FR2775094A1 - Fluid composition simulation method - Google Patents

Abstract

The operation of delumping is based on the fact that the oil reservoir has only two non aqueous phases (liquid and gas) , allowing fluid regrouping and detailed decomposition calculations to be carried out. The oil reservoir is split into a grid system and aggregate fluid compositional simulation using components and pseudo components carried out. For each grid section, modeling is carried out on the vaporized fraction with the regrouped fluid under constant equilibrium. For each grid step, a molar quantity is produced and evaluation of the detailed fluid composition carried out.

Description

Simulation method to predict over time
a detailed composition of a fluid produced by a reservoir
The present invention relates to a simulation method for predicting, as a function of time, the detailed composition of a fluid produced by a reservoir and more particularly the detailed composition of a fluid contained in and produced by a petroleum deposit in which are implanted a or several production wells.

 The compositional simulation of an oil field is commonly used to provide the deposit's production profiles, which in particular make it possible to determine the production scheme best suited to this deposit.

The compositional simulation of a petroleum deposit is implemented not by using a real description of the fluid but by using a fluid modeled by a smaller number of components than the number of components of the real fluid. Indeed, the number of components of the reservoir fluid being relatively large, the modeling with all the components would lead to too much computation time. This prohibitive calculation time has led specialists to group pure components into pseudo-components, for example pseudo-components including nitrogen and methane, pseudo-components including C3 and C4 hydrocarbons, etc. .., and to perform reservoir simulation on a reduced number
N pure components and pseudo-components. Such a reduced composition is referred to as "pooled fluid composition" or "pooled fluid". Generally N represents 5 to 8 components and pseudo-components, which is considered to be sufficient to properly represent the behavior of the reservoir fluid under background conditions.

 Before conducting the compositional reservoir simulation, we represent the reservoir or reservoir in the form of a mesh network, each of which constitutes an elementary volume of said reservoir. The number of meshes can reach several thousands and each mesh has its own properties, such as geometry, porosity, permeability, etc. Moreover, at certain meshes can correspond to at least one injection well or production which is implanted in said reservoir.

 The reservoir simulation makes it possible to calculate for each mesh a certain number of principal variables of the reservoir fluid, said variables possibly being the quantity (number of moles) of fluid, the composition or mole fraction of each component and pseudo-component of the grouped fluid. and the pressure prevailing in each mesh. These variables are known at time t = O (start of exploitation of the deposit) and are then calculated by the simulation of deposit at each moment t or no time m. For each cell, it is possible, from these principal variables, to calculate any other property of the fluid present in the mesh, such as the number of phases, the composition of each phase, etc.

For each mesh with which a production well is associated, it is possible, from these main variables and imposed production constraints, to also calculate the production rate and the composition of the fluid produced by said well.

 The thermodynamic properties of the fluid can be calculated using one of the state equations well known to the specialists and which will not be recalled.

 The N-component model, if it is sufficient to represent the behavior of the reservoir fluid under the background conditions, is no longer suitable for simulating the behavior of the fluid in the facilities for its surface operation, which operation requires knowing a more complex composition. Q components and / or pseudocomponents of the fluid produced by the deposit, Q being greater than N and for example of the order of 20 to 30.

 Heretofore, this detailed component and / or pseudo-component composition was obtained from the pooled composition N assuming that the composition of the pseudo-components remained constant over time.

 Such a procedure is error-prone because the composition of the pseudo-components varies over time, in particular as a function of the pressure of the deposit or following the injection of a gas into the deposit.

 As a result, the detailed composition obtained remains very approximate and does not make it possible to satisfactorily predict the behavior of the fluid in the surface operating conditions for many years after the start of production.

 The object of the present invention is to propose a simulation method for predicting the detailed composition of the fluid produced by a reservoir taking into account the parameters of the reservoir and which is much more precise.

The subject of the invention is a method of the type consisting of:
a) representing the reservoir in the form of a network of meshes (j) each of which constitutes an elementary volume filled with fluid,
b) defining the fluid by N-component and pseudo-component grouped modeling (i) and determining a state equation describing the fluid in this clustered modeling,
c) also defining the fluid by detailed modeling with Q components and / or pseudo-components, Q being greater than N, and determining another state equation describing the fluid in this detailed modeling,
d) producing, in a manner known per se, a compositional simulation of the fluid grouped with N components and pseudo-components (i), said compositional simulation making it possible to calculate at least for each cell (j) and at consecutive time steps (m , m + 1, ...) the vaporized fraction (el). the liquid-vapor equilibrium constants (k ")) of each component (i), the injection or production flow rates (sijm) and for each pair of meshes (j, h) the flow rates of the liquid phases (μm, h) and vapor (U, 3, h) of the N-component fluid and pseudo-components, and is characterized in that it further comprises:
e) determining, at each time step (m) and for each cell (j), the composition of the liquid and vapor phases of the fluid detailed in the composition Q of the fluid, from the values of the vaporized fraction (# jm) and equilibrium constants (ksi)) of the grouped fluid,
f) evaluating for each mesh, at the time step (m + 1), the molar quantity of each of the Q components and / or pseudo-components of the detailed fluid from the values corresponding to the time step (m), the flow rates of the phases of the grouped fluid and liquid-vapor phase compositions of the detailed fluid determined in step e), and
g) evaluating, for each production well, the detailed composition of the fluid produced between times t and t 'corresponding to time steps m and m + 1, from the flows of the phases of the grouped fluid and the compositions of the liquid phase and detailed fluid vapor determined in step e).

