Abstract

This paper describes the use of pulsed photoacoustics to measure ppm concentrations of oil in treated/separated water from subsea and downhole separators. The principle of operation and the system for the pulsed photoacoustic Oil-In-Water Monitor (OIWM) prototype is described. Main test results for the prototype sensor are presented, with focus on the impact on measurement by changes in temperature, pressure and salinity. Following, a model used to compensate such variations is shown. The paper finally addresses the implications of using pulsed photoacoustic measurement principle to measure oil entrainment in produced water from subsea and downhole separators.

Introduction

Significant quantities of produced water are recovered as a bi-product of offshore production of gas and oil. The produced water is obtained from two main sources; formation water from the reservoir and seawater injected into the reservoir during oil and gas production. The produced water is either disposed to sea or re-injected into the reservoir.

A positive verification of the purity of the treated/separated water is an essential parameter for water treatment and separation plants. The industry has for some time evaluated various methods to find a potential method for further development into an in-line, continuous monitoring method for field applications top-side, sub-sea and downhole to replace the infrared spectrophotometric method1 frequently used today, in this paper referred to as IR-analysis. This procedure describes a method where samples are taken manually for laboratory analysis. For subsea separation this implies the mobilisation of a ROV (Remote Operating Vehicle), which is a costly and difficult operation. The availability of a sensor for inline, continuos measurement of the water quality from subsea and downhole separators can drastically reduce the operational costs while improving the control of the separation process - which means less use of chemicals. A pulsed photoacoustic monitoring system can provide an attractive alternatives to today's method as it can be designed to operate inline to continuously give the oil concentration in produced water from topside, subsea and downhole separators. This method has today reached a prototype stage.

The Oil-In-Water Monitor (OIWM) system described in this article is based on pulsed photoacoustic principle2–5. The principle of operation is illustrated in Figure 1. Pulsed laser light is absorbed by the oil in the water stream. When the light is absorbed the optical energy is dissipated as heat which causes sudden local heating. The subsequent thermal expansion generates a pressure wave, which is detected with an acoustic detector. The amplitude of the acoustic peak is proportional to the oil concentration. Hence, by measuring the amplitude one can find the oil concentration.

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