A METHOD FOR FORMING COARSE-SCALE 3D MODEL OF HETEROGENEOUS SEDIMENTARY STRUCTURES
The invention discloses a method for forming a coarse-scale three-dimensional geological model of sedimentary structures, the method being implemented by a computer, and comprising: —forming a fine-scale three dimensional model of the sedimentary structures, by implementing steps of: o modeling a plurality of meshed sedimentary surfaces, the plurality of meshed sedimentary surfaces delimiting superposed layers of lithology, o forming an unstructured grid comprising a plurality of cells, wherein each cell extends between at least two sedimentary surfaces, o attributing petrophysical parameters to each cell of the grid, and o attributing, to at least some of the sedimentary surfaces, a transmissivity reduction coefficient, and —upscaling the fine-scale three dimensional model to obtain a coarse-scale three dimensional model comprising a plurality of cells, wherein each cell is associated to petrophysical parameters determined from the petrophysical parameters of the fine-scale model, and from the transmissivity reduction coefficient of the sedimentary surfaces.
1 . A method for forming a coarse-scale three-dimensional geological model of sedimentary structures, the method being implemented by a computer, and comprising:
forming a fine-scale three dimensional model of the sedimentary structures, by implementing steps of:
modeling a plurality of meshed sedimentary surfaces, the plurality of meshed sedimentary surfaces delimiting superposed layers of lithology,
forming an unstructured grid comprising a plurality of cells, wherein each cell extends between at least two sedimentary surfaces,
attributing petrophysical parameters to each cell of the grid, and
attributing, to at least some of the sedimentary surfaces, a transmissivity reduction coefficient; and
upscaling the fine-scale three dimensional model to obtain a coarse-scale three dimensional model comprising a plurality of cells, wherein each cell is associated to petrophysical parameters determined from the petrophysical parameters of the fine-scale model, and from the transmissivity reduction coefficient of the sedimentary surfaces.
2 . A method according to claim 1 , wherein the sedimentary surfaces are meshed with triangles, and the forming the unstructured grid comprises forming a plurality of tetraedric cells between two successive sedimentary surfaces, such that one face of a tetraedric cell corresponds to a triangular mesh of a sedimentary surface, and the summit of the tetraedric cell belongs to an adjacent sedimentary surface.
3 . A method according to claim 1 , wherein the transmissivity reduction coefficient is comprised between 0 and 1.
4 . A method according to claim 3 , wherein a modeled sedimentary surface having a transmissivity reduction coefficient of 0 represents a thin shale layer.
5 . A method according to claim 1 , wherein the attributing petrophysical parameters to each cell of the grid of the fine-scale model comprises:
determining a number of lithology types within the fine-scaled model and defining each lithology type,
determining a distribution pattern of the lithology types within the grid, and
attributing to each cell petrophysical parameters according to the determined distribution pattern.
6 . A method according to claim 1 , wherein the petrophysical parameters comprise at least porosity and permeability values.
7 . A method according to claim 6 , wherein the upscaling is performed by providing a coarse-scale grid comprising a plurality of cells, each cell having dimensions greater than a plurality of cells of the fine-scale model, and the upscaling of the permeability values is performed by computing equivalent fluid flow values of the cells of the coarse-scale grid from fluid flow values of the cells of the fine-scale grid and inferring equivalent permeability values of the coarse-scale grid.
8 . A method according to claim 7 , wherein the computation of the equivalent permeability values is performed by:
numerically solving—Darcy's equation to obtain, in each cell of the fine-scale model, a fluid head in the cell, said fluid head being determined from fluid head values at the limits of the fine-scale model, inferring a fluid flow value in each cell of the fine-scale model,
computing, from the fluid flow values in each cell and the transmissivity reduction coefficients, an equivalent fluid flow value in a cell of the coarse-scale grid comprising the cells of the fine-scale grid, and
inferring an equivalent permeability value of the cell of the coarse-scale grid from the equivalent fluid flow value.
9 . A method according to claim 1 , wherein the modelling of sedimentary surfaces comprises:
selecting a bedform type to be modelled among a library of previously established bedform types, wherein each bedform type defines a disposition of a plurality of sedimentary surfaces, and
parameterizing the selected bedform type.
10 . A method according to claim 9 , wherein the parameterizing of the bedform type is performed according to at least one of the following parameters:
wavelength of a cyclic geometric pattern of the sedimentary surfaces included in the bedform type,
steepness of said cyclic geometric pattern,
angular orientation of said cyclic geometric pattern,
number of sedimentary surfaces, and
mean thickness between two adjacent sedimentary surfaces.
11 . A computer program product comprising code instructions for performing the method according to claim 1 , when executed by a computer.
12 . A non-transitory computer readable storage medium, having stored thereon a computer program comprising program instructions, the computer program being loadable into a computer and adapted to cause the computer to carry out the steps of the method according to claim 1 , when the computer program is run by the computer.