Patent Publication Number: US-9835012-B2

Title: Simplified compositional models for calculating properties of mixed fluids in a common surface network

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a U.S. National Stage patent application of International Patent Application No. PCT/US2015/020298, filed on Mar. 12, 2015, which claims the benefit of U.S. Provisional Patent Application No. 61/951,825, filed on Mar. 12, 2014, titled “Procedure for Using Simplified Compositional Models for Calculating Properties of Mixed Fluids in a Common Surface Network,” the benefit of both of which are claimed and the disclosure of both of which are incorporated herein by reference in their entirety. 
    
    
     FIELD OF THE DISCLOSURE 
     The present disclosure relates generally to the recovery of subterranean deposits and more specifically to the recovery of subterranean hydrocarbon deposits from multiple reservoirs through a common surface network. 
     BACKGROUND 
     When multiple reservoirs are produced through a common facility network, the capability to integrate the modeling of surface and subsurface can be critical to field development and optimization. The shared facility network imposes constraints that the combined production cannot exceed, determines the pressure drop in the flow lines, and the composition and volume of the sales and reinjection streams. Pressure drop in flow lines is particularly important in deepwater field development, where flow lines are long, and production from multiple reservoirs can flow through the same riser. 
     Often, the fluid characterizations of these reservoirs have been derived independently. In each case, the appropriate fluid representation was selected that provided an optimum combination of accuracy and computational efficiency. The two most common fluid characterizations are the equation of state (EOS) and the black oil model. 
     A hydrocarbon fluid may actually be composed of hundreds of distinct components. When modeling using an EOS, the engineer must specify the number of pseudo-components (typically 5 to 12) and their EOS properties. Pseudo-components are combinations of actual components. Alternatively, black-oil modeling involves specification of a number of common engineering measurements in tables that vary with pressure. However, it is inherently a model with two pseudo-components. The net result is that the different connected reservoirs are being modeled with a variable number of pseudo-components, some of which may be common. However, even the common pseudo-components may have different fluid properties in the different reservoirs. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Illustrative embodiments of the present disclosure are described in detail below with reference to the attached drawing figures. 
         FIGS. 1A and 1B  illustrate examples of production wells suitable for hydrocarbon production and exploration from a subsurface reservoir. 
         FIG. 2  is a block diagram of an exemplary system for simulating fluid production in a multi-reservoir system with a common surface network. 
         FIG. 3  is a diagram illustrating an exemplary of a multi-reservoir system with a common surface network. 
         FIG. 4  is a flowchart of an exemplary method of using EOS compositional models to simulate fluid production and calculate properties of fluids produced in a multi-reservoir system with a common surface network. 
         FIG. 5  is a block diagram of an exemplary computer system in which embodiments of the present disclosure may be implemented. 
     
    
    
