The present invention relates to a method for measuring the true vapour pressure of a hydrocarbon liquid, and to a related system for measuring the true vapour pressure of a hydrocarbon liquid.

The vapour pressure of a liquid is a measure of the tendency of the liquid to change into the gaseous or vapour state, i.e. a measure of the volatility of the liquid. The T rue Vapour Pressure (TVP) is the equilibrium vapour pressure of a liquid when the gas/liquid ratio is effectively zero, and is dependent on the temperature of the liquid. Another common expression for the TVP is the pressure in the container when the first bubble emerges, hence TVP is also known as the ‘bubble point vapour pressure’.

It is useful to know the TVP of a hydrocarbon liquid (e.g. oil) in the field of oil and gas production, since the TVP indicates how the oil will behave under certain conditions experienced throughout the production chain. This can have safety impacts further down the production chain, since a high vapour pressure can lead to pressure build-up, which can cause damage to transportation equipment such as pipelines or vessels.

A conventional way of measuring TVP involves inputting a liquid sample into a specialised machine and expanding the sample into volumes with different vapour/liquid (V/L) ratios. The pressure in the container is measured at each V/L ratio and a graph of V/L against pressure is plotted. The resulting graph is then extrapolated backwards, e.g. using curve fitting, to find the predicted pressure value at V/L = 0, i.e., at the TVP point. However, it is challenging to obtain good accuracy with this method, particularly when the liquid is sampled incorrectly.

Another method for precisely measuring the actual TVP value uses a PVT cell to investigate the compressibility of the fluid as it changes from a liquid to a liquid/gas mixture. However, for this method, samples need to be extracted from the liquid whose TVP is desired to be known and sent to a specialised laboratory for analysis, which is complex and often impractical logistically.

In oil and gas production, due to the challenges and impracticalities of measuring TVP, it is typical to instead design and operate with a good margin to the oil specifications. For instance a typical TVP specification is 0.965 bar. However, this can lead to safety issues since the TVP of the oil is not actually known. The variation in TVP leads to variation in volatility, which can lead to problems with volatile compounds and excessive emissions.

Viewed from a first aspect, the present invention provides a method for determining the true vapour pressure (TVP) of a hydrocarbon liquid, comprising: obtaining a gas from the hydrocarbon liquid; allowing the gas to equilibrate with the liquid in a closed container; analysing a sample of the equilibrated gas to measure the composition of the gas; and determining the TVP of the liquid based upon the gas composition measurement.

The inventors have realised that the TVP of a liquid can be determined based on measuring the composition of a gas with which the liquid is in equilibrium. The use of the gas composition information in this way improves the reliability of the TVP determination compared to traditional extrapolation techniques. Furthermore, difficult lab measurements on the liquid sample itself can be avoided since there is no need to perform direct analysis on the liquid; the liquid phase oil sample does not need to enter the analysis equipment at all. Instead, significantly simpler gas phase measurements are sufficient. Furthermore, there is no reliance on measurements for low V/L, meaning the method is less sensitive than conventional techniques. Another advantage is that air ingress in the sample will be detected during gas composition analysis and can be filtered from the data if needed.

The invention also extends to a system for measuring the TVP of the hydrocarbon liquid based on measuring the gas composition of a gas in equilibrium with the hydrocarbon liquid.

Thus, viewed from a second aspect, the present invention provides a system for determining the true vapour pressure (TVP) of a hydrocarbon liquid, comprising: a closed container for containing the liquid and a gas obtained from the liquid, wherein the container is configured to allow the gas and the liquid to equilibrate; a gas analyser configured to receive a sample of the equilibrated gas and to analyse the sample to measure the composition of the gas; and a processor configured to receive the gas composition data from the gas analyser and to determine the TVP of the hydrocarbon liquid based on the gas composition measurement.

The inventors have therefore developed an improved way of determining the TVP of a liquid which can be performed using relatively simple components. This means the TVP determination system can be easily integrated into existing oil processing systems, making TVP a known quantity rather than an unknown. This enables the oil transportation to be carried out with narrower margins and less uncertainty in the volatility of the oil, thereby improving safety of oil transportation and providing a higher quality oil.

The hydrocarbon liquid may be oil, e.g. from a produced well stream. The hydrocarbon liquid may comprise one or more liquid components which may include heavy hydrocarbons, light hydrocarbons (e.g. methane, ethane, propane, butanes, pentanes, hexanes etc), and/or a mixture of heavy and light hydrocarbons. The liquid may contain dissolved gases.

The gas may comprise one or more gas components, which may include light hydrocarbons and/or inert gases (e.g. N ₂ , CO ₂ , etc). Thus the gas may be or comprise a hydrocarbon gas. The container may be an integrated component of a larger apparatus or system such as an oil processing system. For instance, the container may comprise a separator (e.g. a gas-liquid separator) of an oil processing system. In instances where the oil processing system comprises a series of gas-liquid separators, the container is preferably the final separator in the series. This is because the TVP of the oil is ‘set’ at the final separator before being transported (e.g. to the consumer), and so determining the TVP of the liquid in the final separator provides information about the qualities of the oil during subsequent transportation.

