Multiphase Ejector to Boost Production: First Application in the Gulf of Mexico

Abstract

Abstract A high pressure oil or gas well can be used to enhance both the production and the total recovery from a depleted well using a multiphase ejector. This device does not require any power supply and is characterized by a simple design, absence of moving parts and small dimensions, coupled with a high degree of reliability and low cost. The main disadvantages related with the use of multiphase ejectors are due to the lack of reliable design methods and to the sharp decrease of the performance when operative conditions change. In August 2002 a multiphase ejector has been installed in Allegheny TLP, GOM, to boost the production from a depleted well. In the present paper the design method and the results of field tests of the Allegheny ejector are presented. From a practical view-point the main result of this installation has been an increase of production of 1300 BOPD with an investiment of less than 50,000 USD. Introduction The ejector or Jet Pump, JP, is an artificial lift method which does not require any power supply and is characterized by a simple structural design, absence of moving parts and small dimensions, which allows easy installation and management procedures during fields operations, coupled with a high degree of reliability and low cost of installation, compared with other boosting systems. On the other hand, the ejector is a low-efficiency device: just a small fraction of the power fluid energy (approximately 20-30%) is actually transferred to the low pressure fluid (however it should be remarked that in many cases this energy would be lost through a choke valve). When the ejector is fed with multiphase fluids, significant modelling problems arise and no established methods are available for the design of multiphase ejectors. Design problems are increased by fluid properties changes during field evolution. To cope with all these problems and extend the operative life of the ejector, an advanced multiphase ejector has been developed in order to optimise some of the main geometrical parameters and improve the ejector performances at varying operative conditions. In this paper, the realization and field application of this multiphase ejector are described. Fig. 1 shows the main components of a JP: the nozzle, the mixing chamber followed by the mixing duct and the diffuser sections. Power fluid, at an injection pressure Pd is forced through the nozzle. As the fluid accelerates because of the area reduction, its kinetic energy increases and the pressure decreases to the value Pn at the nozzle exit section. The power and the produced fluids then enter the mixing chamber were pressure reaches its minimum value, Ps. In the mixing chamber, mixing duct and diffuser the pressure increases and the gas-liquid mixture leaves the ejector at a pressure Pl. The main concern about actual multiphase ejectors is the limited capability of the hardware to be adjusted when boundary conditions change (i.e. well depletion, water cut and/or GOR variation, production profiles). To this aim, the present multiphase ejector has been designed to be adaptable in terms of: