GB2506818A

GB2506818A - Monitoring production of a fluid through a production separator train

Info

Publication number: GB2506818A
Application number: GB1401897.2A
Authority: GB
Inventors: Abdelhamid Guedroudj
Original assignee: Petroleum Experts Ltd
Current assignee: Petroleum Experts Ltd
Priority date: 2014-02-04
Filing date: 2014-02-04
Publication date: 2014-04-09
Anticipated expiration: 2034-02-04
Also published as: GB2506818B; GB201401897D0

Abstract

A method of modelling/monitoring production through a production separator train involves determining a scaling factor to convert a measured property of production fluid such that the converted measure property is expressed in terms of it having been passed through a reference separator train and applying said scaling factor to a measured property of said production fluid. The step of determining a scaling factor may comprise creating models of the production separator train and of a reference separator train. These models are then used to determine a property of the production fluid subsequent to it passing through the production separator train, and a property of a production fluid subsequent to it passing through the reference separator train.

Description

A method of monitoring production of a production fluid The present disclosure relates to a method of, and apparatus for, monitoring production of a production fluid through a production separator train in a hydrocarbon production system.

Reservoir modelling techniques are used to monitor production of a production fluid during production operations in a hydrocarbon production system. The production fluid may be a multiphase fluid comprising oil, natural gas and water in varying quantities. However the actual production rate of oil and gas expressed in standard conditions for temperature and pressure and the gas-oil ratio (GOR, usually expressed in standard cubic feet/standard barrel) is dependent on the path taken as the fluid travels to the surface and is separated (the "separator conditions" or "path to surface"). Standard conditions for temperature and pressure may be as defined by the Society of Professional Engineers (SPE). The actual values may be 14.7 psia (101.4kPa) and 60 degrees F (15.56 degrees C).

At the moment, differences observed when comparing models with measured data are attributed to other factors, when in reality they are physically due to the inconsistent path to surface in models and reality of the field. It is desirable that measurements which are taken from the field (and used to match models of reservoirs/production systems) are made consistent with the path to surface which the engineers have chosen in the underlying PVT descriptions embedded in the models themselves. In addition, real time monitoring of fields rely on virtual meters (models that convert measured quantities such as pressure and temperature into rates] and PVT transformation to an agreed path to surface is the only way that consistent rates are calculated at various point in the production system.

SUMMARY OF INVENTION

In a first aspect of the invention there is provided a method of monitoring production of a production fluid through a production separator train in a hydrocarbon production system, said production separator train comprising one or more separator stages; said method comprising: determining a scaling factor operable to convert a measured property of said production fluid, such that the converted measured property is expressed in terms of it having been passed through a reference separator train; and applying said scaling factor to a measured property of said production fluid.

In a second aspect of the invention there is provided an apparatus specifically configured for monitoring production of a production fluid through a production separator train in a hydrocarbon production system, said production separator train comprising one or more separator stages; said apparatus being operable to: determine a scaling factor operable to convert a measured property of said production fluid, such that the converted measured property is expressed in terms of it having been passed through a reference separator train; and apply said scaling factor to a measured property of said production fluid.

It should be understood that references to a production fluid includes any component fluid of said production fluid, for example subsequent to separation of the component from the production fluid.

Other aspects of the invention comprise a computer program comprising computer readable instructions which, when run on suitable computer apparatus, cause the computer apparatus to perform the method of the first aspect; and an apparatus specifically adapted to carry out all the steps of any of the method of the first aspect.

Other non-essential features of the invention are as claimed in the appended dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of example only, by reference to the accompanying drawings, in which: Figure 1 is a simplified diagram of a digital oilfield platform; Figure 2 is a flowchart which illustrates steps of a method according to an embodiment of the present invention; and Figure 3 depicts schematically (a] an example actual separator train comprising three stages and (b) an example reference separator train also comprising three stages.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Fluid simulation tools are widely used in the petroleum industry. An accurate representation of fluid dynamics is important for these models. The applicants have developed a digital oilfield' environment/platform which enables users to combine real time measurements (e.g. pressures, temperatures and rates), with mathematical models representing different physical aspects of the application field (e.g. reservoir models or well models). This environment can help users analyse data during field operations, and then operate the field to exploit the maximum value from assets.

The digital oilfield platform) illustrated in Figure 1, can be divided into four main parts: 1. The core of the system is the mathematical representation of the field, comprising a series of different physical models 110. These models 110 provide the calculation engine for engineering and business workflows, to perform operations (e.g. forecasting "what if" scenarios, optimisation, well surveillance etc.). An integrated production model may consist of models representing the different parts of the system, such as the reservoir, wells, surface pipelines and equipment and any downstream models (for example, process facilities). These models 110 are integrated to form a single dynamic model from which the impact of each individual element on each of the other elements can be seen.

