Heavy Oil Case Study
Received 5 June 2006, Revised 13 July 2006, Accepted 13 July 2006, Available online 9 October 2006.
Abstract
Two-phase flow Metering using a Coriolis mass flow metering for two-phase (gas/liquid) flow is an emerging theme of both academic research and industrial application. The key issue is maintaining flow-tube operation. After that, modeling and correcting for the errors induced in the mass flow and density measurements.
Experimentally derived data is used to illustrate that these errors vary most notably with gas void fraction (GVF) and liquid flow rate. But other factors such as flow-tube geometry and orientation, and fluid properties such as viscosity are also influential. Undoubtedly a universal Two-Phase Flow Metering correction model is the ultimate research goal. While there is currently no obvious candidate to explain the range of behaviors observed. This paper describes and demonstrates an empirical methodology. This has proven effective in developing good correction models for a given choice of Coriolis flow tube and flow mixture.
A growing proportion of the world’s oil reserves may be described as “heavy”, implying high density and high viscosity. Of the various metering challenges heavy oil poses, one of the most significant is its ready entrainment of gas. After that, difficulties entailed in separating gas from the oil. Accurate Two-Phase Flow Metering measurement of heavy oil is therefore an especially desirable technical goal.
Trials
Trials were carried out at the National Engineering Laboratory (NEL), Scotland on a 75mm flowmeter using a high viscosity oil. Flowrates from 1kg/s to 10kg/s were examined, with a gas void fraction (GVF) up to 80%. The resulting models were tested online in a commercial Coriolis mass flow meter and demonstrated good performance for both
steady and slugging two-phase flows, with the corrected measurements typically within 1%–5% of the nominal mass flow and density. Field trials in Venezuela have confirmed the performance of this two-phase solution.
While research continues into the development of a generic two-phase correction, this case study demonstrates that the current state of the art can provide, for economically important fluids, tailored models with good Two-Phase Flow Metering performance.
Introduction
Coriolis mass flow metering has been established as the most accurate of the commonly used industrial flow measurement technology since its introduction in the mid-1980s [1]. Coriolis meters operate by oscillating a flow-tube (typically 1–300mm in diameter), at the natural frequency of a selected mode of vibration, the so-called drive mode. Two sensors monitor the flow-tube vibration as the process fluid passes through.
The frequency of oscillation (in the range of 50Hz–1kHz depending on flow-tube geometry) is determined by the overall mass of the vibrating system. Hence, for a given flow-tube, this varies with the density of the process fluid. Accurate determination of the frequency of vibration thus enables the process fluid density to be calculated. The geometry of the flow tube is arranged so that Coriolis forces act to give a phase difference between the two sensor signals. Which is essentially proportional to the mass flow of the process fluid (this may approach 1ton/s for the largest sizes).
The Flow Tube
While the flow-tube is essentially a mechanical device with a few electrical transducers (sensors and the transmitter is an electronic and computational device that drives and monitors the flow-tube, and which calculates the measurement data. A long-term research program between the University of Oxford has been developing all-digital transmitter technology [2]. [3], [4] with various improvements including fast response time [5], [6] and an ability to operate in two-phase flow [2], [3], [7], [8], [9].
The transmitter architecture includes audio-quality analog-to-digital converters (ADCs) and digital-to-analog converters (DACs), with typical performance characteristics of 24-bit samples delivered at 48kHz.
Architecture
This architecture is used in the so-called Oxford or research transmitter used for experimental and development purposes as well as its commercial variant, the CFT-50 product. The research transmitter has additional features, such as a comprehensive web interface. Also, a local hard drive, additional software for automating experiments, and dozens of additional analog and digital i/o channels. Additionally, they are not available in commercial devices. However, given the common core architecture, the research transmitter can be used to develop new capabilities (e.g., A new two-phase flow model) that are readily transferred into the commercial meter.
Reizner
Reizner [10] provides a good background to the problems associated with metering two-phase flow with Coriolis meters. In brief, it is technically difficult to maintain flow-tube oscillation during two-phase flow, as the condition induces very high and rapidly fluctuating damping (up to 3 orders of magnitude higher than for single-phase conditions). When the transmitter is unable to maintain oscillation, the meter is described as “stalled”. As a result, no (valid) measurement can be provided. Even where stalling is averted, large measurement errors may be induced in the mass flow and density measurements.
Two-Phase
For clarity, the term “two-phase flow” is here taken to mean any mixture of a gas and a liquid. For example, air and water, or air and oil—and is not restricted to mixtures where the gas and liquid are of the same chemical composition.
