Abstract

A novel technique has been developed to detect the onset and increase of calcium carbonate scale deposition from oilfield production fluids. This method utilizes an attenuated total reflectance (ATR) probe, called the scale sensor, to detect scale formation in-situ. Because this method is based on existing technology and requires little supporting equipment, a simple cost-effective monitoring device could be placed in many locations within a production system. This method could provide a system wide, in-situ, real-time response to calcium carbonate scale formation at its earliest stages.

As a first step towards evaluating the effectiveness of this new technology, the scale sensor was field tested at a west Texas oilfield. The scale sensor was observed to detect the deposition of calcium carbonate under static and flowing conditions in actual produced fluids. In the static tests, crude oil, suspended solids and other endogenous materials did not affect the scale sensor response. Under flowing conditions, like those found in the production flow lines at the west Texas oilfield, scale sensor response was observed to correlate to the chemical injection rate of both the scale inhibitor and dispersant. Based on the results from the scale sensor, the scale inhibitor injection point will be moved to the wellhead shortly.

Introduction

Calcium carbonate scale commonly occurs in multiple locations within a production system. These locations include, but are not limited to, production wells, injection wells, flowlines, valves, vessels, separators, and pumps. Scale may form as a result of commingling, depressurization, or heating of the produced fluids contained therein. The result of scale deposition in oil and gas production systems is increased costs due to equipment failures, manpower-related maintenance of scaled equipment, and lost production. Application of a reliable real-time and in-situ sensor that could detect the onset and rate of calcium carbonate scale deposition could greatly reduce equipment failures by informing operators of an imminent scale problem. Corrective actions prompted by such an early warning would prevent excessive scale build-up on production equipment, thus reducing the rate of scale related failure rates.

At present, scaling tendency and scale occurrence in oilfield produced fluids are typically assessed by relying on analysis of produced water samples and/or using an in-situ scale coupon. Laboratory determinations of real-time scale deposition were recently reported using a rotating disk electrode (RDE) technique1, tapered optical fibers,2–4 and a "near real-time" sensor.5Recent literature regarding the on-line real-time detection of oilfield scale in actual production systems has included pulse neutron measurements,6 gamma ray attenuation,7 ultrasonic,8 nuclear attenuation,9 and a dual-energy-venturi multiphase flow measurement.10

Attenuated total reflection (ATR) spectroscopy is a direct measurement surface analysis technique well suited for oilfield use. This technique is ideally suited for obtaining spectroscopic information in direct contact with samples, which are either too thick or opaque for transmission spectroscopy. ATR is an energy-loss mechanism wherein incident light energy strikes the crystal-sample interface at such an angle that total internal reflection occurs. While a great proportion of the energy is reflected back into the crystal, a small portion of the energy is transferred from crystal surface to the sample medium.11 Because this technique requires that the measurement surface referred to as the crystal surface is in contact with the sample, this technique is also ideally suited for process type environments such as those found in oilfield production tubulars. Unlike the other aforementioned methods, ATR devices require very little supporting equipment. For example, the scale sensor used for this study was powered by a 5V-power supply.

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