Abstract

Since the early 1990's scale inhibitor precipitation technology has been routinely used throughout the North Sea to provide improved placement and extended treatment lifetimes when compared to conventional aqueous-based scale inhibitor squeeze treatments. Controlled precipitation technology is based upon the deployment of a specially formulated scale inhibitor package containing an organic additive that breaks down thermally and acts as a pH modifier resulting in in-situ scale inhibitor precipitation. However, the reliance on thermal degradation of the organic additive to cause precipitation means that this technology is limited to use in wells at temperatures > 85°C.

This paper describes the development of a low temperature, scale inhibitor precipitation delivery system. The new system is based upon the use of novel enzyme technology to break down the pH modifier using a non-thermal mechanism, thus allowing product deployment at low temperatures previously unavailable to the thermal degradation systems. Laboratory studies have so far indicated that the enzyme is effective at inducing scale inhibitor precipitation at both 40°C and 80°C.

A description of the laboratory development and evaluation of the new enzyme precipitation technology including enzyme assay, full analysis of the precipitation mechanism and core flood studies to evaluate formation damage potential and retention and release characteristics will be presented.

This paper will also highlight how the use of the enzyme precipitation process can increase squeeze lifetimes when compared to traditional adsorption squeeze treatments at low temperatures.

Introduction

Scale is usually defined as the precipitation of solid minerals from production fluids, a reaction that can occur anywhere from the reservoir to topside production facilities during production. The type and severity of scale formation is directly dependent on ionic and gas compositions of formation waters and injection waters, pressure, temperature and other environmental conditions. Each producing well can therefore potentially have it's own unique scaling regime. If left untreated, continuous buildup of scale can severely limit production potential, in some cases to the point where the system would have to be shut down temporarily to allow physical or chemical removal of the solid scale. In the majority of cases, especially in offshore conditions, this would not prove to be an economically feasible proposition, hence the need to prevent scale by the introduction of scale inhibitors into the production process. The most widely used method of combating the formation of scale downhole is to inject these scale inhibitor chemicals into the near-wellbore region, known as "squeezing", which prevents or slows the nucleation and growth of the scale crystals over time. In a typical aqueous treatment, a large volume of scale inhibitor, known as a "slug" is injected into the reservoir and retained in the formation by one of two main mechanisms:

  1. Adsorption by ionic charge to the rock surface (hydrogen bonding or calcium bridging)

  2. Precipitation of the inhibitor into rock pore spaces.

When production is resumed the scale inhibitor is released into the produced fluids to prevent/retard the formation of scale. The selection of type of squeeze treatment employed is dependant on a number of different different variables including the water chemisitry and scaling regime, the physical characteristics of the target well (completion type, temperature, pressure, porosity, lithology and production rates etc.) and the treatment lifetime required.

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