The “TVP specification” of an oil is typically defined as the TVP of the oil at the maximum of the following temperatures: the actual oil storage temperature, the actual oil transportation temperature, and a reference temperature. The reference temperature may be, for instance, 25°C, 30°C or 38°C. Selecting the maximum of these temperatures to be the TVP specification gives some allowance for the oil to be slightly heated by the environment as it is transported or stored. Thus, the present system and method preferably determines the TVP specification, i.e. the TVP of the oil at the temperature at which the specification is defined.

As well as gas composition, the TVP calculation may additionally use other parameters, such as the temperature and/or the pressure of the gas at equilibrium. Thus, the container may comprise one or more sensors configured to monitor conditions in the container. These may include a temperature sensor for measuring the temperature of the container, and/or a pressure sensor for measuring the pressure of the container. The method may comprise determining the pressure and/or the temperature of the container when the gas and the liquid are in equilibrium.

The measurements (e.g. pressure and/or temperature) of the equilibrated gas do not necessarily have to be taken while the gas is in the container. Thus one or more of the sensors may alternatively be located outside the container, for instance in a gas outlet pipe through which the gas outputted from the container flows.

The parameters used in the TVP calculation may include volume and/or volume flow of the hydrocarbon liquid and/or the gas. The system may comprise suitable sensors for measuring these parameters, e.g. flow rate sensors and/or liquid level sensors.

The container is enclosed or sealed so as to contain the liquid and gas and prevent leakage therefrom, apart from the presence of any inlets and outlets which permit the controlled entry and exit of fluids as necessary. A closed container allows the gas and the liquid to equilibrate.

The container may comprise an inlet for receiving the liquid, e.g. from an inlet pipe. The container may also include one or more outlets for outputting liquid and/or gas from the container. The inlet and/or outlet pipes preferably comprise a valve for controlling, e.g. selectively preventing or allowing, the passage of liquid or gas therethrough. In some embodiments the one or more valves may comprise a binary valve, i.e. an on-off valve which either prevents or permits the flow of fluid therethrough. This enables the system to perform analysis on discrete samples of liquid, e.g. in a “batch process”. In other embodiments the valve(s) may comprise a variable valve for adjusting or regulating the flowrate of the fluid. This enables the system to perform analysis on a continuous flow of liquid, with the flow of fluids into and out of the container being regulated by the valves to maintain desired flow rates. The valve(s) may be controlled by a controller.

In some embodiments, the gas with which the liquid equilibrates is input into the container, e.g. via an inlet. In this case, obtaining a gas from the liquid may comprise separating the gas from the liquid. The gas may be input into the container using the same inlet as the liquid, and optionally at the same time as the liquid, e.g. by inputting a mixed gas-liquid fluid stream which comprises gas components and liquid components. The method may therefore comprise inputting a hydrocarbon fluid stream comprising a mixture of the gas and the liquid into the container, and separating the hydrocarbon fluid stream into the gas and the liquid in the container. The container may comprise one inlet configured to receive both the liquid and the gas. This is particularly advantageous when the liquid whose TVP is desired to be known is the separated liquid from a gas-liquid separator, since the container in which the gas and liquid equilibrate can be the pre-existing gas-liquid separator and thus the TVP determination system can be easily integrated into the existing system.

In other embodiments, the gas may not be input into the container in gaseous form, but instead may be created by expanding the liquid within the container so that gas evaporates therefrom. In this case, obtaining a gas from the liquid may comprise expanding the liquid within the container to form the gas. This is advantageous in instances where only a liquid sample is available, rather than a mixed gas-liquid fluid stream. For instance, customers who purchase oil typically only have access to the liquid oil itself and not to the separator from which the oil originated. Expanding the liquid can be done for instance by decreasing the pressure of the container at constant temperature or by changing the temperature of the liquid at constant pressure. This will cause some liquid to vaporise until the vapour pressure is reached. The ratio of vapour (i.e. gas) and liquid in the resulting mixture may be designated by the variable x = V/L, where V is the vapour volume and L is the liquid volume.

In such embodiments, the method may comprise expanding a first portion of the liquid to a first x value (V/L ratio), and expanding a second portion of the liquid to a second x value (V/L ratio). In this case the first and second gases formed by expansion are allowed to equilibrate with the respective portions of the liquid, a sample of each of the equilibrated first and second gas is analysed, and the determination of the TVP is based on the first and second gas composition measurement.

The method may further comprise expanding further portions of the liquid to further x values to improve the reliability of the final result.

The x values may be any suitable value, but must be greater than 0 (i.e. there is at least some gas in the mixture) and are preferably less than 5.0. For instance, suitable values may include x = 0.5, 1.0, 1.5, 2.0... 4.0, 4.5, 5.0, or more preferably x = 0.1 , 0.2,

0.3...4.8, 4.9, 5.0. In one example, the first x value may be 0.5 and the second x value may be 1.0.

It will be appreciated that when the gas and the liquid are present together in the container they will equilibrate, i.e. naturally tend to equilibrium. The gas and the liquid may be determined to be in equilibrium when the relative amount of vapour and liquid is constant, i.e., when there is no net evaporation of liquid or condensation of gas. It will be understood that the equilibrium is a dynamic equilibrium since some condensation and evaporation will still occur, but at equal rates.

