Patent Application
20150135849
This application claims foreign priority benefits under 35 U.S.C. §119(a)-(d) to United Kingdom patent application number GB 1320202.3 filed Nov. 15, 2013, the disclosure of which is hereby incorporated in its entirety by reference in its entirety.
The present invention relates to a system for measuring flow rate in a pipeline and, specifically, configuring a surface jet pump (hereinafter “SJP”) as a means of measuring flow rate, in addition to its normal duty of pressure/production boosting.
The measurement of fluid flow rates produced from a well is an important part of production management and assessment of reservoir behaviour. In many cases, such as offshore oil and gas production, a test separator is installed and production from each well can be diverted to the test separator for measurement of oil, water and gas. In most of such cases the wells can only be tested at the pressure dictated by the production manifold, as separated gas and liquid phases from the test separator are diverted back to the production header.
It is known to install a multiphase meter instead of the test separator; however, there are a number of issues such as cost and lack of measurement reliability which discourage the use of multiphase meters by many operators.
In the case of onshore oil or gas production, the wells are scattered over a large area and diverting each flow into a test separator is only practical where production from the wells reaches a gathering station, i.e. a test separator can be installed at that location. In any event, in these cases the wells can only be tested at the pressures equal to or above the production pressure of the manifold.
Testing wells at pressures below that of the manifold pressure is often essential as a means to evaluate the use of production boosting systems, installed downhole or at surface, to increase production. However, testing wells at pressures below the manifold pressure requires additional facilities to boost the pressure of produced gas and liquids so that the fluids can then be diverted back to the production header or manifold. Such facilities are known in the prior art, e.g. a compact separation and boosting system which enables wells to be tested at pressures below that of the production header.
Selection of the best system to measure production rates of fluids is very much related to site conditions, economics and the operator's attitude to production management. The present invention seeks to utilise available devices to assist in measuring fluid flow rates during production.
Surface Jet Pumps (SJPs), as illustrated by
By contrast, in order to measure flow at the SJP, there are often no flow meters available. Many times, due to the location of the SJP, an operator does not have any flow rate data to estimate production gain, e.g. more flow from low pressure stream or increase in total discharge flow, by the SJP. It is desirable to know the current flow rate passing through the SJP for reasons as outlined above.
According to an aspect of the invention it is proposed to use existing pressure sensors (PTs) on the SJP to approximate flow rates based on correlations and cross-referencing the pressure values.
In practice, flow rate information will be a displayed or stored output, calculated by suitable software that has a database of established relationships between flowrates and pressures at inlets and outlets, based on the geometry of the SJP compiled through experience and experimentation. The software can be prepared and flow rate determined using momentum, conservation of mass and continuity equations, balanced against the pressure forces and velocities in the SJP.
Accordingly, data collected can profile unique behaviour allowing the software to deduce missing information/unknown flowrates. The system can be verified and optimised by use of an actual flow meter but, in the field, the SJP monitored by existing sensors becomes a flow meter.
Once enlightened to the inventive concept, correlations can be developed for more complex multiphase flow situations. In such cases, i.e. multiphase flow, there can be multiple solutions and, therefore, to minimise this issue it is suggested to include other devices, such as separator or other meters.
The present invention is applicable to gas wells which produce below 1% to 2% liquids by volume at conventional operating pressures and temperatures and use surface jet pumps to boost their production. The suggested limit for the liquid flow rate is due to the fact that within this range of liquids the performance of the SJP and values such as low pressure (hereinafter ‘LP’) generated by the SJP will not change significantly.
In the same way, this approach is also applicable to an oil well with free gas limit in low pressure stream of up to 5% by volume. The stated limit for the gas flow rate is due to the fact that within this range of gas the performance of the SJP and values such as low pressure (hereinafter ‘LP’) generated by the SJP will not change significantly
As will be mentioned hereinafter, with the use of separation on the high pressure (HP) or LP stream, the range of operation of this invention can be extended from 0% to 100% of gas in the liquid or vice versa.
In order to predict the flow rate of gas produced from the well, all is needed is the operating pressure of the SJP on the HP and LP inlet side of the SJP and its discharge pressure. As mentioned, software has been developed which enables to calculation and prediction for the flow rate of gas from the well which the SJP receives by monitoring the said operating pressures (HP, LP and discharge pressures of the SJP). This technique eliminates the need to have dedicated flow meters for each well and enables measurement of the flow rate of gas within the same level of accuracy which flow meters offer (i.e. 5% to 10% accuracy). In cases where more than 1% to 2% liquid is produced with gas volumetrically at the operating pressure and temperature, two unknowns (gas flow rate and liquid flow rate) are involved. However, if the flow rate of the gas phase is known, the software can still analyse and predict the flow rate of liquids at the operating conditions.
