Ejectors for Boosting Low-Pressure Oil Wells
- Marco Villa (ENI-Agip) | Giambattista De Ghetto (ENI-Agip) | Francesco Paone (ENI-Agip) | Giancarlo Giacchetta (U. of Ancona) | Maurizio Bevilacqua (U. of Ancona)
- Document ID
- Society of Petroleum Engineers
- SPE Production & Facilities
- Publication Date
- November 1999
- Document Type
- Journal Paper
- 229 - 234
- 1999. Offshore Technology Conference
- 3.2.6 Produced Water Management, 7.3.3 Project Management, 5.2.1 Phase Behavior and PVT Measurements, 5.3.2 Multiphase Flow, 4.2 Pipelines, Flowlines and Risers, 4.1.5 Processing Equipment, 4.1.3 Dehydration, 4.3.4 Scale, 3.1.3 Hydraulic and Jet Pumps, 3.1 Artificial Lift Systems, 4.1.2 Separation and Treating, 4.4.3 Mutiphase Measurement, 4.3.1 Hydrates
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The first field installation of a multiphase surface ejector was successfully accomplished in the Villafortuna oilfield (Italy) in 1996. The ejector increased the oil production rate by 30% and demonstrated the viability of the system as a reliable low cost-small size boosting system that had minimal impact on existing facilities, and is suitable for applications in existing fields or for new development both onshore and offshore.
The article describes the main steps of the R&D project, developed in collaboration with the University of Ancona, as well as the testing process in which different ejector geometries were extensively tested with crude oil and gas in the Agip's multiphase loop of Trecate.
The article describes the installation in the Villafortuna field as well as other studies under development.
The ejector is a static machine in which a high pressure stream (HPS) is mixed with a low pressure stream (LPS) in a properly designed mixing chamber. The mixture passes through a diffuser, leaving the ejector at an intermediate common outlet pressure (Fig. 1).
The ejector is a well known piece of equipment that is used extensively in many industries in all the chemical industry (gas-gas ejectors) and nuclear industry (liquid-liquid jet pumps).
The oil industry limited the use of jet pumps at the bottom hole as an artificial lift system, using water or diesel as the power fluid pumped from the surface. For the bottom hole jet pumps the approach used is to treat the gas phase in the low pressure stream (suction stream) as a "liquid phase." This is usually correct for down hole jet pumps, where the gas carried under the liquid stream is frequently limited to small percent (5 to 10%). This assumption is not valid for surface oil ejectors because the gas greatly exceeds this limit.
As far as we know, at present, no one is using surface ejectors for oil wells. Instead, for gas wells field applications have been successfully realized in recent years by Agip and others. In this article the results of Agip applications are reported.
The installation of an ejector on the surface allows one to use free energy from a high pressure well (if present, of course) to boost a low pressure well, thereby obtaining enhancement of the well production and/or extension of the field life.
In 1993 Agip launched a R&D project in collaboration with the University of Ancona aimed at analyzing the suitability of ejectors as boosting systems for marginal oil wells. Starting with a preliminary analysis of the existing literature, the project progressed with the development of a software design based on a simplified theoretical model. The experimental coefficients of the model have been tuned while testing the ejectors with different geometries in the laboratory of the University of Ancona using water and air at low pressures in a wide range of process conditions. A second tuning followed the test performed in the Trecate Test Loop (TTL) with live hydrocarbons at high pressure and the model is now more user friendly software, able to design a multiphase ejector working with medium and light crude oil, within an error that is smaller than 10%.
The R&D Project
The project was developed in two main steps:
- laboratory tests and code definition;
- field test and code updated.
Laboratory Tests and Code Definition
The studies on multiphase ejectors started from activities previously performed on bottom hole jet pumps using water or diesel as the power fluid pumped from the surface. From the literature1,2 it was clear that the conservative approach used in the past was to treat the gas phase in the suction as "liquid phase." This is correct whenever the gas carried under the liquid stream is limited to small percentage, 5 to 10%. When the gas exceeds these limits, which always occurs for surface ejectors, the equations used for the liquid only have to be modified.
Three theoretical-experimental software design was developed for liquid-gas (LG) ejectors (liquid in the drive stream and gas in the suction stream), liquid-multiphase (LM) ejectors and multiphase-multiphase (MM) ejectors, taking into account the gas void fraction (GVF) in one or both of the drive and suction streams. All of them where based on the equation of energy and momentum conservation in order to evaluate the pressure difference between the upstream and downstream sections. The models were based on the following common assumption:
- a monodimensional approach along the flow direction, and
- a homogeneous mixture of oil and gas in one or both of the steams.
Three main sections were identified: the nozzle, the mixing tube and the diffuser.
As was for the models available in the literature, the equations define the structure of the code. However the relations need experimental evaluation of the loss coefficients for the nozzle, the mixing tube and the diffuser. The test facilities at the University of Ancona (Fig. 2) were updated to allow testing of the LG, LM and MM ejectors. Over 400 tests3 at low pressure have been performed working on six plastic models with different geometrical arrangements using air and water in a wide range of test conditions shown in Table 1.
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