     The illustrated figures are only exemplary and are not intended to assert or imply any limitation with regard to the environment, architecture, design, or process in which different embodiments may be implemented. 
     DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
     Embodiments of the present disclosure relate to using simplified compositional models to simulate fluid production and calculate properties of mixed fluids produced in a multi-reservoir system with a common surface network. While the present disclosure is described herein with reference to illustrative embodiments for particular applications, it should be understood that embodiments are not limited thereto. The description of the present disclosure has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the embodiments in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the disclosure. The illustrative embodiments described herein are provided to explain the principles of the disclosure and the practical application thereof, and to enable others of ordinary skill in the art to understand that the disclosed embodiments may be modified as desired for a particular implementation or use. The scope of the claims is intended to broadly cover the disclosed embodiments and any such modification. Any actual data values listed in the detailed description are provided for illustrative purposes only and embodiments of the present disclosure are not intended to be limited thereto. Thus, the operational behavior of embodiments will be described with the understanding that modifications and variations of the embodiments are possible, given the level of detail presented herein. 
     In the detailed description herein, references to “one embodiment,” “an embodiment,” “an example embodiment,” etc., indicate that the embodiment described may include a particular feature, structure, or characteristic, but every embodiment may not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is submitted that it is within the knowledge of one skilled in the art to implement such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described. 
     As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprise” and/or “comprising,” when used in this specification and/or the claims, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. The corresponding structures, materials, acts, and equivalents of all means or step plus function elements in the claims below are intended to include any structure, material, or act for performing the function in combination with other claimed elements as specifically claimed. 
     The disclosed embodiments and advantages thereof are best understood by referring to the drawings, in which like numerals are used for like and corresponding parts of the various drawings. Other features and advantages of the disclosed embodiments will be or will become apparent to one of ordinary skill in the art upon examination of the following figures and detailed description. It is intended that all such additional features and advantages be included within the scope of the disclosed embodiments. Further, the illustrated figures are only exemplary and are not intended to assert or imply any limitation with regard to the environment, architecture, design, or process in which different embodiments may be implemented. 
     The disclosed embodiments relate to using equation of state (EOS) compositional models and/or black oil models to simulate fluid production in a multi-reservoir system with a common surface network. As will be described in further detail below, reservoir fluids from multiple hydrocarbon reservoirs may be produced through a common gathering point or shared facility of the common surface network. Thus, heterogeneous fluids from different reservoirs that flow into the common gathering point may combine or mix together. In an example, the disclosed embodiments may be used to calculate properties of the mixed fluids at the common gathering point or other points within the common surface network during a simulation of fluid production in the multi-reservoir system. One example of a reservoir simulator in which the disclosed embodiments may be implemented is the Nexus® integrated reservoir and surface simulator available from Landmark Graphics Corporation of Houston, Tex. 
     In an embodiment, the simulation may be based in part on production system data including various measurements collected downhole from a well drilled within each hydrocarbon reservoir, e.g., in the form of a production well for an oil and gas reservoir. Further, multiple production wells may be drilled for providing access to the reservoir fluids underground. Measured well data may be collected regularly from each production well to track changing conditions in the reservoir, as will be described in further detail below with respect to the production well examples illustrated in  FIGS. 1A and 1B . 
       FIG. 1A  is a diagram of an exemplary production well  100 A with a borehole  102  that has been drilled into a reservoir formation. Borehole  102  may be drilled to any depth and in any direction within the formation. For example, borehole  102  may be drilled to ten thousand feet or more in depth and further, may be steered horizontally for any distance through the formation, as desired for a particular implementation. The production well  100 A also includes a casing header  104  and a casing  106 , both secured into place by cement  103 . A blowout preventer (BOP)  108  couples to casing header  104  and a production wellhead  110 , which together seal in the well head and enable fluids to be extracted from the well in a safe and controlled manner. 
     Measured well data may be periodically sampled and collected from the production well  100 A and combined with measurements from other wells within a reservoir, enabling the overall state of the reservoir to be monitored and assessed. These measurements may be taken using a number of different downhole and surface instruments, including but not limited to, a temperature and pressure sensor  118  and a flow meter  120 . Additional devices may also be coupled in-line to a production tubing  112  including, for example, a downhole choke  116  (e.g., for varying a level of fluid flow restriction), an electric submersible pump (ESP)  122  (e.g., for drawing in fluid flowing from perforations  125  outside ESP  122  and production tubing  112 ), an ESP motor  124  (e.g., for driving ESP  122 ), and a packer  114  (e.g., for isolating the production zone below the packer from the rest of well  100 A). Additional surface measurement devices may be used to measure, for example, the tubing head pressure and the electrical power consumption of ESP motor  124 . 
       FIG. 1B  is a diagram showing an alternative embodiment of the production well  100 A of  FIG. 1A , which includes many of the same components as well  100 A but has been adapted for artificial gas lift. As shown in  FIG. 1B , a production well  100 B further includes a gas lift injector mandrel  126  in addition to the above-described components of well  100 A. In an embodiment, gas lift injector mandrel  126  is coupled in-line with production tubing  112  for controlling a flow of injected gas into a portion of production tubing  112  located above-ground or at the surface of the well near wellhead  110 . Although not shown in  FIG. 1B , the gas lift production well  100 B may also include the same type of downhole and surface instruments as shown for production well  100 A in  FIG. 1A  for providing the above-described measurements. 
     As shown in  FIGS. 1A and 1B , each of the devices along production tubing  112  couples to a cable  128 , which may be attached to an exterior portion of production tubing  112 . Cable  128  may be used primarily to provide power to the devices to which it couples. Cable  128  also may be used to provide signal paths (e.g., electrical or optical paths), through which control signals may be directed from the surface to the downhole devices as well as telemetry signals from the downhole devices to the surface. The respective control and telemetry signals may be sent and received by a control unit  132  at the surface of the production well. Control unit  132  may be coupled to cable  128  through blowout preventer  108 . In an embodiment, field personnel may use control unit  132  to control and monitor the downhole devices locally, e.g., via a user interface provided at a terminal or control panel integrated with control unit  132 . Additionally or alternatively, the downhole devices may be controlled and monitored by a remote processing system  140 . Processing system  140  may be used to provide various supervisory control and data acquisition (SCADA) functionality for the production wells associated with each reservoir in a field. For example, a remote operator may use processing system  140  to send appropriate commands for controlling wellsite operations to control unit  132 . Communication between control unit  132  and processing system  140  may be via one or more communication networks, e.g., in the form of a wireless network (e.g., a cellular network), a wired network (e.g., a cabled connection to the Internet) or a combination of wireless and wired networks. 
     As shown in  FIGS. 1A and 1B , processing system  140  may include a computing device  142  (e.g., a server) and a data storage device  144  (e.g., a database). Although only one computing device and one data storage device are shown in  FIGS. 1A and 1B , it should be appreciated that processing system  140  may include additional computing devices and data storage devices. Computing device  142  may be implemented using any type of computing device having at least one processor, a memory and a networking interface capable of sending and receiving data to and from control unit  132  via a communication network. In an embodiment, computing device  142  may be a type of server. Examples of such a server include, but are not limited to, a web server, an application server, a proxy server, and a network server. In some implementations, computing device  142  may represent a group of computing devices in a server farm. 
     In an embodiment, control unit  132  may periodically send wellsite production data via a communication network to processing system  140  for processing and storage. Such wellsite production data may include, for example, production system measurements from various downhole devices, as described above. In some implementations, such production data may be sent using a remote terminal unit (RTU) of control unit  132 . In an embodiment, data storage device  144  may be used to store the production data received from control unit  132 . In an example, data storage device  144  may be used to store historical production data including a record of actual and simulated production system measurements obtained or calculated over a period of time, e.g., multiple simulation time-steps, as will be described in further detail below. 
     While production wells  100 A and  100 B are described in the context of a single reservoir, it should be noted that the embodiments disclosed herein are not limited thereto and that the disclosed embodiments may be applied to fluid production from multiple reservoirs in a multi-reservoir production system with a common surface or gathering network, as will be described in further detail below with respect to  FIG. 3 . Thus, a plurality of surface control units similar to control unit  132  may be used to send production system data from the respective wellsites of different reservoirs in the production system to processing system  140 . In addition to the above-described SCADA functionality, processing system  140  may be used to process the received data and simulate fluid production in the multi-reservoir system, as will be described in further detail below. 
