Measurement Of Steam Quality Using A Neutron Densitometer
Authors: Wan P.T. 
Published: March 2010

The need for reliable and cost-effective methods to monitor steam quality at the wellhead of steam-injection projects has existed for many years. A neutron densitometer, based on the principle of thermal neutron transmission, has been designed and calibrated to measure steam quality at the wellhead. The meter is non-intrusive, portable and safe/or field use. By combining the meter with a flow measuring device, such as a flow nozzle the steam quality and mass flow rate at the wellhead can be determined.

This paper describes the principle of operation of the neutron densitometer for monitoring steam quality. Because of the large mass attenuation coefficient of thermal neutrons in water, thermal neutrons are well suited for measuring the high void fraction (or low liquid fraction) conditions that exist at the steam qualifies of interest.

The neutron densitometer has previously been calibrated for high-pressure test conditions. This paper also describes the results of recent low-pressure calibration tests. The range of test conditions studied were pressures of 1 MPa to 6.5 MPa, flow rates of 0.25 kg/s to 2.5 kg/s, and steam qualities of 0.1 to 0.9 in 0.051 m and 0.076 m (2 in. and 3 in. nominal) schedule-80 pipes. Both horizontal and vertical flow configurations were tested.

Introduction

Steam injection is one of the most widely used enhanced recoveries techniques available today for heavy oil. When the injected steam is condensed to water in the petroleum reservoir, the latent heat of vaporization heats the surrounding heavy oil, thereby reducing its viscosity. It is possible with reasonable temperature increases, to reduce the viscosity of some crudes to one-hundredth or one thousandth of their former values. Once the heavy crudes are heated, they can be pumped to the surface by conventional equipment.

The need for reliable and cost-effective methods to monitor steam quality at the wellhead of steam-injection projects has existed for many years. Steam is produced from a steam generator typically at 0.7 to 0.8 quality. This steam is usually injected simultaneously into several wells through a distribution network. Depending on the steam quality, mass flow rate, junction geometry, operating pressure and pipe size, the distribution of steam into different wells can be highly uneven, resulting in uneven heat input to the petroleum reservoir.

Several investigations(1–3) have been conducted to study different techniques for measuring steam quality at the wellhead. The Insight ™ steam-quality monitoring system, developed by AECL Research in partnership with two major oil companies, is a neutron densitometer that operates on the principle of thermal neutron transmission to measure steam quality. It has been calibrated to measure steam quality for a wide range of two-phase- flow conditions. The meter has previously been calibrated for high-pressure test conditions (shown in Table 1) at the laboratory of Kernforschungszentrum Karlsruhe (KfK), Germany, in 1985(2). The meter was subsequently field-tested in a heavy oil field in Alberta in 1986. This paper presents the results of recent low-pressure calibration tests performed in 1989.

Principle of Operation

Neutrons, like gammas, are exponentially attenuated by matter.

Equation (1) (Available In Full Paper)