2. The physical models 110 are kept in a Model Catalogue 120. The users can then track the most accurate and up to date representation of the actual field. The Model Catalogue 120 (MC) acts as a model management, control and auditing system which will provide the correct and valid model to either users or engineering workflows running in an automated fashion. The system will also keep track of the model versions over time. Therefore, the workflows can be run on historical data as well as being able to track model evolution.

3. The orchestration of algorithms, referred herein as workflows (both automated and on-demand) is carried out by the Workflow Manager 130 (IFM -Integrated Field Management]. The workflow manager is responsible for the execution of the workflows, the scheduling of any automated workflows and for providing an interface for users to interact with said workflows if they form part of a diagnostic or decision making process.

4. The top layer corresponds to Integrated Visualisation 140 [IVM -Integrated Visualisation Management). The role of IVM is to pre/post-process data and to present all the information regarding the current situation of the field, historica' trends, potential/optimisation resufts and any other information to the users, managers and operators in a visual and easy to read display.

The visualisation layer can display high-level data as well as specific components (e.g. wells, pipelines, overall field key performance indicators,

etc.) in the field.

Hydrocarbon fluids are complex multiphase fluids comprising various hydrocarbon chains. Different fractions of gaseous and liquid phases may coexist in the fluid, depending on the fluid condition. The volume of a particular fluid mass and the rate of volumetric flow vary depending upon the temperature and pressure at which they are measured (due to phase transfer and compressibility). It is therefore preferable to define rates in any system in terms of volumetric rates of oil and gas obtained at specific reference conditions for temperature and pressure, referred to as Standard Conditions (SC].

In order to bring fluids down to standard conditions from high pressure production systems, the fluids are passed through one or more separator units [also referred to as path to surface' or separator train'). These separator units allow the fluid to equilibrate at a certain pressure and temperature before separation into gas and liquid streams. It can be shown that the volumetric oil rate, volumetric gas rate and the gas-oil ratio (GOR) of a given fixed composition are dependent upon the fluid path to surface conditions. For example, if a certain mass flow rate of a given composition is passed through three different separator trains, the volumetric rate of both oil and gas measured at the surface (and therefore the Gas Oil Ratio [GOR)) differs for each separator train. The modelling results show that the path to surface can change the measured GOR by over 30%.

As already mentioned, an accurate modelling of fluid dynamics (i.e. accuracy and robustness of the physical models implemented in the simulation tool) is important for oil and gas industry. These models need to be updated in order to include the latest production data so as to ensure a close representation of real' production systems. For well models, this can be achieved by comparing the results of a well test to the calcukted performance of the well model operating under the same conditions. This benchmarking/ matching process requires the flow rates, GOR, water cut and operating pressures to be measured in the field at the time of the test and then transferred to the model as a well test For reservoir models, this can be achieved by updating the history of the models to ensure that the same volume of fluid which has been extracted from the real reservoir has also been extracted from the models. This benchmarking/ matching process requires the flow rates, GOR, water cut (WC) and operating pressures to be measured in the field throughout the life of the reservoir and, then, transferred to the model as a production history.

A standard practice used in the industry is to define a reference "path to surface" which is used to describe all flow rates from the models. This reference path to surface is commonly chosen to be the same as the production path to surface (i.e. the path to surface the fluid takes during production in the fie'd); however it can differ from this. During well tests, the path to surface is often far less complex then the reference path and therefore, test rates are often measured through these test separators.

If the fluid properties (e.g. flow rates, GOR or water cut) used during this history matching process are incorrect) then the models will be adjusted to match conditions which never actually occurred. Thus, the models become no longer representative of the real system and) therefore, the use of these models to run real time workflows and/or well surveillance leads to erroneous results.

Consequently, a method or workflow is required to help ensure that all data used in the models is more representative of the flowing conditions, assuming that the reference path used within the models is the same as the real' path to surface.

Described herein is a method which corrects (i.e. express in terms of the reference path to surface) measured flow rates obtained under conditions other than the reference path to surface.

The method disclosed herein may be used in a real time production system, for example the digital oilfield pktform, when the measured data is first brought into the system. Consequently, users or other workflows which require measured surface rates [or GOR and WC] can access, from a central location, the rates (or GOR and WC) corrected using the method disclosed herein, rather than every workflow having to correct these rates individually.

The new method uses fundamental thermodynamic properties obtained from fully characterised equation of state models [EoS) in order to carry out the calculation.

Figure 2 is a flow diagram describing a method according to an embodiment of the invention. At step 210, the current real time data is collected and used to interpret the actual separator train conditions. The actual real time data may comprise input data and output data of the actual separator trains. It may also comprise data measured at intermediate stages of the separator train. The input data may comprise, for example, the volumetric flow rate and/or fluid composition ratio. The intermediate and output data may comprise rates and/or volumes of the separated fluids, at each stage. Given this information, the actual separator train conditions can be estimated if needed. Actual separator pressures and temperatures are typically available, but not always. In reality, depending on the field, some data may be available, while other data may not be. Consequently, it is useful to be able to back calculate missing pieces of information before doing the PVT transformation At step 220, the model is set up with the actual separator train conditions as determined at step 210, and with the reference separator train conditions, which are known.

