mfm meter principles

7/30/2019 MFM Meter Principles

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4.1.2 Vx Meter

This meter is manufactured by Framo of Norway. The Vx meter was developed in

conjunction with Schlumberger who now act as its agents and are starting to use it for

routine well testing operations.

4.1.2.1 Principle of Operation

The meter consists of a blind flange which makes the fluids take a sharp 90 degree turn

upwards into the measurement section. The effect of this is to mix the fluids into a fairly

homogeneous mixture but full homogeneity is not a requirement for the meter.

The fluids then pass through a venturi which is used to measure the mass flow rate of the

fluids. Coincident with this venturi is a nuclear densitometer which measures the density

of the fluids with 2 separate gamma ray frequencies.

Each wavelength has a different rate of absorption which is dependent on both the

density and a quantity known as the “mass attenuation coefficient” (MAC) of each fluid.

It this that is exploited to measure not only the density, but also the mixture proportions

of the fluids.

Physically it is known that.

N = No e -ρµl

(1)

where

N = the gamma ray count rate at the detector

No = the gamma ray count rate at the detector when the pipe is full of vacuum (or air)

ρ = density of the fluid in the pipe

µ = MAC of the fluid in the pipe

l = the length of fluid penetrated (in effect the radius of the venturi throat)

both the density of the fluid in the pipe and the MAC are the length averaged values of

the individual oil, water and gas values. So the equation above can be expressed.

N = No e –(ρoµ

o lo + ρw

µw lw

+ ρg

µg lg )

(2)

Where the subscripts o, w, and g symbolise oil, water, gas.

and

l = length fraction of the specific phase in the radius of the venturi.

Many of the parameters in this equation are known, to wit :

N is measured during metering No is measured during calibration and form then on calculated to allow for source decay

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ρ’s are known from PVT experiments

µ’s are measured during calibration, for each hydrocarbon / water system.

Thus the only unknowns are the length fractions of each component. Of course to solve

for each one requires 3 equations, these are readily supplied since there are two equation

similar to eqn 2 (one for each gamma frequency) and the third simply states that

lo + lw + lg = 1 (3)

With the venturi measuring the mass flow rate and the average density being measured

by the nuclear densitometer, the total volume of fluid passing through the meter is known

and thus the volume of each phase can then be calculated.

No claims are made by the manufacturers that the meter can calculate the densities of the

individual components, high accuracy PVT is required and needs to be updated and

checked on a regular basis. In addition, the MACs of the various components need to be

measured by filling the venturi with 100% of each of the phases in turn and measuring

the count rate. Using eqn (1) allows the MAC to be calculated. When using this meter as

an infield test device this process needs to be repeated periodically to ensure accuracy is

maintained.

In the case were water salinity is changing on a regular basis this could be awkward, but

it would be possible to adjust the input density of the water by measuring the salinity of

the water with a handheld refractometer (it takes seconds) and looking up the density in a

set of tables. MACs can be calculated theoretically if the composition is known, so using

a variety of mixtures of injection and formation water a look-up table could be

established to allow input of the MAC of any given salinity also.

One of the advantages of this meter over other MPFMs discussed in this document is that

all the measurements are taken at the same point in the pipe because the nuclear

densitometer is located directly across the venturi. This ensures that the data used to solve

for the flow rate is all referenced to the same section of fluid and prevents problems that

might otherwise arise from slugs of liquid and gas being measured at the same time.

4.1.2.2 Accuracy

The Vx meter accuracy depends the GVF of the fluid stream and is quoted as follows.

The larger of 300 bbls / day and 2.5% of reading (GVF 0 – 98%) for liquidsand

The larger of 1% of reading and 3400 acf/d (GVF 0 – 30%)

the larger of 3% of reading and 10,200 acf/d (GVF 30 – 60%)




for gas.

3% absolute (GVF 0 – 70%)

4% absolute (GVF 70 – 80%)

5% absolute (GVF 80 – 90%)8% absolute (GVF 90 – 95%)

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for water cut

These values are quoted with a 90% confidence level (2_and_a_bit sigma) and indicate

that the meter works best in the top half of its working range and at high pressures where

GVFs are lower.