As a further aspect of the invention, there are multi beam ultrasonic gas flow meters which, even in the presence of liquids, can measure the velocity of the gas phase within +/−5% accuracy, but so far these meters cannot predict the liquid flow rate. Accordingly, a combination of a multi beam ultrasonic meter or an equivalent meter, with a SJP also enables the flow rate of the liquid phase to be computed. This is achieved by feeding the information on the gas flow rate (obtained from the gas meter) to software used for predicting the performance of the SJP. In this case the software has one unknown to solve (the liquid flow rate). Therefore, via the combination of a SJP and a multi beam gas meter the flow rate of produced gas and liquids can be calculated.
By way of background, a standard SJP device is illustrated by reference to
It was recognised by the inventors that a SJP installed to reduce the back pressure on selected wells can provide valuable information on the productivity and characteristics of the wells at a given flowing wellhead pressure. In order to measure or predict the production rate of gas from a gas well in absence of a test separator or flow meter, the surface jet pump can provide such valuable information by monitoring/recording the LP pressure which it has generated. The LP inlet pressure of the SJP is always measured easily, using pressure gauges or pressure transmitters which are always part of the system for monitoring the performance of the SJP.
The HP nozzle 11 alone can infer the HP flow rate passing through it by knowing the temperature and pressure value on the HP stream (along with correlation data accessible by the control software and its specific internal dimension). The pressure difference (and correlations) between the inlet LP stream and the discharge stream can indicate the LP flow rates across the SJP body (by knowing its specific critical internal dimension). A combination of these two aspects can be used for flow rate estimation.
In order to estimate flowrate in a complex mixture of gas-liquid (multiphase flow) flow, it is possible to use flow detection sensors or other commercially available devices such as densitometers, GVF (Gas Volume Fraction) meters, that are further added to the proposed system.
An alternative option is to include a limited number of flow meters (M) adjacent to the SJP to reduce cost compared to a full complement of flow meters. Particularly, using any two out of three flow meters (M) as shown in
In practice flow meters will most likely be associated with the HP line and LP line respectively. It is an aspect of the invention that a boosting SJP is supplied with meters integrally installed, but with the minimum number of components that can still provide data for flow rate in all parts.
Referring to
As a variation to the above described systems, there could be cases where there is a separator 15 upstream of the SJP which separates the LP gas and liquid phases as shown in
Flow rate prediction is intended to be implemented by suitable software. The software developed and validated for the design and prediction of surface jet pump performance enables the mass and momentum balance of the fluids passing at different points through the SJP to be calculated. This is achieved by splitting the internal parts of the SJP into several sections; where for each section mass and momentum balance equations are generated, taking into account the fluid properties of gas and liquids passing through the SJP.
The equations generated are then solved using a powerful mathematical model. The software therefore enables one of the unknowns such as the generated LP pressure or LP gas flow rate to be predicted.
In cases where some liquid is produced with LP gas, if the LP pressure at the inlet to the surface SJP is measured and is therefore known, and the LP gas flow rate is known or measured by other means such as a multi beam ultrasonic flow meter, then the remaining unknown will be the flow rate of the liquids, which the software is able to predict as the example in Table 1 below shows.
Case 1 shows the performance of the SJP with only LP gas passing through the SJP under a given motive (HP) pressure . The underlined values where calculated by the software. Case 2 shows the estimation of LP liquid flow rate when LP pressure and LP flowrates are entered along with other pressures. In these examples the software has predicted the LP liquid flow rate at the given LP gas flow rate and the measured LP pressure for each case with different LP flow rate.
Referring to
Hence the total flow passing through the SJP is HP+LP=80 at the discharge pressure read at the outlet of the SJP.
In software, the geometry of the device can be changed, and the flowrates re-calculated if needed (in other words, reproduce these curve). Hence, this SJP is not only a production booster, but it can be a meter too, according to the invention.
The curves in
The operating points (OP) on the curves is shown to link conditions in
Table 1 shows (as output of software) a more complex situation where liquid phase is involved and either gas or liquid is measured by some other means. Such a state is not easy to show in graphical form due to another dimensions involved of this complex system. Hence Table 1 shows that, by adding input parameters (which are not underlined) into the software, some other unknown can be estimated (in order for the SJP to become a meter) by balancing the internal equations.
For case, 1, just like the curves, flow rate is estimated; whereas for case 2, unlike
It will be clear that the curves in
| Number | Date | Country | Kind |
|---|---|---|---|
| 1320202.3 | Nov 2013 | GB | national |
