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Nested Hybrid Grids

🧪 Experimental Feature

This module is currently experimental. It may undergo breaking changes in future versions without notice.

Table of Contents

Overview

The nestedhybridgrid module creates nested hybrid grids where a selected region of a coarse grid is replaced by a refined (subdivided) sub-grid. The two grids are merged into a single grid and connected through Non-Neighbour Connections (NNCs).

The typical workflow is:

  1. Define a coarse grid and a region property that marks cells to refine with value 1.
  2. Use the :class:`~fmu.tools.nestedhybridgrid.NestedHybridGrid` to produce the merged grid and an NNC table. You may need to export the NNC file to csv at this stage.
  3. Do rescaling from the original gridmodel (e.g. a finer geogrid) to the merged grid, e.g. by using software like RMS.
  4. Make another script, that computes transmissibilities with :meth:`xtgeo.Grid.get_transmissibilities` passing the NNC table.
  5. Export the NNC transmissibilities for the flow simulator.

Quick-start example

The example here runs within RMS, but similar workflows can be created for file i/o.

from fmu.tools.nestedhybridgrid import NestedHybridGrid

# Create nested hybrid grid (refine region 1 by 2×2×1)
nhg = NestedHybridGrid.from_rms(
    project,
    grid_name="Simgrid",
    region_name="Refinement_region",
    refinement=(2, 2, 1),
    properties=["Zone"],  # Optional list of properties to transfer to the output grid
)

# store nested grid with properties in RMS
nhg.to_rms(project, "NestedHybrid")

# write the NNC pandas to disk; this will be applied for computing NNC's in the next script
nhg.nnc_table.to_csv("path_to_some_csv_file.csv", index=False)

The next step is to do a rescaling from the original geogrid to the merged grid using e.g. the RMS tool.

Further, we need to create NNC transmissibilities and generate file for flow simulator:

import pandas as pd
import xtgeo
from fmu.tools.nestedhybridgrid import (
    nnc_to_flowsimulator_input,
    nnc_to_gridproperty,
)

GNAME = "NestedHybrid"

# Load grid and region property which may be stored in RMS
nested = xtgeo.grid_from_roxar(project, GNAME)

# load the NNC table
nnc_table = pd.read_csv("path_to_some_nnc_file.csv")


# Load rescaled property input for transmissibilities and compute
permx = xtgeo.gridproperty_from_roxar(project, GNAME,"PERMX")
permy = xtgeo.gridproperty_from_roxar(project, GNAME,"PERMY")
permz = xtgeo.gridproperty_from_roxar(project, GNAME,"PERMZ")
ntg   = xtgeo.gridproperty_from_roxar(project, GNAME,"NTG")  # defaults to 1 if no NTG

# compute transmissibilities. Note that flow simulators do this for the normal cells/faults
# so strictly speaking, only nnc_hybrid is needed here.
tranx, trany, tranz, nnc_fault, nnc_hybrid, rbnd = nested.get_transmissibilities(
    permx, permy, permz, ntg, nnc_table=nnc_table
)

# Export NNC keyword for Eclipse / OPM Flow
nnc_to_flowsimulator_input(nnc_hybrid, "some_path/NNC_HYBRID.INC")

# Or map NNCs onto grid properties for visualisation
tx_nnc, ty_nnc, tz_nnc = nnc_to_gridproperty(nested, nnc_hybrid)
tx_nnc.to_roxar(project, GNAME, "TRANX_NNC_QC")  # etc

Concepts

NNC table

The NNC table captures which coarse (mother) cells connect to which refined cells — information that xtgeo needs to compute transmissibilities across the refinement boundary. It is accessed via the property nnc_table on the :class:`~fmu.tools.nestedhybridgrid.NestedHybridGrid` instance and is of type :class:`~pandas.DataFrame` with columns:

Column Description
I1, J1, K1 Mother cell indices (1-based)
I2, J2, K2 Refined cell indices (1-based)
DIRECTION Face direction from the mother cell toward the refined cell (I+, I-, J+, J-, K+, K-)

This table is passed to :meth:`xtgeo.Grid.get_transmissibilities` via the nnc_table parameter. The transmissibility computation uses geometric face-overlap calculations (Sutherland–Hodgman algorithm) and two-point flux approximation (TPFA).

Eclipse / OPM Flow export

:func:`~fmu.tools.nestedhybridgrid.nnc_to_flowsimulator_input` writes the NNC keyword in Eclipse format. The output file can be included in the simulator deck:

INCLUDE
  'NNC_HYBRID.INC' /