During petroleum production, both gas and liquid phases flowing in the same pipe may lead to flows that exhibit several spatial distributions for these phases – the flow patterns.. These flow patterns depend on operating variables, namely the liquid velocity and the gas velocity, the fluid properties and, some geometrical aspects such as the pipe diameter, inclination and its wall roughness. Hydrodynamic aspects of these flows are strongly dependent on respective flow pattern.
Among the possibly occurring flow patterns, in the multiphase flows that take place during offshore petroleum production, the so called intermittent flow is the one most often found. Such flow is characterized by the alternating flow of liquid and gas phases. In more details, liquid phase is present like a piston – liquid piston – taking entirely the cross section of the pipe and also as film – wetting entirely or partially the pipe wall – and, having the gas phase heavily present as full gas pocket – that rides on the referred liquid film – and also distributed as small bubbles in the liquid phase. Once the gas pocket velocity is higher than liquid piston velocity, an intermittent liquid-gas flow is generated in the pipe. One shall notice that, among other aspects, the average gas volumetric fraction (GVF), in a volume per volume (v/v) basis, will differ considerably from the in-situ GVF value in a point of interest in such pipe flow scheme (e.g., in the suction of an Electrical Submersible Pump – ESP – eventually equally present in such petroleum production scheme. Pistons liquid occupying the entire diameter of the tube are separated by gas bubbles, which contain a stratified liquid layer flowing into the bottom. Thus, the average fraction of the gas flows is likely to be very different from the instantaneous gas flow fraction.
The objective of this work is to experimentally investigate the behavior of an intermittent multiphase liquid-gas flow that takes place upstream of an ESP, which composes an offshore petroleum production scheme. Based on the acquired results and associated interpretation, a flow homogenizer device – aiming at the homogenization of such two phase intermittent flow – was conceived, built and also experimentally tested.
Classical ESPs, which are in essence composed by a series of radial/centrifugal impellers and diffusers pairs, are not tailored to work in scenarios with high GVF values. An experimental apparatus was assembled in order to avoid the development of the pattern of intermittent flow. For development of experimental work, a test section has been assembled in the Laboratory of the Research Center of Petrobras (CENPES). In this test section similarity conditions were respected so that the pipe has an internal diameter of 19 mm and the reproduction of accidents before entering the homogenizer system was considered. Air and water were used as the working fluid. In a second stage of the research the system was subjected to a flow condition with oil and gas.
The results showed that it was possible to get homogeneous flow at the inlet of the pump, even in case of intermittent flow occurrence. This study aims to prevent the occurrence of the phenomenon of gas-lock in situations where the gas void fraction of the system is still within the operational envelope of the pump.