At step 230, the EoS model is used to calculate fluid properties resulting from passing the measured data through the actual separator train. At step 240 the EoS model is used to calculate fluid properties resulting from passing a unit amount (e.g a number of moles of fluid) through the reference separator train.

In steps 230 and 240, the process may comprise calculating thermodynamic properties such as surface gas volume' SGV and/or surface oil volume' SOV, for both actual and reference separators trains. This may be done using fully characterised EOS models, and may be achieved by means of the so-called flash calculations which are well known in the art. The properties of the fluid, obtained at each separator stage, are stored and can be used later in the transformation of the pressure-volume-temperature [PVTJ properties.

At step 250 the results obtained in steps 230 and 240 are used to determine a scaling factor between current operation conditions and reference conditions. The scaling factor may be used to transform the measured rates into equivalent rates which would have been obtained had they passed through the reference separator train instead of the actual separator train.

At step 260, these equivalent rates are output as corrected rates which can be used by other operators or elements of the system. The composition of the fluid can then be calculated for different stages of the production process.

The above method maybe applied to measured data as it is brought into the system.

As a consequence, all other elements of the system which require measured surface rates (or GOR and WC) will be able to use these corrected' rates from a central location rather than correct these rates individually.

Figure 3 shows (a] an example actual separator train comprising three stages and (b] an example reference separator train also comprising three stages. The conditions, i.e. pressure and temperature, at each stage are denoted as where x is 1 and 2 respectively for the first two stages of the actual separator train and x is 3 and 4 respectively for the first two stages of the reference separator train. At the final stage of both separator trains, the conditions Psc, Tsc are the Standard Conditions. At each stage of the actual separator train, the surface gas volume SGVa(n] and the surface oil volume SOVa(n) is shown, where n is the stage number.

Similarly, at each stage of the reference separator train, the surface gas volume SGVr(n] and the surface oil volume SOVr(n] is shown, where n is the stage number.

These values are calculated atsteps 230 and 240 of the flowchartof Figure 2.

In an embodiment, measurement data D1, (e.g. volumetric rate etc.] are input to the separator trains. The output of the actual separator train is the standard condition gas rate for the actual conditions Qga@sc and the standard condition oil rate for the actual conditions Qoa@sc. Similarly, the output of the actual separator train is the standard condition gas rate for the actual conditions and the standard condition oil rate for the actual conditions Qnrsc.

At step 250 a scaling factor for the oil rate can be calculated from (for example] the ratio of SOVr(m] and SOVc(m], where m denotes the final stage (3 in this example].

It follows therefore that the oil rate at standard conditions from the reference separator Qrir@sc can be calculated from the oil rate at standard conditions through the actual separator conditions Qoa@sc (that is the measured standard condition oil rate] by: Q Th.. = __________ * SOVr(3) SOVc(3) Also, the gas oil ratio of the reference separator GOR1.0ris: -SGVr(1)+SGVr(2)+SGVr(3) (IORRC1 -SOVr(3) The GOR is the most commonly used parameter to describe the fluid in discussions between engineers. Although it is simply a ratio of phase rates, it gives engineers a feel of the quality of the fluid they are dealing with. Inputs into physical models are normally on the basis of the main phase rate (e.g. liquid or gas) and then the water and gas fractions (e.g. Water Cut and G0R) rather than all three phase rates (oil, water and gas) separately.

Normally the gas rate will be calculated based upon the corrected oil rate and GOR but could also be calculated as shown below: Q = _________ * SGVr(3) SGVc(3) One or more steps of the methods and concepts described herein may be embodied in the form of computer readable instructions for running on suitable computer apparatus, or in the form of a computer system comprising at least a storage means for storing program instructions embodying the concepts described herein and a processing unit for performing the instructions. As is conventional, the storage means may comprise a computer memory (of any sort), and/or disk drive, optical drive or similar. Such a computer system may also comprise a display unit and one or more input/output devices.

The concepts described herein find utility in all aspects of surveillance, monitoring, optimisation and prediction of hydrocarbon reservoir and well systems, and may aid in, and form part of, methods for extracting hydrocarbons from such hydrocarbon reservoir and well systems.

It should be appreciated that the above description is for illustration only and other embodiments and variations may be envisaged without departing from the scope of the invention.