Allowing the gas and liquid to equilibrate, i.e. to come into equilibrium, may comprise keeping the conditions of the container (e.g. volume, temperature and/or pressure) constant or stable for a predetermined amount of time. This may include not heating or cooling the container and not changing the volume of the container. In embodiments in which there is a continuous flow of fluid into and out of the container, the flow rates may be kept constant to allow equilibrium to be established. This allows the natural process of equilibrium to occur.

The predetermined amount of time may be based on (preferably equal to or longer than) the time in which equilibrium is typically reached, which may be generally known or determined experimentally. For example, in cases where the container comprises a separator, the predetermined amount of time may be equal to or greater than the retention time in the separator. It may be assumed that equilibrium has been reached after the predetermined amount of time, or the method may comprise determining whether equilibrium has been reached, such as by detecting the level of the liquid in the container and determining that equilibrium has been reached when the liquid level is approximately constant for a predetermined threshold of time.

The gas analyser may comprise any suitable apparatus which is configured to determine the composition of a gas, e.g. a gas chromatograph, IR or FTIR spectrometer, Raman spectrometer, etc. Gas chromatography is a standard gas analysis method with high accuracy. Gas chromatographs are particularly advantageous in the present system due to their reliability and accuracy. However, other techniques such as IR, FTIR or Raman spectroscopy may offer other advantages such as relatively small size and/or low cost spectrometers, enabling them to be easily integrated into existing systems.

Once equilibrium has been reached, a portion or sample of the equilibrated gas is input into the gas analyser for analysis of the composition thereof. In embodiments in which multiple (i.e. at least two) portions of the liquid are expanded to different V/L ratios, the method may comprise analysing a portion of the equilibrated gas from each expanded mixture.

The gas analyser may be incorporated into or in fluid communication with the container which contains the liquid and gas. For instance the system may be configured to divert at least a portion of an outputted gas stream from the container into the gas analyser, e.g. via a conduit extending between a gas outlet of the container and an inlet of the gas analyser. The system may therefore be able to do the required sampling and analysis in situ.

Alternatively, a sample of the gas can be extracted from the container, e.g. in a pressurized cylinder or other suitable sampling equipment, and the gas analysis can be carried out remotely from the container, such as in a laboratory equipped with the gas analyser. The gas outlet may therefore be connectable to a sampling device, and the gas analyser may be configured to receive the gas sample from the sampling device. This allows the same gas analyser to be used for analysis of liquids from multiple systems.

Analysis of the gas determines information about the gas composition, preferably identifying chemicals, or groups of chemicals (e.g. based on the number of carbon atoms) that are present in the gas and the relative amounts of each detected component. The gas composition data may be expressed in terms of mole fractions, i.e. the ratio of the number of moles of a particular gas component relative to the total number of moles of gas.

The gas analyser is configured to transmit the gas composition data to the processor. Thus the processor receives the gas composition data as an input parameter for the calculation of TVP. The processor may comprise any suitable numerical calculation or processing means.

A higher proportion of heavier components (i.e. longer chain molecules) in the equilibrated gas may be associated with or correspond to a lower TVP, and a lower proportion of heavier components in the equilibrated gas may be associated with or correspond to a higher TVP. This is because lighter molecules contribute more to the TVP since they tend to evaporate first. Thus, if there is a bigger proportion of lighter components compared to heavier components present in the fluid, then the TVP (i.e. the point at which the first bubble appears) will be higher, because the pressure of the hydrocarbon fluid does not need to be decreased as much before the first bubble appears. Thus, for liquids having a relatively high TVP, the proportion of light gas molecules in the equilibrated gas will be higher and the proportion of heavy molecules will be lower.

As noted previously, as well as the gas composition, the determination (e.g. numerical processing) for the TVP calculation may additionally be based on the temperature and/or pressure of the equilibrated gas. Thus the processor may be arranged to receive the temperature and/or pressure measurements from the sensors. The processor may therefore receive, as input parameters, the gas composition data and at least one of the temperature and pressure of the gas at equilibrium.

The processor may execute an algorithm which relates gas composition data, and optionally other input parameters (e.g. temperature, pressure), to TVP. Thus, the algorithm may be based upon data relating composition to TVP and/or mathematical relationships that may be empirical or based upon first principle models.

The particular algorithm used may vary depending on various factors, such as whether the gas was input into the container or generated by expansion of the liquid as described previously.

The algorithm may include the step of summing values for each component of the gas. The values which are summed may comprise the partial pressures of each gas component. Partial pressure of a component is defined as total pressure multiplied by mole fraction of the component. Thus calculating the TVP based on the gas composition may comprise determining a partial pressure of a gas component, wherein each partial pressure is determined by multiplying the total pressure by the corresponding mole fraction of that component.

The partial pressure of each component may be based on (e.g. depend on) the temperature and the pressure of the container at equilibrium. The partial pressure of each gas component may also depend on other characteristics, such as the molar enthalpy for dissolving that gas component in the liquid and the ideal gas constant.

Each partial pressure may be proportional to the total pressure of the equilibrated gas at equilibrium.

Each partial pressure may be a function of temperature. Thus, when the partial pressures are summed, the final TVP is also dependent on temperature. In a specific example, each partial pressure may be proportional to the exponential of the negative inverse of the temperature of the container at equilibrium.