 According to another characteristic of the present invention, the steps e, f and g are implemented at the same time as performing the compositional simulation of the grouped fluid.

 According to another characteristic of the present invention, the results of step d are stored and subsequently used for the implementation of steps e, f and g.

 The method according to the present invention is implemented for a reservoir constituted, for example, by a petroleum deposit. In known manner, a reservoir simulation is performed. For this purpose, the deposit is represented in the form of a mesh network, some of the meshes or a group of meshes being associated with one of the production wells implanted in the oil field to be exploited.

 To reduce the calculation time, the compositional simulation of the deposit is performed on a limited number N of components and pseudo-components, for example from 5 to 8, as described above, these components and pseudo-components being selected according to the nature of the deposit.

The grouped N-component and pseudo-component fluid is described by a state equation which could be for example that of PENG
ROBINS ON adapted to said grouped composition.

At the beginning of the exploitation of the deposit, we know, by measurements carried out beforehand, for each mesh j of the network, the main variables at time t = 0 or no time m = 0. These main variables are: the quantity of fluid: fJ the molar fraction of the component i: z i with i zizi <N the pressure: p,
To go from the time step m to the time step m + 1, the compositional simulation comprises several steps detailed below.

dans lesquelles bi est la fugacité du composant i.In a first step and for the time step m, a liquid-vapor equilibrium calculation (flash) is performed on the grouped fluid, for each mesh j to determine: the vaporized fraction 0, the molar fraction of each component i in the oil phase (x "with i = 1 ... N) and the molar fraction of said component i in the gas phase (yt with i =
These various fractions are determined with the following system of equations:

where bi is the fugacity of the component i.

avec i variant entre 1 et N, afin de les utiliser dans une autre étape dénommée "delumping" qui sera explicitée ultérieurement.According to a first characteristic of the invention, the vaporized fraction OT is memorized. and equilibrium constants

with i varying between 1 and N, in order to use them in another step called "delumping" which will be explained later.

 In a second step and for each mesh, some of the properties of each liquid and gas phase are evaluated, namely density (pOm, and) viscosity, saturation and relative permeability.

 In a third step, for each mesh j and at time step m, the coefficients of the equation for the pressure are calculated.

The equations for the pressure of a mesh j at time t + At or at time step ml, are:

- le terme source

To solve these equations, the coefficients to calculate are: - the total compressibility CTS (fluid + rock), - the generalized transmissivities

- the source term

 In these equations: h are the meshes close to the mesh j, h EJ (j), a, m is the partial molar volume of the component i in the mesh j at the time step m, 7i, mJ and h, m are the mobilities of the phases oil (o) and gas (g), t ,, is the transmissivity between the centers of meshes j and h, I being the upstream mesh.

 In a fourth step, the equations for the pressure at time t + At, that is to say at the time step m + 1, are solved for all meshes.

These equations form a linear system of the type:
A. p = B

J being the number of meshes of the network.

 These equations are solved by a standard iterative process.

In a fifth step, the flow rates of the phases between each pair of adjacent meshes and for each well associated with, for example, the law of DARCY, are calculated at the time step m + 1.

1 being the upstream mesh with l = j or h.

 According to another characteristic of the invention, the flow rates of the phases which will be used in the delumping are stored or stored.

In a sixth step, the quantity f and the composition z of the combined fluid, in each mesh j and at the time step m + 1 are evaluated from the values at the time step m, the flow rates of the phases evaluated at the fifth step and the compositions of the phases calculated in step 1. To do this, we apply the conservation laws defined by the following equations:

 By an iterative process, the above steps are repeated to calculate the principal variables at times t + 2At, t + 3At, etc ... or else at time steps m + 1, m + 2, m + 3, etc. ..., the increment At being able to be constant or variable.

 According to the present invention, a detailed modeling is defined with Q components and / or pseudo-components as well as another state equation describing the fluid thus modeled.

 For this purpose, the components and / or pseudo-components whose characteristics are to be known and which are necessary to represent the production fluid in the surface conditions are chosen beforehand. The number Q which is greater than the number N of the grouped composition is generally of the order of 20 to 30. The composition of the detailed fluid with Q components and / or pseudo-components is known at the time step m = 0.

 The data stored in the first and fifth steps are used to extend them to the Q components and / or pseudo-components of the detailed composition.

 Since the flow can be in any direction between neighboring meshes, each mesh can not be processed independently. That is why, at each time step, all the meshes are treated together.