In this paper, it is assumed that the transmitter can maintain flow-tube oscillation through continuous two-phase flow or batching to or from an empty flow-tube [3], [8], [9]. The focus is restricted to modeling and correcting for two-phase flow errors in the mass flow and density measurements, which are derived from the resonant frequency and phase difference properties of the flow tube.
The development of theoretical models to explain the observed frequency and phase difference
Phase Flow
Phase flow is difficult. The so-called “bubble” model developed by Hemp and Sultan [11] considered the inertial losses generated by a single bubble surrounded by much denser fluid when passing through a vibrating pipe. This model predicts monotonic, negative errors which are a function only of the gas void fraction (GVF), i.e. The proportion of gas by volume in the two-phase.
For clarity, it is to be understood that the mass of the gas is assumed to be negligible. Hence the nominal mass flow of the two-phase mixture is equal to the mass flow of the liquid phase only. However, when assessing the density error, it is more useful to consider the meter’s ability to measure the density of the two-phase mixture. Rather than of the pure liquid itself. Thus, with a liquid density of (say) 1000kg/m3 and a GVF of 10%, assuming the gas has negligible mass, the nominal mixture density is 900kg/m3. In these conditions, a density error of say −5% would mean that the meter was reporting a mixture density of only 855kg/m3.
As illustrated in the following section, the bubble model is successful in predicting the general characteristics.
Errors
Two-phase flow errors (i.e., Negative and increasing in magnitude with GVF), the pattern of errors observed experimentally is much more complex, at present poorly understood, and varies with several different parameters.
Several manufacturers are now acknowledging the technical possibilities of handling two-phase flow with Coriolis mass flow meters [13]. For example, Seeger gives experimentally derived mass flow errors [14]. Also, exploring such parameters as surface tension and viscosity. The current paper is intended to demonstrate that, even though the factors generating two-phase flow errors remain poorly understood. It is possible to provide online corrections leading to good two-phase performance.
Witnessed Trials
Specifically, customer-witnessed trials have taken place at a national flow laboratory in which satisfactory, corrected two-phase flow measurement has been achieved using a Coriolis mass flow meter with gas void fractions ranging from 0% to 80%, and subsequent field trials have been adjudged to be successful.
Experimentally observed two-phase flow errors for air–water mixtures
The Oxford team has investigated two-phase flow errors using a variety of flow-tube designs and fluids. Their experience suggests that while for low-viscosity fluids the negative errors predicted by the bubble model form a good first approximation of the experimentally observed errors, there are additional influencing factors that the model excludes. The underlying assumptions of the model include no interaction between bubbles and no interaction between the bubbles and the flow tube.
Two-phase flow correction methodology
The definition of two-phase flow correction adopted in this paper is the application of compensation to the raw mass flow and density readings. This was done for the effects of two-phase flow, to generate improved (corrected) readings, where the correction is based solely on data available within the flowmeter. It is however reasonable to allow single-valued configuration parameters (such as the nominal single-phase liquid density, perhaps with a temperature coefficient of expansion)
Trial at the National Engineering Laboratory, UK
A growing proportion of the world’s oil reserves may be described as “heavy”. Of the various metering challenges heavy oil poses, one of the most significant is its ready entrainment of gas. It also showed the difficulties entailed in separating gas from the oil. Accurate two-phase measurement of heavy oil is therefore an especially desirable technical goal.
Venezuelan oil is characterized by very high viscosity (up to 10,000 cSt). Longstanding field trials in upstream oil applications by Chevron were conducted.
Results of Two-Phase Flow Metering
The data collection strategy was chosen to reflect the likely pattern of behavior in the oilfield application. Generally, flow rates are low, but low-frequency slugging behavior (as described below) can occur leading to bursts of high flow. Accordingly, for model development, a range of steady-state flows from 1kg/s up to 10kg/s were examined. During this, model testing was carried out using either steady-state low flows (up to 4kg/s) or deliberately engineered low-frequency slugging flow.
Field trials
The commercial meter incorporating the new two-phase correction model has been tested at a heavy oil field in Venezuela. The wells themselves are up to 2km distant from the flow stations where metering occurs. Consequently, the pipelines are oversized to keep pressure drop manageable despite the high viscosity of the oil. Typical average production rates for the well are 100–1200 barrels per day. This equates to approximately 0.18–2.2l/s, which is low for the 75mm diameter flow tube.
Discussion
This paper has described a methodology for developing a two-phase flow correction for gas/liquid mixtures, illustrated with a case study of high-viscosity oil.
Examples of different mass flow and density error data have been provided. This demonstrates that the general characteristics of the error surface against true mass flow and GVF under different conditions—flow-tube orientation and flow-tube geometry—are broadly similar for the same fluid, water.
J.R. Reizner
Batch accuracy: The good, bad, and ugly of Coriolis
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