The TVP as a function of temperature may be calculated according to the equation: where TVP(T ) is the True Vapour Pressure at temperature T of the fluid in Kelvin;

P ₃ is the pressure of the gas at equilibrium, T ₃ is the temperature of the gas at equilibrium in Kelvin, y _{i 3} is the mole fraction of component / in the gas phase at equilibrium; AH _t is the molar enthalpy for dissolving component / in the liquid, and R is the ideal gas constant.

This equation may be primarily applicable to embodiments in which the gas is input into the container as part of a mixed gas-liquid fluid stream.

The TVP specification T _s can be obtained by inputting the specification temperature T = T _s into equation (1). The equation therefore relates the TVP at the container temperature T3 to the TVP at the temperature at which the TVP specification is defined, T _s .

The molar enthalpy for dissolving each component may be a known value or determined experimentally, and may be stored in a look-up table and accessed by the processor.

In some instances, e.g. those utilising the above equation, the algorithm may be based on the Clausius-Clapeyron equation. The Clausius-Clapeyron equation describes the relationship between the temperature of a liquid and its vapour pressure, and can be written as follows: where p is the vapour pressure, T is the temperature of the system in Kelvin, H _vap is the molar enthalpy of vaporization of the liquid, and R is the ideal gas constant. Basing the TVP calculation on the Clausius-Clapeyron equation relies on the assumption that the gas phase is ideal, and that the equation applies to every component in the gas mixture. The method may also rely on the assumption that the enthalpy of dissolving a component in the liquid phase is constant in the relevant temperature interval, although this is not an essential assumption and merely decreases the complexity of the calculation.

In other embodiments, particularly embodiments in which two or more samples of the liquid are expanded to different V/L ratios, a different algorithm or numerical processing method may be used.

In these cases, the TVP calculation may be based on the difference between partial pressures at the two or more V/L ratios. An exemplary algorithm for this case and a derivation thereof are described below. In the following equations, the superscript values depict the value of x = V/L, and the subscript values indicate whether the quantity is a component / of the oil (oil, i), a component / of the gas (gas, i) or the total oil amount (oil).

The TVP determination is based on summing partial pressures of each component of the gas. Each partial pressure may be written as: where P, is the partial pressure of component / in the gas, K ,· is the equilibrium constant for component /, x, is the molar fraction of the component / of the oil, n ₀ u is the number of moles of oil (i.e. liquid), and n ₀ nj is the number of moles of component / of the oil. n ₀ n may be calculated as the sum over / of n ₀ i .

The TVP is calculated by summing the partial pressures at x = 0, since this is the point at which the amount of vapour reaches 0. Since this cannot easily be directly measured, an equation for determining the TVP in terms of measurable quantities may be derived as follows. In this example the values x = 0.5 and x = 1.0 are used, but other choices of x values could be used.

Firstly, the difference in partial pressures at the first and second x values is obtained using equation (3):

This equation can be rearranged to give an expression for Ki/noil:

This can be input into equation (3) to obtain an expression for the partial pressure Pi V/L for component i at general x = V/L: pl.0 _ p0.5

P ^V ' ^L = V/L l - 1.0 _ 0.5 n oίΐ,ί (6) ngas,i ⁿ gas,i

Then, to estimate the TVP (which is based on the sum of the partial pressures at V/L = 0), starting from the partial pressure difference between x = 0.5 and x = 0:

This can be rearranged to give an expression for the partial pressure at x = 0 of component /:

And thus an equation for the TVP can be obtained by summing the partial pressure for each component /:

Hence the TVP can be obtained based on inputs of the partial pressures at two different values of x (in this example, x = 0.5 and x = 1.0); and the number of moles of each component of the gas at the two different values of x. The calculation of TVP can therefore be based on the gas composition by using the gas composition to determine the molar ratio of each gas component.

The above equation is based on the example values of x = 0.5 and x= 1; however other values of x could be used. Furthermore, more than two values of x can be used in order to increase the reliability of the TVP estimation. For instance, the above general equation can be repeated with other pairs of x values in order to obtain a number of TVP estimations, and an average can be calculated.

To obtain the TVP specification, the method may be carried out at the specification temperature. For instance, the system may be configured such that the equilibrium temperature is the specification temperature, so as to directly obtain the TVP specification using equation (9). Alternatively, the method may be carried out at another temperature, e.g. the container or separator temperature, to obtain the TVP at that temperature along with the partial pressures of each component at that temperature. Equation (1) can then be used to relate the calculated TVP to the TVP at the specification temperature. This method provides some advantages over carrying out the method directly at the specification temperature. For instance, by monitoring the difference between the calculated TVP at the separator temperature and the actual pressure in the separator, one can assess the approach towards equilibrium in the container, or detect if there is any gas carried under as bubbles within the oil (e.g. if the separation efficiency is less than 100%). A temperature regulation system or temperature adjustment device may be provided to achieve and maintain the desired temperature within the system.

The above method relies on the assumption that the total number of moles in the liquid phase (n ₀ n) is constant. In practice there may be a small reduction of n ₀ u as x=V/L increases, but n ₀ u = constant is a very good approximation at least up to V/L = 4. This method may also rely on the assumption that the pressure is low, e.g. below 2-3 bara, because at higher pressure, the number of moles in the gas phase may be significant even at V/L = 4. Also, at high pressure, one may also need to account for non-ideality of the gas phase, and hence may need to use a more advanced equation of state than the ideal gas law. Thus the present method is preferably for low pressures, e.g. below 2-3 bara.

