DEVICE FOR MEASUREMENT OF CHARACTERISTIC REFLECTION COEFFICIENT FOR ELECTROMAGNETIC WAVES IN MULTIPHASE FLOUR The invention includes a device for measuring the characteristic reflection coefficient of high-frequency electromagnetic waves (normally above 100 kHz) in fluids and multiphase flow regimes. In this application,'multiphase flow regimes'means volume flows in pipes mainly containing varying fractions of oil, gas and water. This type of flow regime is typical in crude oil production. High-frequency electromagnetic waves are reflected by the interface between the device and the fluid (the combination of fluids and gases). The reflection coefficient is a manifestation of the dielectric properties (permittivity) of the medium to be measured. The relative permittivity of water is 80 and the relative permittivity of oil is 2. For comparison, the relative permittivity of gas is 1. The device may be used in a measuring arrangement to distinguish between fluids of different permittivity. The invention is designed as an open-ended coaxial probe and can replace the capacitive measuring principles used previously, which require installation of plate electrodes inside the pipe.

Measuring of permittivity in tanks and pipes is important for the characterisation of fluids and can for example be used for calculating the percentage of the water content in the fluid phase. In the production of crude oil, the fractions of gas and water will increase with the age of the oil wells.

Particularly large gas fractions appear in wells where production is driven by gas pressure and in wells producing condensate. In some oil wells, the fraction of free gas may exceed 95 per cent. Free gas in the medium has previously been a complicating factor when measuring reflection coefficients/permittivity. The probe may be used effectively where gas fractions range from 0 to 95 per cent.

The patent US-4503383 has already described a device (coaxial probe) and a procedure to determine interfaces in tanks containing two different fluids.

The coaxial probe is designed with level measurements in mind. Therefore, it is relatively long (60-90 cm) so that it can be inserted into tanks. The central conductor is not in the same plane as the outer conductor and the excitation signals are low-frequency signals. The procedure is a type of bulk measuring (measuring on the basis of a cross section) unlike local, detailed measuring.

A vertical multiphase flow with high gas fractions produces an annular flow regime with a fluid film along the walls of the pipe. According to the invention, the design and placement of the probe are optimised for measuring the complex reflection coefficient of high-frequency electromagnetic waves in a local area near the pipe wall beside the terminal surface of the probe. The penetration depth of the electric field can be changed by changing the diameter of the outer conductor.

The device is designed taking into account the high pressures and the high temperatures common in crude oil production tubings, but may also be used in characterisation of fluids in storage tanks and gas free flow regimes in pipes.

Figure 1 shows the invention installed in the wall (10) of a pipe or a tank.

The probe has a flat terminal surface which is in the same plane as the inner wall of the pipe/tank. The design contributes to the electric field being reduced to a small area beside the terminal surface of the probe. The device (the probe) has a coupling for a coaxial cable (1). The inner conductor (2) is insulated from the outer conductor (3) with a heat resistant, insulating plastic material (4) with a very low permittivity. Further, the probe consists of a female threaded sleeve (5) and a screw (6). The screw presses against a conical metal ring (7) squeezing the conically formed insulation material (9) against the inner conductor. An O-ring (8) provides a tight connection between the probe and the outside of the pipe wall/tank wall.