Claims

Claims 1. A method of monitoring production of a production fluid through a production separator train in a hydrocarbon production system, said production separator train comprising one or more separator stages; said method comprising: determining a scaling factor operable to convert a measured property of said production fluid, such that the converted measured property is expressed in terms of it having been passed through a reference separator train; and applying said scaling factor to a measured property of said production fluid.
2. A method as claimed in claim 1 wherein said step of determining a scaling factor comprises: creating a model of said production separator train; creating a model of a reference separator train; using said mod& of the production separator train to determine at least one property of said production fluid subsequent to said production fluid passing through said production separator train; using said model of the reference separator train to determine at least one property of a production fluid subsequent to said production fluid passing through said reference separator train; and using said at least one property of said production fluid subsequent to said production fluid passing through said production separator train and said at least one property of a production fluid subsequent to said production fluid passing through said reference separator train to determine said scaling factor.
3. A method as claimed in claim 2 comprising the steps of: obtaining measured production fluid data; determining characteristics of said production separator train from said measured production fluid data; and using the determined characteristics of said production separator train in the step of creating a model of said production separator train.
4. A method as claimed in claim 2 or 3 wherein said step of using said model of the production separator train to determine at least one property of said production fluid subsequent to said production fluid passing through said production separator train comprises using said model on measured production fluid data.
5. A method as claimed in any of claims 2 to 4 wherein said step of using said model of the reference separator train to determine at least one property of a production fluid subsequent to said production fluid passing through said reference separator train comprises using said model on a unit amount of said production fluid.
6. A method as claimed in any preceding claim comprising the initial step of measuring said production fluid to obtain said measured production fluid data.
7. A method as claimed in any preceding claim wherein said production separator train is operable to separate said production fluid into two or more component fluids.
8. A method as claimed in claim 6 wherein said production separator train is operable to separate said production fluid into oil, gas and water.
9. A method as claimed in any preceding claim wherein said production separator train is operable to reduce the pressure and temperature of said production fluid to standard conditions.
10. A method as claimed in any preceding claim comprising the step of determining one or more properties of said production fluid at different separator stages of said reference separator train.
11. A method as claimed in any preceding claim wherein said measured property of said production fluid comprises a measured property of a component fluid of said production fluid.
12. A method as claimed in any preceding claim wherein said measured property of said production fluid comprises a volumetric rate of said production fluid or a component fluid thereof
13. A computer program comprising computer readable instructions which, when run on suitable computer apparatus, cause the computer apparatus to perform the method of any preceding claim.
14. A computer program carrier comprising the computer program of claim 13.
15. An apparatus specifically configured for monitoring production of a production fluid through a production separator train in a hydrocarbon production system, said production separator train comprising one or more separator stages; said apparatus being operable to: determine a scaling factor operable to convert a measured property of said production fluid, such that the converted measured property is expressed in terms of it having been passed through a reference separator train; and apply said scaling factor to a measured property of said production fluid.
16. An apparatus as claimed in claim 15 being further operable to: create a model of said production separator train; create a model of a reference separator train; use said model of the production separator train to determine at least one property of said production fluid subsequent to said production fluid passing through said production separator train; use said model of the reference separator train to determine at least one property of a production fluid subsequent to said production fluid passing through said reference separator train; and use said at east one property of said production fluid subsequent to said production fluid passing through said production separator train and said at least one property of a production fluid subsequent to said production fluid passing through said reference separator train to determine said scaling factor.
17. An apparatus as claimed in claim 16 being further operable to: obtain measured production fluid data; determine characteristics of said production separator train from said measured production fluid data; and use the determined characteristics of said production separator train in creating a model of said production separator train.
18. An apparatus as claimed in claim 16 or 17 being operable such that said using of the model of the production separator train to determine at least one property of said production fluid subsequent to said production fluid passing through said production separator train comprises using said model on measured production fluid data.
19. An apparatus as claimed in any of claims 16 to 18 being operable such that said using of the model of the reference separator train to determine at least one property of a production fluid subsequent to said production fluid passing through said reference separator train comprises using said model on a unit amount of said production fluid.
20. An apparatus as claimed in any of claims 15 to 19 being operable to measure said production fluid to obtain said measured production fluid data.
21. An apparatus as claimed in any of claims 15 to 20 wherein said production separator train is operable to separate said production fluid into two or more component fluids.
22. An apparatus as daimed in daim 21 wherein said production separator train is operable to separate said production fluid into oil, gas and water.
23. An apparatus as claimed in any of claims 15 to 22 wherein said production separator train is operable to reduce the pressure and temperature of said production fluid to standard conditions.
24. An apparatus as claimed in any of claims 15 to 23 being operable to determine one or more properties of said production fluid at different separator stages of said reference separator train.
25. An apparatus as claimed in any of claims 15 to 24 wherein said measured property of said production fluid comprises a measured property of a component fluid of said production fluid.
26. An apparatus as claimed in any of claims 15 to 25 wherein said measured property of said production fluid comprises a volumetric rate of said production fluid or a component fluid thereof