The inventors have further discovered that this calculation method can also be extended to obtain an estimated “VPCRx-curve”, that is, a general curve that describes how the vapour pressure of the liquid behaves as a function of x = V/L. ‘VPCR’ stands for Vapour Pressure of Crude Oil.

For instance, starting with the partial pressure difference between x = V/L and x = 0: v

1000 V g ^L as (10) RT This can be rearranged to give an expression for the partial pressure of component I at general V/L: pl.O _ p0.5 pV/L ₌ pO ₊ 1000

1.0 0.5 RT n 'gas,i ⁿ gas,i

Which can then be rearranged to give:

(12)

1000 V g ^v a ^/ s ^L i RT

The total curve can then be obtained by summing these partial pressures over components i and inputting equation (9) for the TVP:

This allows the determination of the pressure at any V/L ratio.

It is noted that the factor of 1000 in above equations 10-13 is present due to the choice of units of kPa for pressure. If other units are selected, then this factor may vary.

For instance, if units of bara or Pa are chosen, this factor would be 10000 or 1 respectively.

The inventors further discovered a surprising advantage in that the above TVP (and VCPRx) calculation method can be simplified by grouping certain gas components together in the same “bin” or group, without substantial loss in reliability. Thus, for instance, the method may characterize hydrocarbons based upon their C number (number of carbon atoms), e.g. using a “bin” for each C number. Moreover, this can be further simplified by grouping C numbers together, such as all those above a given value, e.g. by grouping propane and heavier hydrocarbons as “C3+”. The inventors have found that the accuracy of the TVP measurement is enhanced compared to traditional TVP estimation methods, even with this grouping. The reason for this is believed to be because the lighter hydrocarbons contribute much more to the TVP, whereas the contribution of the heavier components is less and hence they can be grouped without substantial loss of accuracy. Therefore this method advantageously requires less detailed information about the gas composition to estimate the TVP.

The calculation device (e.g. processor) may be configured to perform any or all of the above calculation steps.

The system may be configured to output any gas remaining in the system after the gas analysis has been performed. The gas may go to further processing. If the gas is not needed for further processing, the system may be arranged to purge or vent the gas into the atmosphere or surroundings. The container may further comprise an outlet for outputting the liquid. The liquid outlet may be arranged to output the liquid for further processing or transportation elsewhere.

Certain preferred embodiments of the present invention will now be described, by way of example only, with reference to the following drawings, in which:

Figure 1 shows a schematic diagram of a first embodiment of the present invention;

Figure 2 shows a schematic diagram of a second embodiment of the present invention;

Figure 3 shows a schematic diagram of a first implementation of the embodiment of Figure 2;

Figure 4 shows a schematic diagram of a second implementation of the embodiment of Figure 2; and

Figure 5 shows a graph of example results obtained using the present method.

Figure 1 shows a schematic diagram of a system 1 for determining the TVP of a liquid according to a first embodiment of the present invention. The system 1 comprises a gas-liquid separator 2, a gas chromatograph 4 and a processor 6.

The separator 2 comprises a fluid inlet 8 arranged to receive an incoming fluid stream comprising a mixture of gas and liquid components. The separator 2 is configured to separate the fluid stream into a gas and a liquid in a conventional manner, and to allow the separated gas and liquid to come into equilibrium with each other. The separator 2 further comprises a gas outlet 10 arranged to output the separated gas, and a liquid outlet 12 arranged to output the separated liquid.

The separator 2 also comprises temperature and pressure sensors 14 which are arranged to measure the temperature and pressure of the separator, respectively. The sensors 14 are configured to transmit the measurements to the processor 6.

The gas chromatograph 4 is arranged to receive a portion of the outputted gas stream from the separator 2. The gas chromatograph 4 is configured to analyse the portion of the gas to determine the composition of the gas, and to output the gas composition data to the processor 6.

The processor 6 is configured to receive, as input parameters, the gas composition data from the gas chromatograph 4 and the temperature and the pressure measurements from the sensors 14. The processor 6 is further configured to analyse the input parameters to calculate the TVP of the liquid.

In use, the system 1 functions as follows. A gas-liquid fluid stream is input into the gas-liquid separator 2 via the inlet 8. The separator 2 separates the fluid stream into a gas phase and a liquid phase, which will naturally come into equilibrium with each other within the separator 2. The sensors 14 measure the temperature and the pressure of the separator 2 when the gas and liquid are in equilibrium and output these measurements to the processor 6. Once the gas and liquid have separated and equilibrated, the gas is output via the gas outlet 10 and the liquid is output via the liquid outlet 12. A portion of the outputted gas is inserted into the gas chromatograph 4 for compositional analysis, and the remaining gas goes to further processing. The outputted liquid is transported elsewhere, e.g. to a consumer via a transport vessel or pipeline.

The gas chromatograph 4 analyses the gas portion to determine the components of the gas and the relative proportions thereof, and outputs the gas composition data to the processor 6.