       FIG. 2  is a block diagram of an exemplary system  200  for simulating fluid production in a multi-reservoir system. For example, system  200  may be used to implement a processing system, e.g., processing system  140  of  FIGS. 1A and 1B , as described above, for processing wellsite data sent by a surface control unit (e.g., control unit  132  of  FIGS. 1A and 1B ) of a production well associated with each reservoir in the production system. As shown in  FIG. 2 , system  200  includes a reservoir simulator  210 , a memory  220 , a user interface (UI)  230  and a network interface  240 . Reservoir simulator  210  includes a fluid model generator  212 , a flow simulator  214  and a data presentation unit  216 . In an embodiment, reservoir simulator  210  and its components (including fluid model generator  212 , flow simulator  214  and presentation unit  216 ), memory  220 , UI  230  and network interface  240  may be communicatively coupled to one another via an internal bus of system  200 . 
     In an embodiment, system  200  can be implemented using any type of computing device having at least one processor and a processor-readable storage medium for storing data and instructions executable by the processor. Examples of such a computing device include, but are not limited to, a desktop computer, a workstation, a server, a cluster of computers (e.g., in a server farm) or similar type of computing device. Such a computing device may also include an input/output (I/O) interface for receiving user input or commands via a user input device (not shown). The user input device may be, for example and without limitation, a mouse, a QWERTY or T9 keyboard, a touch-screen, a graphics tablet, or a microphone. The I/O interface may also include a display interface for outputting or presenting information on a display (not shown) coupled to or integrated with the computing device. 
     While only reservoir simulator  210 , memory  220 , UI  230  and network interface  240  are shown in  FIG. 2 , it should be appreciated that system  200  may include additional components, modules, and/or sub-components as desired for a particular implementation. It should also be appreciated that reservoir simulator  210  and its components may be implemented in software, firmware, hardware, or any combination thereof. Furthermore, it should be appreciated that embodiments of reservoir simulator  210 , or portions thereof, can be implemented to run on any type of processing device including, but not limited to, a computer, workstation, embedded system, networked device, mobile device, or other type of processor or computer system capable of carrying out the functionality described herein. 
     In an embodiment, system  200  may use network interface  240  to communicate with different devices and other systems via a network  204 . Network  204  can be any type of network or combination of networks used to communicate information between different computing devices. Network  204  can include, but is not limited to, a wired (e.g., Ethernet) or a wireless (e.g., Wi-Fi or mobile telecommunications) network. In addition, network  204  can include, but is not limited to, a local area network, medium area network, and/or wide area network such as the Internet. 
     In an embodiment, system  200  may use network interface  240  to send and receive information to and from a wellsite control and monitoring device, e.g., surface control unit  132  of  FIGS. 1A and 1B , as described above, via network  204 . Such information may include, for example, production system data sent from the wellsite control and monitoring device to system  200  via network  204 . Likewise, various control signals and commands may be sent by system  200  to the wellsite control and monitoring device via network  204 , e.g., for purposes of controlling wellsite operations or requesting wellsite production system data from the device. In some implementations, such control signals may be in the form of telemetry signals sent using a telemetry transceiver integrated within network information  240  of system  200 . 
     In an embodiment, the control signals or commands sent by system  200  to the device at the wellsite may be based on input received from a user  202  via UI  230 . User  202  may interact with UI  230  via a user input device (e.g., a mouse, keyboard, or touch-screen) and a display coupled to system  200  to configure, control or monitor the execution of production system simulation. In accordance with user input received by reservoir simulator  210  via UI  230 , production system data may be requested and received from a wellsite control and monitoring device via network  204 , as described above. The data received from the device may be processed and used by reservoir simulator  210  in the production system simulation. The results of the simulation may then be presented by presentation unit  216  to user  202  via UI  230 . 
     In an embodiment, memory  220  may be used to store the production system data from the device in the above example in addition to various other types of data accessible by reservoir simulator  210  and its components (including fluid model generator  212 , flow simulator  214  and presentation unit  216 ) for implementing the production system simulation functionality disclosed herein. Memory  220  can be any type of recording medium coupled to an integrated circuit that controls access to the recording medium. The recording medium can be, for example and without limitation, a semiconductor memory, a hard disk, or similar type of memory or storage device. In some implementations, memory  220  may be a remote cloud-based storage location accessible to system  200  via network interface  240  and network  204 . 
     In the example shown in  FIG. 2 , the data stored in memory  220  may include production data  222 , fluid data  224  and simulation data  226 . As will be described in further detail below, reservoir simulator  210  may use a combination of production data  222 , fluid data  224  and simulation data  226  to derive a desired set of operating points for a given time-step of the production system simulation. 
     Production data  222  may include, for example, actual and/or simulated production system measurements. Actual production system measurements may include, for example, surface and downhole well measurements from various production wells in the multi-reservoir system. Such measurements may include, but are not limited to, pressure, temperature and fluid flow measurements taken downhole near the well perforations, along the production string, at the wellhead and within the gathering network prior to the point where the fluids mix with fluids from other reservoirs. Likewise, the simulated measurements may include, for example and without limitation, estimates of pressure, temperature and fluid flow. Such estimates may be determined based on, for example, simulation results from one or more previous time-steps. 
     Fluid data  224  may represent different reservoir fluid components (e.g., heavy crude, light crude, methane, etc.) and related properties including, for example, their proportions, fluid density and viscosity for various compositions, pressures and temperatures, or other data. In an embodiment, fluid data  224  may include black oil and/or EOS model data, e.g., in the form of one or more data tables, representing the fluids of each reservoir within the multi-reservoir production system. 
     In an embodiment, fluid model generator  212  may generate a fluid model for each reservoir in the multi-reservoir system based on corresponding production data  222  and fluid data  224 . For example, fluid model generator  212  may determine parameters for each fluid component or group of components of the reservoir based on actual and simulated production system measurements (e.g., from one or more prior simulation time-steps) and fluid component characterizations associated with each reservoir. The resulting model for each component/group can then be applied to known state variables to calculate unknown state variables at each simulation point or “gridblock” within the reservoir, at the wellbore perforations or “sandface,” and within the common gathering network of the production system. These unknown variables may include, for example and without limitation, each gridblock&#39;s liquid volume fraction, solution gas-oil ratio and formation volume factor. 
     In an embodiment, the resulting fluid component state variables, both measured and calculated, may be provided as inputs to flow simulator  214  for simulating the flow of fluids through the multi-reservoir production system. Additional inputs to flow simulator  214  may include, for example, various floating parameters, fixed parameters and characterization data related to the production system and constraints thereof. The floating parameters may include, for example, various enhanced oil recovery (EOR) parameters including, but not limited to, gas lift injection rates, reservoir gas injection rates and reservoir liquid injection rates. Examples of fixed parameters may include facility constraints (e.g., a production capacity limit) and default production rates for individual wells. Reservoir characterization data may include, for example, geological data describing reservoir formations (e.g., log data previously collected during drilling and/or prior logging of the well) and formation characteristics (e.g., porosity). The above-described fluid component state variables along with the other simulation inputs, parameters and production system constraints may be stored in memory  220  as simulation data  226 . 
     In an embodiment, flow simulator  214  may employ set of a fully-coupled equations to perform the simulation and determine optimal operating settings for the production system such that production of the reservoirs can be maximized over time without exceeding any facility constraints. The equations are characterized as “fully-coupled” because all the equations for all the reservoirs and the gathering network may be solved simultaneously, rather than solving the reservoir and gathering network separately and iterating between the reservoir and gathering network solutions to determine appropriate boundary conditions for each set of equations (i.e., loosely-coupled). 
     In an embodiment, the fully-coupled equations may be used with any of various numerical analysis techniques (e.g., a Newton-Raphson method) to determine a set of mass and/or volume balance values for each gridblock. The equations also may be used to determine the flow of fluids through the production system and provide a solution that includes operating settings that honor the various production system constraints, e.g., one or more facility constraints, gathering network constraints, well constraints, or reservoir constraints. Further, the equations may be used by flow simulator  214  to determine updated fluid properties (e.g., updated fluid component mass and volume values for each gridblock) at the end of the simulation time-step. At least some of the updated parameters may be provided, for example, as previous time-step data for subsequent simulation time-steps. In addition, the simulation performed by flow simulator  214  may be repeated for each of a plurality of different time-steps, where the simulation results for a given time-step are used to update the simulation for the next time-step. 
     With the state of the fluids known throughout the production system, the flow of fluid can be simulated using mass/volume balance equations representative of the reservoir, of perforations in the wellbore and of the gathering network. In an embodiment, the facility equations representing the gathering network include molar balance equations at the nodes, hydraulic equations, constraint equations, and composition equations. The independent variables for the facility equations include pressure and composition for the nodes, and molar flow rates for the connections. 
     The full system of equations can be expressed as follows: 
     
