APPARATUS AND PROCESS FOR MEASURING MULTI-PHASE MASS FLUID FLOW

Abstract: Mass flow of a multi-phase fluid is determined by feeding the fluid into a chamber through a positive displacement device and measuring the density of the fluid. The fluid is removed from the chamber at the same volume rate as that at which it is introduced. The density of the fluid in the chamber is measured together with the number of volume increments passed.

Description: APPARATUS AND PROCESS FOR MEASURING MULTI-PHASE MASS FLUID FLOW The present invention relates to the measurement of multi-phase flow. It is often important to know the total mass flow of a fluid stream containing more than one phase, for example one containing two or more liquid phases and/or liquid gas phases. Thus in crude' oil production the fluid at the wellhead may comprise a combination of crude oil, water, and gas (either dissolved or as a separate phase). In multiwell crude oil production systems it is often necessary to apportion the total production on a per well basis. Therefore measurement of total mass flow from each individual well is required. At present the total volumetric flow is usually obtained indirectly from a knowledge of the pressure measured upstream of the choke, and the choke size. Well tests are performed to obtain gas/oil ratios and samples are taken from laboratory analysis to ascertain water content and phase densities. The mass flow of each phase may then be calculated. These techniques are imprecise and a more direct method of measuring multiphase mass flow is desirable. US 4 272 982 discloses a multi-phase flow meter for measuring the flow of a fluid containing both liquid and gas components. The meter comprises a structure corresponding to a sliding vane positive displacement meter together with a gamma ray densitometer for determining the density of a segment of the flow trapped between two sliding vanes. The use of a rotating cylinder with sliding vanes is convenient In that interphase velocity slippage is prevented and a clear path is provided for the gamma rays from one side of the device to the other except for the very short time during which a vane passes between the gamma ray source and detector. However the fluid enters the positive displacement device along an axis at right angles to the rotation of the cylinder and thus impinges directly on the sliding blades. The fluid may contain pockets or "slugs" of gas and of liquid and the consequent transient forces acting on the blades may be high, leading to damage to the blades, which may distort to prevent them sliding within the rotating cylinder. It is therefore desirable to provide an apparatus for measuring multiphase flow which is capable of greater robustness. The present invention provides an apparatus for the measurement of multiphase mass flow of a fluid comprising a first rotatable axial flow positive displacement device for feeding fluid to a chamber, said chamber being associated with a radiation densitometer for determining the average density of the fluid, and a second rotatable axial flow positive displacement device for removing fluid from the chamber at the same volume rate as that at which it is fed. According to a further aspect of the invention there is provided a process of determining the mass flow of multi-phase fluid which comprises feeding the multi-phase fluid into a chamber through a positive displacement device, measuring the density of the fluid in the chamber with a radiation densitometer, removing the multi-phase fluid from the chamber at the same volume rate as that at which it Is fed to the chamber, measuring the number of volume increments passing through the positive displacement devices and summing the number of increments and the density of each volume passed by the positive displacement devices to determine the mass flow passed through the device. The two devices for feeding and removing fluid from the measurement chamber are positive displacement devices. Thus fluid cannot pass through the devices unless they are allowed to rotate and the components of each Increment of the fluid cannot travel at different rates through the positive displacement device. The devices are axial flow devices i.e. the fluid enters the device flowing parallel to the axis of rotation of the device. The general direction of flow In the device is parallel to the axis of rotation and thus the transient forces generated in multiphase flow are generally tangential to the rotors and do not directly impinge on them. An example of a suitable rotating axial flow positive displacement device comprises pairs of helical meshing rotors, one in each positive displacement device, and which are fixed to rotatably mounted shafts which are parallel to one another. The second rotating axial flow positive displacement device may be caused to remove fluid from the chamber at the rate at which it is fed by being coupled to a shaft or shafts common to the first rotating axial flow positive displacement chamber. The density measuring device must be arranged so as to measure the average density over the confined volume. This may be achieved by moving the confined volumes past a density measuring device which Is fixed. The density of the confined volume of fluid may be determined by a radiation densitometer. Radiation densitometers, e.g. Gamma ray densitometers are well known to those skilled In the art and a detailed description of their operation is not necessary. The response time of the density measuring device used must of course be sufficient to measure the density within a period determined by the transit time of the confined volume increments through the density measurement chamber. A single gamma ray source or a plurality of sources and detectors may be used In the gamma ray densitometer. The apparatus may be provided with means for measuring the pressure and temperature in the chamber. A specific embodiment of the present invention will now be illustrated by reference to the accompanying drawing which represents a three dimensional diagramatic view of one apparatus, in accordance with the present invention. The apparatus comprises a double helical rotor type positive displacement meter in which progressing cavities are formed, displacing known volumes of. fluid between the rotors. The meshed helical rotor assembly (1) is divided into two sections (la and lb) separated by a cylindrical chamber (2) through which are passed the shaft(s) (3) coupling the two sections. A gamma ray source (4) is arranged to pass a beam of gamma rays across the chamber to a corresponding detector (5) on the other side. Thus each volume increment trapped between the rotors of the entry section (la) travels within its cavity at the same forward velocity as the previous and following increments of flow. Interphase slippage is thereby prevented. On entering the central densitometer chamber (2), unilateral acceleration of different phases in the fluid is constrained by the exit section rotors (lb) which, because they are directly synchronised with the entry rotors (la) ensure that the velocity of the flow departing from the chamber is identical to that of the entry flow. The gamma ray beam (4) is arranged to measure the average density of the fluid passing through the chamber taking due account of the additional density of the rotor shaft(s). The two positive displacement devices are mounted within a tubular body 6 which forms an Inlet 7 and an outlet 8. The chamber 2 is defined between the body 6 and the two positive displacement devices. Facilities (not shown) are also provided to measure pressure and temperature within the chamber. The mass flow passing through the device Is obtained from the product of the sum of the Incremental confined volumes, and the fluid density. Appropriate calculations involving the temperature and pressure measurement may be performed to obtain the volume ratio of gas to fluid at atmospheric pressure and other parameters of interest.

Claims: Claims: 1. An apparatus for measuring multi-phase mass flow of a fluid comprises a first rotatable axial flow positive displacement device for feeding controlled volumes of fluid to a chamber, said chamber having associated therewith a radiation densitometer for determining the average density of the fluid In the chamber, and a second rotatable axial flow positive displacement device for removing fluid from the chamber at the same volume rate as that at which it is fed. 2. An apparatus according to claim 1 wherein In each rotatable axial flow positive displacement device comprises two double helical rotors, and pairs of rotors, one in each axial flow positive displacement device, are fixed to rotatably mounted shafts which are parallel to one another. 3. The apparatus according to claims 1 or 2 wherein the radiation densitometer is a gamma ray densitometer. 4. The apparatus according to claims 1 or 2 wherein the apparatus is provided with means for measuring the temperature and pressure in the chamber. 3- A method of determining the mass flow of multi-phase fluid which comprises feeding the multi-phase fluid into a chamber through a positive displacement device, measuring the density of the fluid in the chamber with a radiation densitometer, removing the multi-phase fluid from the chamber at the same volume rate as that at which it is fed to the chamber, measuring the number of volume increments passing through the positive displacement devices and summing the number of increments and the density of each volume passed by the positive displacement devices to determine the mass flow passed through the device.

Inventors: GOLD ROY CHARLES, GOLD, ROY, CHARLES
Applicants: BRITISH PETROLEUM CO PLC, THE BRITISH PETROLEUM COMPANY P.L.C.
Application year: 1986
Main classification: G01F1
IPC codes: G01F1, G01F3