In-Line Measurement of PPM Concentration of Oil Contained in Produced Water Streams
Authors: Whitaker Timothy S. , Terzoudi Voula 
Published: October 2008

As operators seek to re-inject produced waters back into reservoirs for disposal it is critical to be able to measure the concentration of oil remaining within the produced water. Excessive oil concentration can reduce the permeability of the reservoir, hinder re-injection, and prevent full recovery of the hydrocarbons.

As subsea processing systems become a realistic alternative to traditional methods, instrumentation is required which will measure the concentration of oil in re-injection water. The measurement is extremely useful in providing information to assist in optimising the separation process, and also provides an alarm to indicate when process upsets may cause problems in the reservoir.

An on-line oil-in-water monitor, which exploits the wellknown photo-acoustic response of hydrocarbons to measure oil concentration, is described. Independent tests of an advanced prototype monitor to determine the effects of common environmental factors on the measurement are discussed.

The response of the monitor is then discussed by comparison with traditional techniques, such as IR analysis. The relative merits of sample based techniques and on-line measurement are illustrated, with particular emphasis on the operability aspects of such instrumentation in a subsea environment.

Introduction

In the field of environmental monitoring there is increasing interest in the monitoring of hydrocarbons, which may be found in the marine environment as pollutants from the offshore oil-production process. Produced water is a byproduct of crude oil production, which is either discharged over board from offshore installations or is re-injected into the crude oil reservoir, and is obtained from two main sources: seawater injected into the reservoir during oil production processes and naturally occurring reservoir formation water. Produced water is known to contain varying concentrations of crude oil, production chemicals, solids and gases, and its disposal introduces environmental, operational, and economic considerations.

A current interest in offshore oil production is towards subsea facilities that utilise the re-injection of produced water. This offers both environmental advantages because it avoids the disposal into the sea and economic benefits because it extends the cost-effective lifetime of established fields and allows exploration and production in more inhospitable regions.

Current methods used for monitoring concentration of oilin-water offshore use sampling techniques to either pass a sample to a measurement cell or remove a sample to a laboratory for analysis. These techniques are prey to errors in obtaining a sample which is representative of the flowing stream, and in conditioning the sample for analysis.

The standard technique for analysis of hydrocarbons uses near infra red (NIR) transmission spectroscopy and requires pre-treatment of samples. Further complications are caused by the presence of background absorption by water in this spectral region. Although these difficulties may be overcome in the laboratory environment, the technique of transmission spectroscopy does not transfer successfully into industrial online measurement requirements.

An alternative technique, a photo-acoustic monitoring system has been developed for applications requiring in-line monitoring.