       
         
           
             
               
                 
                   
                     
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     where R denotes the residuals, and A the Jacobian for a Newton iteration of the production system simulation. A contains the derivatives of the residuals with respect to the variables x, where x r  includes gridblock moles and pressures, x p  includes perforation flow rates, and x f  includes facility node compositions and pressures and the total molar flow rate of the facility connections. The first row of equations represents the reservoir equations (simulating fluid flow through the reservoir), the second row represents the perforation equations (simulating fluid flow through the perforations), and the third row represents the facility equations (simulating fluid flow through the gathering network). 
     In an embodiment, the reservoir equations include molar balance equations of the form:
 
 R   ri   =F   i   in   −F   i   out   −a   i   +G   i −Σ pεp     r     Q   rpi   (2)
 
     where the residual R ri  of component i for each reservoir gridblock r is driven to zero at full convergence of the equations. For component i, F i   in  and F i   out  are the molar flow rates across reservoir gridblock faces, a i  is the rate of accumulation, G i  is the rate of generation and Q rpi  is the perforation flow rate (positive for production, negative for injection) between a reservoir gridblock r and a wellbore through perforation p. The Q rpi  are summed over the perforations within gridblock r. The independent variables are the mass (in moles) of each component i, and the gridblock pressure. In addition to the molar balance equations, in at least some illustrative embodiments a volume balance equation operates to constrain the pore volume so that it equals the fluid volume. This can be written in residual form as:
 
 R   r,nc     r     +1   =V   Pr   −V   Fr   (3)
 
     where nc r  is the number of reservoir pseudo-components, V Pr  is the pore volume and V Fr  is the fluid volume for gridblock r. 
     In at least some illustrative embodiments, the perforation equations are expressed as flow equations for each perforation within a reservoir gridblock. Starting with the simpler case of a single reservoir and a gathering network with the same number of pseudo-components, the perforation equation for producing perforations can be expressed using the flow equation, 
     
       
         
           
             
               
                 
                   
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     where Q rpi  is the perforation flow rate of fluid pseudo-component i for perforation p within gridblock r, C p  is the wellbore constant (equal to the well index multiplied by the permeability-thickness product), ΔΦ p  is the permeability-thickness product (i.e., the potential difference from the reservoir to the wellbore for perforation p), and where for phase m within gridblock r, krel rm  is the relative permeability, μ rm  is the viscosity, ρ rm  is the density, and z rmi  is the mole fraction of fluid pseudo-component i. Similarly, the perforation equation for injecting perforations can be expressed using the flow equation,
 
 Q   rpi   =C   p λ p   inj ρ p   inj ΔΦ p   z   rpi   (5)
 
     where λ p   inj  is the fluid mobility (e.g., the sum of the gridblock phase mobilities or an endpoint mobility), ρ p   inj  is the perforation-injected fluid density, and z rpi  is the component mole fraction at a node in the wellbore. 
     The above-described simulation assumes a configuration of the production system in which multiple reservoirs are coupled to a common surface or gathering network. Such a gathering network may include, for example, a plurality of nodes with connections between the nodes and various reservoir gridblocks. Nodes may represent physical locations of relevant components or devices (e.g., separator  310  of  FIG. 3 , as will be described below) within the gathering network and/or the production wells of various reservoirs. Connections may represent pipes or flow control devices, for example, pumps, compressors, valves, or similar types of devices. An example of such a production system configuration is shown in  FIG. 3 . 
       FIG. 3  is a diagram illustrating an exemplary multi-reservoir system including a common surface or gathering network. As shown in  FIG. 3 , a group of N reservoirs  302 - 1  through  302 -N are coupled together through a gathering network  320 . Individual well lines  304  ( 1  through N) from each well couple to a corresponding reservoir node  306  ( 1  through N), with each node coupling through a reservoir line  305  ( 1  through N) to a common node  308 . Common node  308  may provide, for example, mixed fluids produced from reservoirs  302 - 1  to  302 -N through riser  309  to a processing facility  300 . The mixed fluids that are produced at common node  308  through riser  309  may include fluids produced from any number of reservoirs  302 - 1  to  302 -N, for example, all of the reservoirs or any subset thereof. In the example shown, processing facility  300  includes a separator  310  that receives the mixed product from facility riser  309  and separates the product into water, oil and gas. These separated products are respectively stored in water storage  312 , oil storage  316  and gas storage  314  for later use and/or delivery further downstream (e.g., to a refining facility). Alternatively, some of the separated product may be used to assist with the removal of product from the reservoir. For example, a portion of the separated gas and/or water may be reinjected into one or more reservoirs as part of an enhanced oil recovery (EOR) operation, as indicated by the dashed arrows in  FIG. 3 . 
     Maximizing fluid production in the multi-reservoir production system of  FIG. 3  may involve controlling the production of each individual well such that the combined production of the wells, or a selected group of the wells, provides the greatest possible amount of hydrocarbon (e.g., oil and/or gas) production within the operating limits of processing facility  300  and without exceeding any production system constraints. In an embodiment, optimal well operating points that maximize fluid production over time and enable processing facility  300  to operate within its limits may be determined from the results of a simulation of fluid production in the multi-reservoir system. For example, reservoir simulator  210  of  FIG. 2 , as described above, may be used to identify the optimal well operating points from a simulation of fluid production in the multi-reservoir system of  FIG. 3  based on production system measurements, reservoir characterizations and constraints related to reservoirs  302 - 1  to  302 -N and processing facility  300 . In some implementations, such operating points may be expressed as a solution to a simultaneous set of fully-coupled equations, as described above. 
     In addition to using simulation results to determine optimal well operating points and maximize fluid production in the multi-reservoir system, a reservoir engineer (e.g., user  202  of reservoir simulator  210  of  FIG. 2 ) might be interested in improving the computational efficiency of the simulation itself and the accuracy of the simulation results. As will be described in further detail below, the fluid modeling and production simulation techniques disclosed herein may allow such improvements to be achieved for the simulation by using a modified or simplified compositional model representing the mixed fluids produced from the multiple reservoirs of the above-described multi-reservoir production system. 
     Referring to  FIG. 3 , each of reservoirs  302 - 1  to  302 -N may be associated with an EOS model, e.g., in the form of one or more EOS data tables, representing the fluids within that reservoir. In this example, each reservoir may have at least two fluid components, e.g., an oil component and a gas component, which can be produced into gathering network  320 . In an embodiment, it may be assumed for purposes of the simulation that the gas components of reservoirs  302 - 1  to  302 -N are identical while the oil components maintain their separate identities for each reservoir. However, it should be noted that the gas components also may retain their separate identities in some implementations, e.g., for more flexibility when dealing with condensate reservoirs. 
     As will be described in further detail below with respect to the process shown in  FIG. 4 , an EOS characterization of reservoir fluids may be used to produce an EOS model for each of a plurality of reservoirs within the multi-reservoir system. Additionally, some of the reservoir fluids may be represented by black oil models. In this case, an additional step is required to transform the black oil models to EOS models so that for each reservoir the basis of the mixing will be the mixing of EOS models. While the EOS models for the reservoirs in the multi-reservoir system may have at least some light components in common, the disclosed embodiments do not require the EOS model for each reservoir to have the same EOS components as in conventional approaches, which utilize a master EOS model with a component set that can be expanded from the component sets of individual reservoirs. The EOS model in accordance with the disclosed embodiments may have at least one heavy component, also referred to herein as a “marker component,” that is unique to each reservoir in the multi-reservoir system. Further, while the embodiments disclosed herein may perform delumping, the delumping is performed without using full EOS modeling for calculating the fluid properties of the mixed fluid. Instead, the disclosed embodiments use the marker component to generate property tables as a function of pressure and the marker component. 
       FIG. 4  is a flowchart of an exemplary method  400  of using a simplified compositional model for determining the properties of mixed fluids produced in a multi-reservoir system having a common surface network. For discussion purposes, method  400  will be described using the above-described multi-reservoir system of  FIG. 3  but is not intended to be limited thereto. As shown in  FIG. 4 , method  400  includes steps  402 ,  404 ,  406 ,  408 ,  410 ,  412  and  414 . However, it should be noted that method  400  may include additional steps to perform the techniques disclosed herein, as desired for a particular implementation. The steps of method  400  may be implemented by, for example, reservoir simulator  210  of  FIG. 2 , as described above, but method  400  is not intended to be limited thereto. 
     Method  400  begins in step  402 , which includes matching an EOS characterization of fluids with an EOS model for each of a plurality of reservoirs (e.g., reservoirs  302 - 1  to  302 -N of  FIG. 3 ) in the multi-reservoir system. The EOS model for each reservoir may represent different components of the reservoir&#39;s fluids in their EOS form. The different fluid components represented by the EOS model for each reservoir may include, for example, one or more light fluid components that are common across all of the reservoirs in the production system and at least one heavy fluid component (or marker component) that is unique to each reservoir. In an embodiment, the common light fluid components may be gas components and the unique heavy oil component may be an oil component. 
     Method  400  then proceeds to step  404 , in which fluid production in the multi-reservoir system may be simulated at different points in the common surface network. The simulation points may correspond to, for example, different nodes (e.g., nodes  306 - 1  to  306 -N of  FIG. 3 ) in the common surface network, as described above. In step  406 , it is determined for each simulation point in step  404  whether there are mixed fluids produced from different reservoirs. In step  408 , the results of the determination in step  406  may be used to decide whether method  400  will proceed to step  410  or step  412 . In an example, if it is determined in step  406  that the fluids at a particular simulation point in the network are produced from only a single reservoir, method  400  may proceed from step  408  to step  410 . Step  410  may include calculating fluid properties using the EOS model (from step  402 ) corresponding to the reservoir from which the fluids are produced. 
     Alternatively, if it is determined in step  406  that there is a commingling or mixing of fluids from different reservoirs at the particular simulation point in question, method  400  may proceed from step  408  to step  412 . Step  412  includes generating one or more interpolation tables representing the mixed fluids produced from the different reservoirs via the common surface network, based on the corresponding EOS model for each of the different reservoirs. In an embodiment, the interpolation tables may be generated as a function of pressure and the marker components of the different reservoirs in this example. In step  414 , the generated interpolation tables from step  412  may be used to calculate the properties of the mixed fluids. 
     In an embodiment, the mixed fluid properties may be calculated by performing a table look-up using any of various table look-up techniques based on the marker component and the one or more interpolation tables. A benefit of using such table look-up techniques is that the interpolation tables generated in step  412  may be used in place of phase equilibrium calculations, which can often be the most expensive part of phase behavior calculations for the mixed fluids. For example, the interpolation tables may provide a simplified compositional model that can be used to replace the equilibrium flash calculations associated with conventional phase behavior calculation techniques. The phase splits of the mixed fluids may be determined using such a simplified flash replacement, and a standard EOS based table look-up technique may be used to calculate the individual phase properties. Examples of such EOS flash replacement techniques that may be used include, but are not limited to: 
     1) use of compositionally dependent K-value calculations in which K-values are tabulated as functions of pressure, temperature and the composition of one or more components including, for example, one or more marker components, and the K-values are used to determine the phase equilibrium while phase properties are calculated using standard EOS techniques; 
     2) use of EOS interpolation techniques for simplified phase equilibrium calculations in which saturation pressure, K-values at the saturation pressure, liquid and vapor compressibility factors are tabulated as functions of pressure, temperature, and the composition of some of the components including the marker components while EOS calculations may be used to determine other fluid properties; and 
     3) use of the interpolation tables as a function of compositional tie-lines relating oil and gas compositions. 
     In the latter technique, the phase mole fractions and the phase compositions may be tabulated as a function of pressure, temperature and overall component compositions. Then, standard EOS techniques may be used to calculate other phase properties. Some or all of the components may be included, e.g., as interpolants, in addition to all of the marker components. 
     The disclosed embodiments will now be further described using the following examples of possible data structures that may be defined for the individual reservoirs and for the mixed fluid models described above. 
     For example, Table 1 below shows the black oil data for a reservoir 1. The black oil data in Table 1 may provide, for example, a black oil model description of the fluids in reservoir 1, which can be input to a reservoir simulator, e.g., reservoir simulator  210  of  FIG. 2 , as described above, for performing the simulation. As shown in Table 1 below, the main data columns of this input table may be for the pressure (in psia), the solution gas oil ratio (Rs) in units of MSCF/STB, and the oil formation volume factor (Bo) in units of STB/RB. While only these three main columns are shown below, it should be noted that embodiments are not intended to be limited thereto and that the table may have additional columns representing other black oil data parameters including, for example and without limitation, gas FVF, solution gas-oil ratio, oil viscosity, and gas viscosity. Additionally, undersaturated data may be associated with at least one of the pressures. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Original Black Oil Model Data for Reservoir 1 
               