To carry out the delumping, it is assumed that at each time step m, the molar fraction of the vaporized phase in each mesh is
independent of the number of constituents of the fluid, # jm = # jm - the number of moles of liquid and vapor flowing between the meshes and
in or out of the wells is independent of the number of constituents of the
fluid,
UoJh = Uouh and Ugh =
Sojm = sojm and Sgjm = sgjm
The uppercase letters are used to represent the detailed fluid and the lowercase letters are used to represent the grouped fluid.

comme indiqué par C. LEIBOVICI dans son article "A Consistent
Procedure for Pseudo-Component Delumping" paru dans FLUID PHASE
EQUILIBRIA, 117 (1996), 225-232.The calculation of equilibrium constants can be done from the expression

as indicated by C. LEIBOVICI in his article "A Consistent
Procedure for Pseudo-Component Delumping "published in FLUID PHASE
EQUILIBRIA, 117 (1996), 225-232.

où aI et bI sont les paramètres de l'équation d'état pour le composant I et où les paramètres #jm,ssjm et &gamma;jm sont calculés à partir des constantes d'équilibre pour le fluide regroupé (kt) en minimisant la fonction:

For a two-parameter state equation, such as
PENG ROBIN SON, the generalized equation of LEIBOVICI can be written for the detailed fluid in the following way:

where aI and bI are the parameters of the state equation for component I and where the parameters # jm, ssjm and &gamma; jm are calculated from the equilibrium constants for the pooled fluid (kt) by minimizing the function :

As a result, when the detailed composition ZIJ is known in the mesh j at the time step m, the detailed compositions of the liquid and vapor phases can be estimated in the same mesh j and for the same time step m.

 I being the component and / or pseudo-component of the detailed composition. These compositions can be normalized so that their sum is equal to 1 if necessary.

dans lesquelles j'=j pour un débit positif de la maille j vers la maille h ou dans le puits et j'=h pour un débit négatif, j' correspondant au fluide injecté pour les puits d'injection où S sera négatif.When the compositions of the detailed phases are known for all the meshes of the network at the time step m, it is possible to estimate the detailed compositions zlmJ '' at the consecutive time step m + 1 from the equations below.

in which j '= j for a positive flow from the mesh j to the mesh h or in the well and j' = h for a negative flow, j 'corresponding to the fluid injected for the injection wells where S will be negative.

 Thanks to the present invention, a detailed composition with Q components and / or pseudo-components is obtained which makes it possible to better define the predicted production profiles, with greater precision compared with the previous methods. This greater precision of the projected production profiles allows a more reliable choice of development scheme (type and size of surface installations, number of wells, etc.), resulting in an increase in profitability and a decrease in economic risk.

 In the foregoing, certain values obtained during the simulation of the deposit are stored or stored for later use in the delumping. It goes without saying that these same values could be used directly if one decides to carry out the delumping at the same time as said simulation of deposit.

Claims (1)

1. Simulation method for predicting, as a function of time, the detailed composition of a fluid produced by a reservoir, of the type consisting in: a) representing the reservoir in the form of a mesh network (j) each of which constitutes an elementary volume filled with fluid, b) defining the fluid by modeling grouped with N components and pseudo-components (i) and determining a state equation describing the fluid in this grouped modeling,  c) also defining the fluid by detailed modeling with Q components and / or pseudo-components, Q being greater than N, and determining another state equation describing the fluid in this detailed modeling,  d) producing, in a manner known per se, a compositional simulation of the fluid grouped with N components and pseudo-components (i), said compositional simulation making it possible to calculate at least for each cell (j) and at consecutive time steps (m , m + 1, ...) the vaporized fraction (û,), the liquid-vapor equilibrium constants (k;) of each component (i), the injection or production flow rates (sp) and for each mesh pair (j, h) the flow rates of the liquid (Uojh) and vapor (UeJh) phases of the fluid with N components and pseudo-components, and it is characterized in that it further comprises:  e) determining, at each step, the time (m) and for each cell (j) the composition of the liquid and vapor phases of the fluid detailed in the composition Q of the fluid, from the values of the vaporized fraction (OT) and equilibrium constants (kit)) of the grouped fluid,  f) evaluating for each mesh, at the time step (m + 1), the molar quantity of each of the Q components and / or pseudo-components of the detailed fluid from the values corresponding to the time step (m), the flow rates of the phases of the grouped fluid and liquid-vapor phase compositions of the detailed fluid determined in step e), and  g) evaluating, for each production well, the detailed composition of the fluid produced between times t and t 'corresponding to time steps m and m + 1, from the flows of the phases of the grouped fluid and the compositions of the liquid phase and detailed fluid vapor determined in step e). 2. Method according to claim 1, characterized in that the steps e, f and g are implemented at the same time as performing the compositional simulation of the grouped fluid. 3. Method according to claim 1, characterized in that the results of step d are stored and subsequently used for the implementation of steps e, f and g.