Abstract

Increased lifetime of chemical treatments is an important achievement both from an economical - field productivity issue as well as from an environmental point of view. Whatever type of reservoir potential enhanced treatment efficiency would be beneficial. Normally, this could be obtained by different strategies of employments, like precipitation squeezes, enhanced adsorption and bridging techniques.

However, in HPHT fields, e.g. field with temperature higher than 150°C, an additional obstacle would be the thermal stability of the chemicals. In nature, the thermal stability of products is often in conflict with or in contradiction to the property of high biodegradation, e.g. environmental friendliness.

In the present paper a new, multi-task method with a high throughput screening system for thermal ageing is presented. Combined with this set up is a new dynamic test -rig at HPHT conditions, directly testing the efficiency of the thermally aged products. Further analytical aspects of the degraded chemicals are analyzed on liquid-chromatographic mass spectroscopic, LC-MS, techniques. In order to gain more in-depth knowledge of the overall chemical efficiency, all data are evaluated using a multivariate analytical tool. This combined system is used for qualifying chemicals to be used in North Sea HPHT fields.

Three scale inhibitor products, two classified as yellow and one classified as a red chemical, were evaluated for field application.

In this paper, the detailed experimental data and a new procedure for thermal ageing tests will be presented.

Introduction

Our search for new oil and gas reserves takes us to deeper reservoirs, which means higher pressure and temperature. Therefore the characteristics of scale inhibitor (SI) chemicals with respect to thermal stability are becoming more and more relevant.

With Statoils zero discharge philosophy in mind, there will be an increasing need for effective, environmentally acceptable scale inhibitor chemicals. Also following the new imposed directive from The Norwegian Pollution Control Authority the petroleum industry has to switch to "green" chemicals by 2005. Green chemicals are substances used and discharged, which are considered to Pose Little Or No Risk (PLONOR)1 to the environment. To be classified as a green or yellow chemical there are stringent demands to toxicity and biodegradation. In order to be classified as a green chemical all components must figure on the PLONOR-list.

This will require two completely different properties from the chemical. If a chemical are to be effective at elevated temperatures, it has to be stable, or the degradation products have to be effective at this temperature. And if a product is stable, it is usually not that easily biodegradable.

The objective of this work has been to find and qualify chemicals for use at high temperatures. The different chemicals were exposed to high temperatures for a period of time, and afterward tested for their ability to prevent scaling. The chemicals were only tested for inhibition against barium and strontium scale, even though for HPHT applications, inhibition against calcium scale is more relevant. This method was chosen because the procedure for testing against barium scale gives better sensitivity in the results.

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