The values quoted suffer from the same issue as the AGAR unit, in that (for oil) the 300

bbls / day fixed error is only small for rates above 12,000 bbl / day. Below this, the

percentage error increases steadily until at 3000 bbl / day the allowable error is 10%

which is on the edge of acceptability for allocation purposes.

It will be impossible to operate these at high pressures since they are forced to be

downstream of the choke. It is expected that they will operate at around 800 – 1200 psi

resulting in GVF values of between 30-60% for EKT and EMN TAGI formation fluids,

between 60 and 90% for EMK and EME TAGI formation fluids and in excess of 90% for

the gas condensates produced from the RKF and Strunian sands (see section 5 below).

This will result in gas rates being measured with an accuracy of around 3% for the lowGOR oil and 10% for the high GOR oils. This is considered barely satisfactory for wells

producing mostly oil.

In the case of gas condensates where gas rate accuracy (primary produced phase) will be

only +/- 15 – 20% these meters cannot be considered acceptable. Oil wells in which gas

breakthrough has occurred or gas lift is in place will have a similar problems with gas

rate measurements and GOR calculations will be excessively in error.

The turn down ratio of this meter is claimed to be 50:1 on a mass flow rate basis. It is

possible to replace the venturi in any given meter in about half a day including the

necessary calibration steps, but this is likely to be too long to be done on a routine basis.

Another option of having two meters (one for low rates, one for higher rates) at each

metering station that needs it is feasible but would involve double cost, as well as pipe

and valve requirement to allow switching between meters.

Finally it has recently become possible to equip the Vx with a wide range dP cell for the

Venturi meter. This allows a smaller meter to be used to measure higher rate with a larger

turn down ratio.

4.1.2.4 Likely Future Developments

Schlumberger have spent many millions on the development of this meter over the past

few years. It is now being sold by them with the intention that it should become their

primary well testing tool over the next few years.

Schlumberger are a large company with a large research and development budget and

regularly contribute to the literature on multi-phase metering.

Discussions with Schlumberger reveal commitment to developing and improving the

meter in the next few years to allow gas rate measurements at high GVFs to be improved

significantly. Although it cannot currently (April 04) be used to meter lean condensates,

there is an intention to release a wet gas version of the meter which will use identicalhardware but a new version of the software. Accuracy specifications of this device are

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not yet available but the new software is intended to be used for GVFs in excess of 98%.

Switching the function of a unit from one version to the other will require only reloading

of the software.

This wet gas function of the meter will not resolve the gas rate metering problems in the

90 – 98% range but it is likely that in the next few years, new software will improve thesituation.

4.1.3 Roxar

This meter is manufactured by Roxar of Norway who is better known for their ultrasonic

meters often used on flare lines and their water cut meters. Roxar have in the past had a

reputation for excessive cost due to the use of very expensive dual microwave technology

for measuring the velocity of the fluid stream. Recently this has been replaced with

capacitive sensors which are cheaper, to perform the same task.

4.1.3.1 Principle of Operation

The meter consists of a blind flange which makes the fluids take a sharp 90 degree turn

upwards into the vertical measurement section. The effect of this is to mix the fluids into

a fairly homogeneous mixture but full homogeneity is not a requirement for the meter.

The rest of the meter consists of

A gamma densitometer to measure the density of the fluids. This works similarly to the

Vx meter densitometer, except it has only one gamma frequency and gives no

information about the composition of the mixture.

A dual capacitance sensor to measure relative permittivity of the fluid mixture. This,

combined with the densitometer data, allows an estimation of the oil water and gas cuts,

in oil continuous flow. The readings from the two capacitance meters are also cross

correlated to allow the velocity of flow to be determined.

An inductance sensor to measure the capacitance of the fluid mixture. This combined

with the densitometer data allows an estimation of the oil water and gas cut, in water continuous flow.

Finally there is a venturi meter to measure the mass flow of fluid. This provides a

measure of redundancy to the cross correlation based measurement of fluid velocity.

However, use of this measurement alone does not take into account the slippage between

oil and gas in the meter. To allow for this, input from the cross correlation meter is used.