The processor 6 then determines the TVP using numerical processing of the gas composition data, the temperature and the pressure of the separator 2 at equilibrium using equation (1) as described previously.

The system 1 therefore enables the determination of the TVP of the separated liquid phase by only performing composition analysis on the separated gas phase. No analysis of the liquid phase itself is required. The TVP information can then be provided e.g. to the consumer or transporter of the oil, giving them vital information as to the behaviour and characteristics of the oil and thus ensuring that appropriate safety measures are in place without needing to allow for wide oil specifications.

Figure 2 shows a schematic diagram of a system 200 according to a second embodiment of the present invention. The system 200 comprises a gas-liquid separator 202, an expansion container 204, a gas chromatograph 206 and a processor 208. This system and corresponding method may be preferred if equilibrium is not reached in the separator 202, or if there are gas bubbles entrained in the oil from the separator 202.

The separator 202 comprises a fluid inlet 210 arranged to receive an incoming fluid stream comprising a mixture of gas and liquid components. The separator 202 is configured to separate the fluid stream into a gas and a liquid. The separator 202 further comprises a gas outlet 212 arranged to output the separated gas, and a liquid outlet 214 arranged to output the separated liquid.

The system is under temperature control to maintain a constant temperature. The expansion container 204 is arranged to receive a portion (i.e. a sample) of the outputted liquid stream. The expansion container 204 is configured to expand the liquid sample to obtain a mixture of gas and liquid, and to allow the mixture to equilibrate. The expansion container 204 comprises a pressure sensor 216 that is arranged to measure the pressure of the equilibrated gas, and to transmit the measurement to the processor 208.

The gas chromatograph 206 is arranged to receive a portion of the gas from the expansion container 204, analyse the portion of the gas to determine the composition of the gas, and to output the gas composition data to the processor 208. The processor 208 is configured to receive, as input parameters, the gas composition data from the gas chromatograph 206 and the pressure measurements from the sensor 216. The processor 208 is further configured to numerically process the input parameters to calculate the TVP of the liquid.

In use, the system 200 functions as follows. A gas-liquid fluid stream is input into the gas-liquid separator 202 via the inlet 210. The separator 202 separates the fluid stream into a gas and a liquid. The gas is output via the gas outlet 212 and the liquid is output via the liquid outlet 214.

A portion of the outputted liquid is input into the expansion container 204. The remaining outputted liquid is transported elsewhere, e.g. to a consumer via a transport vessel or pipeline.

The liquid is expanded within the expansion container 204, forming a mixture of gas and liquid. The gas and liquid naturally come into equilibrium with each other since the conditions in the expansion container 204 are stable. The sensor 216 measures the pressure of the equilibrated gas and transmits the measurement to the processor 208.

A portion of the equilibrated gas is inserted into the gas chromatograph 206 for compositional analysis. The gas chromatograph 206 analyses the gas portion to determine the components of the gas and the relative proportions thereof, and outputs the gas composition data to the processor 208.

This process is then repeated with a second sample of the outputted liquid, wherein the second sample is either input into the first expansion container 204 (after the first sample has been purged therefrom) or input into a second separate expansion container 204 to enable simultaneous processing of the liquid samples. A preferred method is to split one liquid sample in two parts.

Thus, a second portion of the outputted liquid is input into an (or the) expansion container 204 and expanded therein to form a mixture of gas and liquid at a second V/L ratio. The gas and liquid naturally come into equilibrium with each other since the conditions in the expansion container 204 are stable. The sensor 216 measures the pressure of the equilibrated gas and transmits the measurement to the processor 208. A portion of the equilibrated gas is inserted into the gas chromatograph 206 for compositional analysis, which is preferably the same gas chromatograph 206 as for the first sample to reduce costs and unnecessary duplication of components. The gas chromatograph 206 analyses the gas portion to determine the components of the gas and the relative proportions thereof, and outputs the gas composition data to the processor 208.

The processor 208 then determines the TVP using numerical processing of the gas composition data from the sample at the first V/L ratio, the gas composition data from the sample at the second V/L ratio, and the pressure of the expansion cha ber(s) 204 at equilibrium for each sample. The processor may use equation (9) as described previously.

This system 200 enables the determination of the TVP of the separated liquid phase by expanding a sample of the separated liquid phase at two (or more) different V/L ratios and performing composition analysis on the resulting gas phases. This system 200 therefore enables the determination of the TVP of the separated liquid phase by only performing composition analysis on the separated gas phase. No analysis of the liquid phase itself is required.

While this example has been described with reference to the outputted liquid from a gas-liquid separator 202, it will be appreciated that any liquid sample could be used.

Indeed, an advantage of this embodiment over the first embodiment is that the method can be applied to any liquid sample, since the gas is created by expansion of the liquid sample rather than by separating a mixed gas-liquid fluid stream. As described previously, this embodiment may also be preferred if there is not equilibrium in the separator 202, or if there are gas bubbles entrained in the oil from the separator 202.

Figure 3 shows a schematic diagram of a system 300 according to a third embodiment of the present invention.

The system 300 of Figure 3 receives and analyses an oil sample, which could be an oil sample from a gas-liquid separator. Hence the system 300 can be a specific implementation of the embodiment of Figure 2.

The system 300 comprises a first container 302, a second container 304, a gas circulation pump 306 and a gas analyser (e.g. a gas chromatograph) 308.