            
           
           
               
               
               
            
               
                 Pressure (psia) 
                 Rs (MSCF/STB) 
                 Bo (STB/RB) 
               
               
                   
               
            
           
           
               
               
               
            
               
                 3000 
                 1.2 
                 1.3 
               
               
                 2000 
                 0.8 
                 1.2 
               
               
                 1000 
                 0.4 
                 1.1 
               
               
                 14.7 
                 0.00001 
                 1 
               
               
                   
               
            
           
         
       
     
     Table 2 below shows the fluid component properties for a reservoir 2, as represented by an EOS model of the reservoir&#39;s fluids. As shown below, Table 2 may include data columns for key fluid properties of interest during the simulation including, for example and without limitation, molecular weight, critical temperature, and critical pressure. It should be noted that the table for a full EOS model description may include additional columns of data, for example, columns for an acentric factor, critical volume, parachors, and volume translation factors for each component. A separate table of interaction coefficients may also be used. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 EOS Model Data for Reservoir 2 
               
            
           
           
               
               
               
               
            
               
                   
                   
                 Critical 
                 Critical pressure 
               
               
                 Component Name 
                 Molecular weight 
                 temperature (R) 
                 (psia) 
               
               
                   
               
            
           
           
               
               
               
               
            
               
                 P1 
                 18 
                 300 
                 550 
               
               
                 P2 
                 35 
                 549.8 
                 850 
               
               
                 P3 
                 44.1 
                 665.7 
                 616.3 
               
               
                 P4 
                 65 
                 820 
                 485 
               
               
                 P5 
                 90 
                 950. 
                 438. 
               
