Abstract

The newly developed Dielectric Steam Quality Sensor (DSQS) utilizes a unique approach to measuring steam quality. The DSQS actually measures the electrical impedance of the wet steam in an annular cross section between the sensor's electrode and housing. Based on the sensor's dimensions, geometrical cell constants relate the measured impedance of the steam to the dielectric constant and resistivity of the two phase medium. The documented electrical properties of water and vapor, established mixing laws, and the measured impedance are correlated with the liquid volume fraction. The more common "Steam Quality" expression is then computed using specific volumes of the phases at saturation conditions. The DSQS was developed specifically for use in oil industry thermal recovery steam distribution systems. Application points include steam generator discharge, injector wellhead, steam headers, and other selected locations in the piping network. Direct use of the DSQS technology is also anticipated in geothermal industry steam collection systems.

This paper presents the theory of operation, the mechanical design, associated instrumentation and results of extensive testing. Laboratory testing conducted in Texaco's Steamflow Research Facility, provided guidance on the optimal mechanical design and demonstrated the concept of relating measured impedance to steam quality. Early field testing was accomplished in the Kern River Field to verify performance in oil field operating conditions. These tests showed the effect of saline field water on the DSQS system and provided the data necessary to development the methodology to incorporate various water resistivities into the impedance measurement. A further evaluation the DSQS system with a broader and more extensive test program was conducted where results were compared to concurrent measurements obtained using portable separator test units. The tests, conducted in various San Joaquin Valley oilfields, document the performance of the DSQS over a broad range of stable and unstable operating conditions.

Introduction

Enhanced Oil Recovery (EOR) operations are increasing the use of steam flooding to improve production rates and overall recovery in heavy oil reservoirs. Although the injection of "dry" or super-heated steam would transfer more heat into the reservoir, operating costs and practical limitations result in the injection of lower quality steam, usually 80% and below. To optimize the effectiveness of the steam flood and to model the steam flood movement, it is important to accurately know the mass rate and quality of the steam being supplied to each injector well.

One can calculate steam quality at the exit of a generator based on consumed fuel, feedwater rate, and associated generator factors. However, as the steam flows through long pipelines and becomes divided into multiple lines, the steam quality at any given point becomes unknown and varies throughout the distribution system. This is due primarily to the fact that "wet" steam is made of two phases, steam vapor and hot water. As the two-phase steam divides at tees or manifolds the proportional mass of the liquid to vapor is not maintained in the outlets. This phenomenon is known as phase splitting and has been the subject of many research studies.

Field personnel will often connect a trailer mounted apparatus to the steam line feeding an injector to measure the steam quality at the well-head. The apparatus on the trailer separates the two phases using settling tanks and demisters. Single phase vapor and liquid measurements are then used to calculate steam quality. While this method is accurate, it is time consuming to produce the final measurement and requires the service of skilled personnel. In addition, a trailer-mounted device is quite expensive and alone could not provide a "snapshot" of all the wells in the entire field.

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