The cross correlation works by a statistical examination of the response from two pairs

of electrodes set some distance apart. The flow in the pipe contains a mixture of large

and small bubbles of gas dependant on the flow regime. The small bubbles are subject to

very little slip and thus travel at essentially the same speed as the surrounding liquid phase. The large bubbles have significant slip and thus travel faster. By comparing the

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two signals from different points in the pipe the speed of liquid and gas can be estimated.

These velocities are used in a modification of the standard Venturi equations to increase

the accuracy of the venturi calculations. Given the average density of the mixture, the

flow rates can then be determined.

At very high GVFs the cross correlation process sometimes breaks down due to annular flow conditions which give a capacitance signal which is too smooth to correlate. In

these conditions the Venturi meter provides the mass flow rate without the benefit of slip

adjustments. Company literature does not mention that this tends to degrade the

accuracy, but presumably it does.

No attempt is made to estimate the densities of oil water and gas, the meter requires

accurate inputs of;

Oil density Oil permittivity

Water density Water conductivity

Gas density

It also requires inputs describing how the oil water and gas density change with

temperature and pressure. This is provided by reliance on Calsep’s PVT simulator

package “PVTsim”.

The meter has a turndown ratio of about 10:1 in mass flow rate terms. This is

controlled by the venturi whose response is proportional to the square of the mass

flow rate. The turn down ratio can be extended by use of a two venturis in the same

meter, with different diameters, or by use of dual dP sensors with different ranges of

operation.

4.1.3.2 Accuracy.

The accuracy of the meter is quoted as in Table 2 below

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Table 2: Roxar Meter Accuracy Specification.

GVF Range Uncertainties

Gas Liquid WLR

0 - WTr

% rel % rel % abs %

0 – 30 8(1) 2 1.530 – 90 3 2

90 – 96 6 5 3

96 – 99 7 4

99 - 100 - -

Where:

GVF = Gas Volume Fraction

WLR = Water in Liquid Ratio

WTr = Transition point oil/water continuous liquid phase

rel % = Relative uncertainties in gas and liquid flow rates(2)

abs % = Absolute uncertainty for Water to Liquid Ratio (WLR) (2)

• For WLR > WTr WLR is 1.5 x listed value

• Uncertainties are given for pressure > 10 barG

• (1)For GVF > 5%

Repeatability: ¼ of measurement uncertainty

Uncertainties: Based on 90% confidence interval

Note that the error is quoted for liquid rate not oil rate. When the error of the liquidrate is combined with WLR error then the oil rate error can be significant when the

water cut is above about 70%.

It is difficult to directly compare the accuracy figures quoted for different MPFMs

since they are all quoted in a different way, but some attempt is made to do this in

Figures 2 and 3 in comparison with both AGAR and the Vx meter. It is noticeable that

the liquid error at higher than 99% GVF is not specified. Roxar also have a wet gas

meter intended to be used at GVFs greater than 95%. This works on a different

physical principle but details have not been forthcoming from Roxar.

As for all MPFMs, these uncertainties assume perfect fluid property values enteredinto the software. Extra errors are induced by errors in the data. Roxar are unique in

providing estimates of the extra errors induced by faulty input data. These are shown

in table 3 below.

Table 3: Roxar Meter Sensitivity to Input Data.

Quantity Change

% rel

Liq. Rate

% rel

WLR

% abs

Gas rate.

% rel

Note

Oil density: +1 % +0.9 % -0.2 % -0.2 % 1

Gas density: +10 % +1.1 % -0.3 % -0.3 % 1

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Water density: +1 % +0.3 % -0.1 % -0.1 % 1

Oil permitt. + 5 % -0.3 % +1.3 % +0.1 % 1

Water conduct + 1 % -0.02 % + 0.9 % - 0.0 % 2

Notes: 1: Given at 80% GVF, 20 % WLR

2: Given at 80% GVF, 80 % WLR

Some of these parameters are more prone to error than others, to wit

Meter condition oil density is very difficult to calculate to better than +/- 3%

A gas density of +/- 5% should be achievable

Water density in Groupement Berkine circumstances could vary by as much as 10%

although it’s very easy to measure the salinity and thus work out a density and

conductivity.

Oil permittivity is known to a high degree of accuracy. 5% is easily achievable.

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mfm meter principles

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