The system 300 further comprises an inlet pipe 310 for inputting an oil sample. The oil sample may for instance be a sample of a liquid outputted from a gas-liquid separator. The inlet pipe 310 comprises a first binary valve 312a arranged to control the incoming flow of oil.

The first container 302 comprises a first inlet 314 arranged to receive the oil sample from the inlet pipe 310. The first container 302 further comprises a gas outlet 316 and a liquid outlet 318 arranged to output a gas phase and a liquid phase respectively. The liquid outlet 318 is arranged to drain liquid from the first container 302 after the necessary measurements have been completed. Draining of the liquid is controlled by a binary valve 312e. The first container 302 also comprises a liquid level sensor 324a for detecting the level of liquid in the container 302 and a temperature sensor 324c for measuring the temperature in the first container 302.

The second container 304 comprises an inlet 320 that is connected to the gas outlet 316 of the first container 302. A binary valve 312b is arranged to control the flow of gas from the first container 302 to the second container 304. The second container 304 further comprises an outlet 322 arranged to output gas therefrom, a pressure sensor 324b for measuring the pressure of the second container 304, and a temperature sensor 324c for measuring the temperature of the second container 304. A purge gas supply 342 is connected to the second container 304.

The gas circulation pump 306 is connected to the outlet 322 of the second container 304. The pump 306 is configured to receive the gas output from the second container 304 and to pump the gas to circulate the gas through the system 300.

The gas analyser 308 is arranged to receive a portion of the gas output by the pump 306 and to analyse the gas composition thereof.

The system 300 comprises a gas outlet pipe 326 for purging gas from the system after the measurement is complete, and a return pipe 328 for circulating gas back to the first container 302. The gas outlet pipe 326 and the return pipe 328 comprise binary valves 312c, 312d to control the flow of fluid therethrough. A vacuum pump 340 is arranged on the gas outlet pipe 326.

Operation of the system 300 proceeds as follows. Throughout the process, the shaded region (e.g. the first and second containers 302, 304 and the pump 306) is kept at constant specified temperature by a temperature regulation system (not shown).

In an initial starting condition, the system 300 is cleaned by purging the system 300 with inert gas from the purge gas supply 342, typically argon or nitrogen, and then evacuated to very low vacuum with the vacuum pump 340. All valves are then closed.

The oil sample is input into the first container 302 through valve 312a. It is assumed that the oil comes from a source with enough pressure to cause the oil to enter the container 302. Oil is filled to a predefined level in the first container 302, with the predefined level being determined by the desired x=V/L for that measurement. The level of oil is monitored by the liquid level sensor 324a. Upon entry into the first container 302, gas will flash from the oil and fill the head space within the first container 302.

When enough oil has filled the container 302, the valve 312a is closed. As soon as the liquid level measurement is stable enough, the valve 312b is opened. This will cause more gas to flash from the oil, and the second container 304 will be filled with gas. The pressure within the second container 304 is monitored by the pressure sensor 324b.

When the liquid level as measured by liquid level sensor 324a is stable enough again and the pressure measurement as measured by the pressure sensor 324b does not vary too much, the valve 312d is opened and the gas circulation pump 306 is started. The purpose of the pump 306 is to circulate the gas within the system 300 and bubble it through the oil in order to make sure that the gas phase is homogenous, and that true equilibrium between the gas and the oil is achieved. While the gas is circulating, the temperature of the oil and gas is monitored by temperature sensors 324c. The gas circulation continues until the temperature is as specified by the temperature regulation system and the pressure 324b is constant enough. When this is achieved, the valves 312b, 312d are closed in order to separate the oil from the gas. This is necessary because pressure in the second container 304 will decrease when the gas is sampled, and if the oil were in contact with the gas in the second container 304 when the pressure decreases, more gas would flash from the oil and the equilibrium would be disturbed.

The gas is then sampled by the gas analyser 308. Several samples of the gas may be taken in order to improve the statistics of the measurement.

Once the gas has been sampled and analysed, the valves 312e, 312b are opened and the oil is drained from the system 300. The first container 302 may then be cleaned with a solvent in order to ensure all oil is removed.

The system is then back at the initial starting point, where the system 300 is purged with gas from the purge gas supply 342 and evacuated with vacuum pump 340. If the vacuum pump 340 is powerful enough, purging with gas may be unnecessary.

As described above, all valves are then closed and the system is ready to perform a new measurement.

The process is then repeated with a second oil sample. The level of oil in the first container 302 is adjusted to expand the second liquid sample to a different V/L ratio (i.e. x value) so as to obtain gas composition and pressure measurements relating to a different x value.

A processor (not shown) then receives the gas composition data and the pressure data relating to the samples at different x values and numerically processes the data to determine the TVP of the initial oil sample.

It will be recognised that the system of Figure 3 depicts a “batch process” implementation. In other words, in a temperature controlled environment, a discrete oil sample is input into the first container 302, the expansion and analysis process is carried out, the sample is then drained/purged, and then the process repeats in cycles with further samples.

Alternative implementations involve a “continuous” type process. Figure 4 shows an example “continuous” process embodiment of the present invention.

The system 400 comprises a container 402, a temperature adjustment device (e.g. heater or cooler) 404, and a gas chromatograph 406.