               
                 P6 
                 200 
                 1200 
                 255 
               
               
                   
               
            
           
         
       
     
     In an embodiment, an EOS characterization of the mixed fluids from different reservoirs may be generated. Such an EOS characterization may include, for example, a set of light components (CO2, N2, H2S, C1, C2, C3, iC4, nC4, iC5, nC5, C6) and a set of common heavy components (HC1, HC2), two heavy component(s) exclusive to reservoir 1 (R1H1 and R1H2), and at least one heavy component that is exclusive to a reservoir 2 (R2H1). The actual number of light components may be the same, but the number of heavy components may be specific to a particular implementation. The process performs a characterization procedure as described in step 1 above. 
     In one example, a common EOS model may be generated based on a set of reservoir pseudo-components defined using common pseudo-components that overlap between reservoirs  1  and  2 . Each of the “pseudo-components” may represent, for example, any number of real fluid components that are grouped together or “lumped” into a single component that can be processed as an individual unit. The use of overlapping common pseudo-components may enable the application of fully-coupled mass/volume balance equations to multiple reservoirs, wells and the gathering network using a larger but still relatively small number of common pseudo-components (e.g., less than the total of all the reservoir pseudo-components). The common pseudo-components represent a common fluid characterization that includes sufficient components to represent the behavior of multiple fluids in different reservoirs. 
     The common characterization in this example may be based on an expression of the components as common pseudo-components that are defined based on the components&#39; bulk hydrocarbon composition or carbon number up to C45. The set of light components CO2, N2, H2S, CI, C2, C3, nC4, iC4, nC5, iC5 and C6 may be used with their commonly accepted properties, while the C7+ heavy components are defined using a probability distribution function that provides the molecular weight and mole fraction for each carbon number from C7 to C45. It should be appreciated that any of various techniques may be used to define the C7+ heavy components. Once a set of molecular weights and mole fractions are established, a Watson or other type of characterization factor may be calculated for each common pseudo-component, which in turn may be used to calculate the specific gravity of each common pseudo-component. It should be appreciated that any of various techniques may be used to calculate pseudo-component specific gravities and/or other pseudo-component characteristics will become apparent to those of ordinary skill in the art, and all such techniques are within the scope of the present disclosure. The true boiling point (TBP) for each common pseudo-component may also be calculated. The molecular weights, TBPs and specific gravities can be combined using any of a number of correlation techniques to calculate the critical properties needed by the fluid models. Non-zero interaction coefficients may also need to be estimated through correlations. 
     At this stage, a large number of pseudo-components may be used, far larger than the usual number normally used to simulate reservoir and network systems. In order to improve computational efficiency, the components are lumped together in a pseudoization process. For example, in this case the heavy components of reservoir larc lumped together into two pseudo-components R1H1 and R1H2, while the heavy components of reservoir 2 are lumped together into pseudo-component R2H1. 
     The critical properties and the interaction coefficients generated in the above manner may need to be adjusted to adequately match the fluid properties for each reservoir. Regression methods may be applied to adjust the values of the fluid parameters. 
     For calculations of commingled fluids, the extra step of mixing or weaving components from the two EOS models is carried out. In this case, the two EOS models share the same light components and light component properties so only the compositions of the light components in the mixture needs to be adjusted. Using simple weaving, we retain the heavy components R1H1 and R1H2 from reservoir 1, and R2H2 from reservoir 2. The mole fractions are adjusted during the mixing step. 
     The EOS model based on the above-described fluid component characterizations may be represented using a data table similar to Table 3 below, which may be provided as input to a reservoir simulator (e.g., reservoir simulator  210  of  FIG. 2 , as described above). 
     
       
         
           
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 EOS Model Data for Mixed Fluids from Different Reservoirs 
               
            
           
           
               
               
               
               
            
               
                   
                   
                 Critical 
                 Critical pressure 
               
               
                 Component Name 
                 Molecular weight 
                 temperature (R) 
                 (psia) 
               
               
                   
               
            
           
           
               
               
               
               
            
               
                 CO2 
                 44.01 
                 547.6 
                 1070.9 
               
               
                 N2 
                 28.01 
                 227.3 
                 493.0 
               
               
                 H2S 
                 34.08 
                 212.7 
                 1036.0 
               
               
                 C1 
                 16.043 
                 343.0 
                 667.8 
               
               
                 C2 
                 30.07 
                 549.8 
                 707.8 
               
               
                 C3 
                 44.1 
                 665.7 
                 616.3 
               
               
                 iC4 
                 58.12 
                 734.7 
                 529.1 
               
               
                 nC4 
                 58.12 
                 765.3 
                 550.7 
               
               
                 iC5 
                 72.15 
                 828.8 
                 490.4 
               
               
                 nC5 
                 72.15 
                 845.4 
                 488.6 
               
               
                 C6 
                 86.18 
                 913.4 
                 436.9 
               
               
                 HC1 
                 98.55 
                 1004.4 
                 441.5 
               
               
                 HC2 
                 319.83 
                 1490.2 
                 191.1 
               
               
                 R1H1 
                 135.84 
                 1135.1 
                 362.7 
               
               
                 R1H2 
                 206.25 
                 1309.6 
                 266.9 
               
               
                 R2H1 
                 500.0 
                 1670.4 
                 140.3 
               
               
                   
               
            
           
         
       
     
     The EOS model may then be used to generate tables of compositional calculations in which values are tabulated as a function of pressure, temperature and compositions, e.g., in the form of one or more interpolation parameters. It is possible that only some of the components may be required as interpolants, in which case the marker components may be targeted to be the interpolants. For example, in one embodiment, in the above-described example of mixed fluids from two different reservoirs, at least one of the unique “marker” components that is specific to one of the reservoirs, e.g., R1H1, may be selected as the interpolation parameter. Tables 4-6 below are examples of generated tables for different values of the interpolation parameter, e.g., the marker component (R1H1) in this example. The composition of the respective oil and gas phases in each table may be expressed, for example, as a list of 15 numbers corresponding to the values of the remaining components, i.e., excluding R1H1, as shown in Table 3 above. Depending on the calculation mechanism, alternative data such as K-values or phase Z-factors may be tabulated instead of the compositions and phase fractions. 
     
       
         
           
               
             
               
                 TABLE 4 
               
             
            
               
                   
               
               
                 Mole fraction R1H1 = 0 
               
            
           
           
               
               
               
               
            
               
                   
                   
                 Composition of gas 
                 Composition of oil 
               
               
                   
                   
                 phase (a list of 
                 phase (a list of 
               
               
                   
                 Fraction of 
                 15 numbers 
                 15 numbers 
               
               
                 Pressure 
                 gas phase 
                 that sum to 1.) 
                 that sum to 1.) 
               
