production, the precipitated polymer inhibitor was released through a hydrolysis process.

This paper presents a detailed method and field results based on the treatments using the polymer inhibitor and additive package in the oil field. The application of polymer interaction package significantly improved the inhibitor squeeze life. Satisfactory results have been achieved from the field trials. The unique polymer reaction package satisfies the stringent Norwegian environmental regulations of low toxicity, high biodegradation and low bioaccumulation.

Introduction

The Field is located in the North Sea, in the northern portion of the Viking Graben and straddles the border between the Norwegian and UK sectors. The Field was discovered in 1973, declared commercial in August 1974, and started production in 1979. The field has been developed with three condeep production platforms. All of them have concrete gravity base structures incorporating storage cells. Production is from the Brent, Dunlin, and Statfjord reservoirs, with the main reserves in the Brent and Statfjord reservoirs.

Two of the satellite fields are involved in the field scale inhibitor squeeze treatments. The satellites are located 17 and 6 kilometers north and east of the main Field. They are tied back to the main Field through sub sea flowlines.

Typical formation brine chemistry is displayed in Table 1. After seawater breakthrough, both sulphate and carbonate scale has been identified in the near well bore areas and tubing. [1, 2, 3]

More recently scale build up in the 7″ re-entry guide was confirmed during caliper logging in June 2003 in one of the wells. Severe scale build up was detected in the wire line re-entry guide, which prevented access to the formation interval. The scale control strategy for the field has been a combined scale dissolver and scale inhibitor squeeze treatment.

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