The system 400 is arranged to receive a continuous flow of oil via an inlet pipe 408. The inlet pipe 408 comprises an adjustable valve 410a arranged to regulate the flow of the incoming oil to obtain a desired flow rate of oil through the system 400. The adjustable valve 410a may be controlled by a controller (not shown).

The container 402 comprises an inlet 412 that is arranged to receive the incoming oil flow from the inlet pipe 408, a gas outlet 414 for outputting gas and a liquid outlet 416 for outputting liquid. The container 402 further comprises a liquid level sensor 418a arranged to measure the level of liquid in the container 402, and a pressure sensor 418b arranged to measure the pressure in the container 402.

The liquid outlet 416 is connected to a liquid outlet pipe 420 comprising an adjustable valve 410b for regulating the flow of liquid therethrough. The adjustable valve 410b may be set to achieve a desired liquid level in the container 402: too low of a liquid level will give a shorter oil residence time and a risk of gas blow-by to the oil outlet, and too high of a liquid level will give a shorter gas residence time and a risk of flooding container 402. The target level of liquid in container 402 is one which allows enough residence time to reach equilibrium. The liquid outlet pipe 420 also comprises a temperature sensor 418c for measuring the temperature of the liquid, and a flow rate sensor 418d for measuring the flow rate of the liquid. The adjustable valve 410a in the inlet pipe 408 may be controlled or set based on obtaining a desired flow rate of liquid in the outlet pipe 420.

The gas outlet 414 of the container 402 is connected to a gas outlet pipe 422 comprising an adjustable valve 410c for regulating the flow of gas therethrough. The adjustable valve 410c may be set to achieve a desired pressure in the container 402. The gas outlet pipe 422 also comprises a temperature sensor 418e for measuring the temperature of the gas, and a flow rate sensor 418d for measuring the flow rate of the gas. The heating or cooling by the temperature adjustment device 404 may be set or controlled in order to achieve a desired output gas temperature.

The gas analyser 406 is connected to the gas outlet pipe 422 and arranged to receive a portion of the output gas. The gas analyser 406 is configured to analyse the gas portion to determine the composition thereof.

Operation of the system 400 proceeds as follows.

An oil sample, in this case a continuous flow of oil, enters the system 400 via the inlet pipe 408. The first adjustable valve 410a is controlled to regulate the flow rate of the incoming flow of oil. The oil flow passes through a temperature adjustment device 404 where it undergoes heating or cooling as desired. The heated or cooled oil then enters the container 402, wherein some gas flashes out of the liquid.

The gas exits the container 402 via the gas outlet 414, and the liquid exits the container 402 via the liquid outlet 416. The adjustable valve 410b on the liquid outlet pipe 420 is set to maintain a desired liquid level within the container 402, and the adjustable valve 410c on the gas outlet pipe 422 is set to maintain a desired gas output flowrate. The conditions within the container 402 (i.e. the temperature, the volume, and the rate of entry and exit of fluids) are stable, and hence the gas and liquid are in equilibrium with each other within the container 402. The pressure in the container 402 and the temperature of the equilibrated gas are measured using sensors 418a, 418b.

A portion of the output gas is inserted into the gas analyser 406, where the composition thereof is analysed.

The process is repeated at a second x value (V/L ratio) by adjusting the flow rates of the outgoing oil and gas streams as measured by flow rate sensors 418f, 418d.

A processor (not shown) then receives the gas composition data and the pressure and temperature data relating to the samples at the different x values, and numerically processes the data to determine the TVP of the oil.

Advantages of the system 400 as compared to the system 300 include avoiding the need to clean and purge the system between measurements, and the absence of pumps required to circulate the gas and/or evacuate the system between measurements.

However, there are also some disadvantages. For instance, it may be more challenging to be sure that equilibrium is reached in system 400, so it may be necessary to have good range on the flow control valves/sensors and to have good methods to tune the required residence time in the container. It may also be more challenging to keep a constant temperature in the entire system downstream of the temperature adjustment device 404. The systems 300, 400 described with reference to Figures 3 and 4 could for instance be connected directly to the liquid outlet of a gas-liquid separator, e.g. the gas-liquid separator 202 shown in Figure 2. Thus the systems can be integrated into existing oil processing equipment.

Figure 5 shows a graph demonstrating the results attained using the present system and method.

The graph shows estimated VCPRx curves and estimated TVP values (the point where x = 0) as compared to the actual value. The graph includes data from three different scenarios: one in which the gas components are grouped into three bins of ‘up to C6’ (molecules with up to 6 carbon atoms), ‘C7+’ (7 or more carbon atoms), and H2O; one in which the gas components are grouped into three bins of ‘up to C2’, ‘C3+’, and H2O; and one in which the gas components are grouped into three bins of ‘up to CT, ‘C2+’, and H2O.

It can be seen that the TVP as estimated using the present invention is very close to the true value, and hence the reliability of this method is demonstrated.

The graph also demonstrates the surprising result that the estimation provides improved reliability even when grouping multiple heavy hydrocarbons together, since in all three scenarios the estimated TVP value is very close to the actual TVP.. Thus, the gas composition data does not need to be particularly detailed in order to attain reliable TVP estimation with the present invention.