               
                   
               
            
           
           
               
               
               
               
            
               
                 3000 
                 0.01 
                 (0.02, 0.005, 0.0005, 
                 (0.002, 0.0003, 
               
               
                   
                   
                 0.52, . . . , 0.015) 
                 0.00003, 0.2, . . . , 
               
               
                   
                   
                   
                 0.125) 
               
               
                 2000 
                 0.22 
                 (0.02, 0.0048, 
                 (0.002, 0.0003, 
               
               
                   
                   
                 0.00052, 0.533, . . . , 
                 0.00003, 0.19, . . . , 
               
               
                   
                   
                 0.0166) 
                 0.135) 
               
               
                 1000 
                 0.35 
                 (0.02, 0.0048, 
                 (0.002, 0.0003, 
               
               
                   
                   
                 0.00052, 0.543, . . . , 
                 0.00003, 0.19, . . . , 
               
               
                   
                   
                 0.0173) 
                 0.127) 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 5 
               
             
            
               
                   
               
               
                 Mole fraction R1H1 = 0.5 
               
            
           
           
               
               
               
               
            
               
                   
                   
                 Composition of gas 
                 Composition of oil 
               
               
                   
                   
                 phase (a list of 
                 phase (a list of 
               
               
                   
                 Fraction of 
                 15 numbers 
                 15 numbers 
               
               
                 Pressure 
                 gas phase 
                 that sum to 1) 
                 that sum to 1.) 
               
               
                   
               
            
           
           
               
               
               
               
            
               
                 3000 
                 0.015 
                 (0.02, 0.005, 0.0005, 
                 (0.002, 0.0003, 
               
               
                   
                   
                 0.52, . . . , 0.015) 
                 0.00003, 0.2, . . . , 
               
               
                   
                   
                   
                 0.125) 
               
               
                 2000 
                 0.27 
                 (0.02, 0.0048, 
                 (0.002, 0.0003, 
               
               
                   
                   
                 0.00052, 0.533, . . . , 
                 0.00003, 0.19, . . . , 
               
               
                   
                   
                 0.0166) 
                 0.135) 
               
               
                 1000 
                 0.37 
                 (0.02, 0.0048, 
                 (0.002, 0.0003, 
               
               
                   
                   
                 0.00052, 0.543, . . . , 
                 0.00003, 0.19, . . . , 
               
               
                   
                   
                 0.0173) 
                 0.127) 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 6 
               
             
            
               
                   
               
               
                 Mole fraction R1H1 = 1.0 
               
            
           
           
               
               
               
               
            
               
                   
                   
                 Composition of gas 
                 Composition of oil 
               
               
                   
                   
                 phase (a list of 
                 phase (a list of 
               
               
                   
                 Fraction of 
                 15 numbers 
                 15 numbers 
               
               
                 Pressure 
                 gas phase 
                 that sum to 1.) 
                 that sum to 1.) 
               
               
                   
               
            
           
           
               
               
               
               
            
               
                 3000 
                 0.020 
                 (0.02, 0.005, 0.0005, 
                 (0.002, 0.0003, 
               
               
                   
                   
                 0.52, . . . , 0.015) 
                 0.00003, 0.2, . . . , 
               
               
                   
                   
                   
                 0.125) 
               
               
                 2000 
                 0.32 
                 (0.02, 0.0048, 
                 (0.002, 0.0003, 
               
               
                   
                   
                 0.00052, 0.533, . . . , 
                 0.00003, 0.19, . . . , 
               
               
                   
                   
                 0.0166) 
                 0.135) 
               
               
                 1000 
                 0.41 
                 (0.02, 0.0048, 
                 (0.002, 0.0003, 
               
               
                   
                   
                 0.00052, 0.543, . . . , 
                 0.00003, 0.19, . . . , 
               
               
                   
                   
                 0.0173) 
                 0.127) 
               
               
                   
               
            
           
         
       
     
     In the above example, other interpolation parameters may be added for mixed fluids from different reservoirs, for example, a unique or special marker component of reservoir 2, e.g. R2H1. 
     Accordingly, the disclosed embodiments provide a method for using simplified compositional models for calculating properties of mixed fluids in a common surface network. In contrast to the disclosed embodiments, other approaches with different EOS models and black oil models tied to a common network all try to match the fluid behavior to a single equation of state model using a common set of components. Additionally, the disclosed embodiments also match to EOS models, but without the requirement that all components must exist within each fluid. Further, while other approaches then delump the fluids to the common EOS model at the sandface, the disclosed embodiments are configured to generate a multi-dimension interpolation table with a special marker component from each reservoir as a parameter. These tables are used in an approach which will replace the EOS equilibrium calculations. 
     Thus, advantages of the disclosed embodiments over prior methods include increased computational efficiency and in some cases, more accuracy. Additionally, in parts of the reservoir that have no commingling of fluids from different reservoirs, the original EOS data may be used. Alternatively, the simplified table lookup technique may also be used everywhere for computational efficiency and consistency. In parts of the network where mixed fluids are produced from different reservoirs, the disclosed embodiments may be used as a basis for calculating the properties of the mixed fluids that are produced from the mixing of EOS fluids from different reservoirs in different proportions. The disclosed embodiments allow the operators to keep their original EOS characterization, thereby providing a relatively efficient way for calculating properties of mixed fluids in the common network. 
       FIG. 5  is a block diagram of an exemplary computer system  500  in which embodiments of the present disclosure may be implemented. For example, system  500  may be used to implement system  200  of  FIG. 2 , as described above. The system  500  may be any type of computing device including, but not limited to, a desktop computer, a laptop, a server, a tablet, and a mobile device. The system  500  includes, among other components, a processor  510 , main memory  502 , secondary storage unit  504 , an input/output interface module  506 , and a communication interface module  508 . 
     The processor  510  may be any type or any number of single core or multi-core processors capable of executing instructions for performing the features and functions of the disclosed embodiments. The input/output interface module  506  enables the system  500  to receive user input (e.g., from a keyboard and mouse) and output information to one or more devices such as, but not limited to, printers, external data storage devices, and audio speakers. The system  500  may optionally include a separate display module  511  to enable information to be displayed on an integrated or external display device. For instance, the display module  511  may include instructions or hardware (e.g., a graphics card or chip) for providing enhanced graphics, touchscreen, and/or multi-touch functionalities associated with one or more display devices. 
     Main memory  502  is volatile memory that stores currently executing instructions/data or instructions/data that are prefetched for execution. The secondary storage unit  504  is non-volatile memory for storing persistent data. The secondary storage unit  504  may be or include any type of data storage component such as a hard drive, a flash drive, or a memory card. In one embodiment, the secondary storage unit  504  stores the computer executable code/instructions and other relevant data for enabling a user to perform the features and functions of the disclosed embodiments. 
     For example, in accordance with the disclosed embodiments, the secondary storage unit  504  may permanently store executable code/instructions  520  for performing the steps of method  400  of  FIG. 4 , as described above. The executable code/instructions  520  are then loaded from the secondary storage unit  504  to main memory  502  during execution by the processor  510  for performing the disclosed embodiments. In addition, the secondary storage unit  504  may store other executable code/instructions and data  522  such as, but not limited to, a reservoir simulation application (e.g., a reservoir simulation application) for use with the disclosed embodiments. 
     The communication interface module  508  enables the system  500  to communicate with the communications network  530 . For example, the network interface module  508  may include a network interface card and/or a wireless transceiver for enabling the system  500  to send and receive data through the communications network  530  and/or directly with other devices. 
     The communications network  530  may be any type of network including a combination of one or more of the following networks: a wide area network, a local area network, one or more private networks, the Internet, a telephone network such as the public switched telephone network (PSTN), one or more cellular networks, and/or wireless data networks. The communications network  530  may include a plurality of network nodes (not depicted) such as routers, network access points/gateways, switches, DNS servers, proxy servers, and other network nodes for assisting in routing of data/communications between devices. 
     For example, in one embodiment, the system  500  may interact with one or more servers  534  or databases  532  for performing the features of the disclosed embodiments. For instance, the system  500  may query the database  532  for well log information for creating a reservoir model in accordance with the disclosed embodiments. Further, in certain embodiments, the system  500  may act as a server system for one or more client devices or a peer system for peer to peer communications or parallel processing with one or more devices/computing systems (e.g., clusters, grids). 
     As described above, embodiments of the present disclosure are particularly useful for calculating properties of mixed fluids produced in a multi-reservoir system with a common surface network. In one embodiment of the present disclosure, a computer-implemented method of simulating fluid production in a multi-reservoir system with a common surface network includes: matching equation of state (EOS) characterization of fluids with a delumped EOS model for each of a plurality of reservoirs within the multi-reservoir system, where the delumped EOS model represents different components of the fluids for each reservoir; simulating fluid production in the multi-reservoir system for at least one simulation point in the common surface network, based in part on the delumped EOS model for each of the plurality of reservoirs; determining whether or not fluids produced during the simulation at the simulation point are mixed fluids from different reservoirs in the plurality of reservoirs; when the fluids at the simulation point are determined not to be mixed fluids produced from different reservoirs in the plurality of reservoirs, calculating properties of the fluids using the delumped EOS model corresponding to one of the plurality of reservoirs from which the fluids are produced; and when the fluids at the simulation point are determined to be mixed fluids produced from different reservoirs, generating one or more interpolation tables representing the mixed fluids produced from the different reservoirs via the common surface network, based on the corresponding delumped EOS model for each of the different reservoirs and calculating properties of the mixed fluids based on the one or more interpolation tables. 
     In a further embodiment, the one or more interpolation tables include compositional values that are tabulated as a function of one or more interpolation parameters. In yet a further embodiment, the different fluid components represented by the delumped EOS model for each reservoir include at least one heavy fluid component that is unique to that reservoir. In yet a further embodiment, the different fluid components further include at least one light fluid component that is common amongst the plurality of reservoirs. In yet a further embodiment, the heavy fluid component is a unique heavy oil component and the light fluid component is a common gas component. In yet a further embodiment, the one or more interpolation tables represent the fluids of each of the plurality of reservoirs in proportion to the unique heavy oil component of each reservoir relative to the reservoir&#39;s fluid pressure. In yet a further embodiment, the one or more interpolation tables are used in place of phase equilibrium calculations for the mixed fluids, and calculating properties of the mixed fluids comprises performing a table look-up of fluid properties using the one or more interpolation tables. In yet a further embodiment, the table look-up is performed using at least one of a set of tabulated compositionally dependent K-values, an EOS interpolation, or compositional tie-lines relating to oil and gas compositions of the mixed fluids. 
     In another embodiment of the present disclosure, a system for defining non-linear petrofacies for a reservoir simulation model includes at least one processor and a memory coupled to the processor has instructions stored therein, which when executed by the processor, cause the processor to perform functions, including functions to: match equation of state (EOS) characterization of fluids with a delumped EOS model for each of a plurality of reservoirs within the multi-reservoir system, the delumped EOS model representing different components of the fluids for each reservoir; simulate fluid production in the multi-reservoir system for at least one simulation point in the common surface network, based in part on the delumped EOS model for each of the plurality of reservoirs; determine whether or not fluids produced during the simulation at the simulation point are mixed fluids from different reservoirs in the plurality of reservoirs; when the fluids at the simulation point are determined not to be mixed fluids produced from different reservoirs in the plurality of reservoirs, calculate properties of the fluids using the delumped EOS model corresponding to one of the plurality of reservoirs from which the fluids are produced; and when the fluids at the simulation point are determined to be mixed fluids produced from different reservoirs, generate one or more interpolation tables representing the mixed fluids produced from the different reservoirs via the common surface network, based on the corresponding delumped EOS model for each of the different reservoirs and calculate properties of the mixed fluids based on the one or more interpolation tables. 
     In yet another embodiment of the present disclosure, a computer-readable storage medium has instructions stored therein, which when executed by a computer cause the computer to perform a plurality of functions, including functions to: match equation of state (EOS) characterization of fluids with a delumped EOS model for each of a plurality of reservoirs within the multi-reservoir system, the delumped EOS model representing different components of the fluids for each reservoir; simulate fluid production in the multi-reservoir system for at least one simulation point in the common surface network, based in part on the delumped EOS model for each of the plurality of reservoirs; determine whether or not fluids produced during the simulation at the simulation point are mixed fluids from different reservoirs in the plurality of reservoirs; when the fluids at the simulation point are determined not to be mixed fluids produced from different reservoirs in the plurality of reservoirs, calculate properties of the fluids using the delumped EOS model corresponding to one of the plurality of reservoirs from which the fluids are produced; and when the fluids at the simulation point are determined to be mixed fluids produced from different reservoirs, generate one or more interpolation tables representing the mixed fluids produced from the different reservoirs via the common surface network, based on the corresponding delumped EOS model for each of the different reservoirs and calculate properties of the mixed fluids based on the one or more interpolation tables. 
     While specific details about the above embodiments have been described, the above hardware and software descriptions are intended merely as example embodiments and are not intended to limit the structure or implementation of the disclosed embodiments. For instance, although many other internal components of the system  500  are not shown, those of ordinary skill in the art will appreciate that such components and their interconnection are well known. 
     In addition, certain aspects of the disclosed embodiments, as outlined above, may be embodied in software that is executed using one or more processing units/components. Program aspects of the technology may be thought of as “products” or “articles of manufacture” typically in the form of executable code and/or associated data that is carried on or embodied in a type of machine readable medium. Tangible non-transitory “storage” type media include any or all of the memory or other storage for the computers, processors or the like, or associated modules thereof, such as various semiconductor memories, tape drives, disk drives, optical or magnetic disks, and the like, which may provide storage at any time for the software programming. 
     Additionally, the flowchart and block diagrams in the figures illustrate the architecture, functionality, and operation of possible implementations of systems, methods and computer program products according to various embodiments of the present invention. It should also be noted that, in some alternative implementations, the functions noted in the block may occur out of the order noted in the figures. For example, two blocks shown in succession may, in fact, be executed substantially concurrently, or the blocks may sometimes be executed in the reverse order, depending upon the functionality involved. It will also be noted that each block of the block diagrams and/or flowchart illustration, and combinations of blocks in the block diagrams and/or flowchart illustration, can be implemented by special purpose hardware-based systems that perform the specified functions or acts, or combinations of special purpose hardware and computer instructions. 
     The above specific example embodiments are not intended to limit the scope of the claims. The example embodiments may be modified by including, excluding, or combining one or more features or functions described in the disclosure.