To build ert, you need libecl and libres.

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                   |  Ensemble based Reservoir Tool  |
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1. libecl
2. ECLIPSE utilities.
3. Building libecl
   1. CMake settings you might want to adjust
4. The code:
   1. The different libraries
   2. The general structure
   3. Python wrappers
5. Tests

libecl - Ensemble based Reservoir Tool is a tool for managing an ensemble of reservoir models. The initial motivation for creating libecl was a as tool to do assisted history matching with the Ensemble Kalman Filter (EnKF). Very briefly, the process of using EnKF for history matching can be summarized as:

1. Sample initial reservoir parameters from a (Gaussian) initial distribution.
2. Simulate the ensemble of of reservoir forward in time through a part of the historical period for which data is available.
3. Load the results, compare with the observed data, update the parameters and the state of reservoir by filtering out the most inacurate results, and restart the forward simulations.

This recipe is quite complex technically, and in particular involves the ability to read and write input and output files from the reservoir simulator (i.e. ECLIPSE in the case of libecl), run simulations with arbitrary external programs, plotting data and so on. This implies that a quite significant technical machinery must be in place before the EnKF algorithm as such can be utilizied. This in particular applies to real industry reservoir models, where typically imperfections of all kinds flourish.

Despite the fact that the initial motivation for creating libecl was to be able to use the EnKF algorithm for history matching, libecl is currently more used with the Ensemble Smoother and also purely as a workflow manager, i.e. herding a large collection of reservoir models through the required simulations steps.

libecl has a quite large amount of code devoted to reading and writing the ECLIPSE output files (grid/rft/restart/init/summary). In addition, there is also reasonable support for reading and writing the grdecl input files, but there is no general .DATA file parser. The ability to read and write ECLIPSE output files is valuable in many reservoir applications, and it is possible to only build and use the libecl (with support libraries) library for working with ECLIPSE files. In fact, the default build setup is to only build the ECLIPSE related library and utilities. This part of the libecl distribution can also be built on Windows with Visual Studio (albeit with maaaany warnings) and with MinGW.

CMake is the build system for libecl. The top level CMakeLists.txt file is located in the top level directory of the repository, and this CMakeLists.txt file includes individual CMakeLists.txt files for the different libraries.

Building with CMake is performed like this:

1. Create a build directory, this can in principle be anywhere in the filesystem. One level above the toplevel source directory is a practical choice.
2. Go to the build directory and invoke the command:

ccmake <path/to/directory/containing/CMakeLists.txt>

Go through several 'configure' steps with CMake and generate native build files. 3. Exit ccmake and invoke the native build system, i.e. ordinarily 'make' on Linux. 4. Subsequent builds can be performed using just the native make command, as in step 3.

The main setting you should adjust is BUILD_ERT which is default to OFF, i.e. by default only the ECLIPSE related utilities will be built. The build system has numerous configurations checks; the ECLIPSE utilities should build on Windows, but to build all of libecl you will need a Linux (Posix) system.

The code is mainly a collection of libraries written in C.

The provided libraries are:

• libert_util: This library is a collection of utilities of various sorts; if C++ had been chosen as implementation language, most of these utilities could probably be replaced by standard C++ classes.
• libgeometry: This is a very small geometry library; the main code is a small implementantion of an alorithm to determine whether a point is inside a polyhedron. The ECLIPSE library has some geometry related code which should be moved here.
• libwell: This library will load well information from an ECLIPSE restart file. This is mainly for the purpose of visualization of the existing wells, and can not be used to update or model the well configuration.
• libecl: This library will read and (partly) write the various binary ECLIPSE files, including GRID/EGRID, summary, INIT, restart and RFT files. There is also support for reading an writing grdecl formatted files, but there is no support for general parsing of the ECLIPSE input format.

The code is written in C, but conventions give a 'scent of object orientation'. Most of the code is uses the following conventions:

• Every file 'xxx' implements a data type 'xxx_type' - this naming convention is quite strong.
• All the structure definitions are in the source files, i.e. external scopes must access the data of a structure through accessor functions.
• All functions which operate on a type 'xxx_type' take a pointer to xxx_type as their first argument, the structure closely resemble the 'self' argument used when implementing Python classes.
• Memory management is manual; however there are some conventions:
  • Functions allocating storage have alloc as part of the name.
  • For all functions xxx_alloc() which allocate memory, there should be a matching xxx_free() function to discard the objects.
  • Containers can optionally destroy their content if the content is installed with a destructor.
• In libert_util/src/type_macros.h there is a macro based 'type-system' which is used to runtime check casts of (void *).

Some of the code, in particular the ECLIPSE related functionality, has been wrapped for usage in Python. Using these wrappers, it is quite easy work with ECLIPSE files. The python wrappers are quite well documented both in the directory python/docs and in the Python classes themselves.

The libecl codebase has a small but increasing test coverage. The tests are typically located in a per-library subdirectory tests/. The test framework is based on ctest which basically works like this:

1. In the CMakeLists.txt file testing is enabled with ENABLE_TESTING().
2. Tests are added with:

add_test( test_name test_executable arg1 arg2 arg3 ... )

If the executable exits with status 0 the test has passed, otherwise it has failed. The executable run by the test can typically be an executable built as part of the solution, but can in principle be an arbitrary executable like a dedicated test runner or e.g. the Python interpreter.

The main part of the testing infrastructure are small C applications which are added like this:

add_executable( test_exe test_source.c )
target_link_libraries( test_exe lib )
add_test( test_name ${EXECUTABLE_OUTPUT_PATH}/test_exe commandline_arg1 commandline_arg2 )

Where the first two lines create the test executable in the normal way, and the last line adds it as a test.

In python/test there are several files with Python tests, these files are executable files and they are invoked directly from the command line. A limited number of the tests have been integrated in the ctest system.

The tests in the cmake build system follow the naming convention of the library regarding the functionality which they are testing: For example, all tests for the libecl library use a name starting with 'ecl' and all tests for the tests for the config library are prefixed by 'config'. The ctest options -R and -E can be used to include and exclude tests based on their name

ctest -R ecl      # Run all tests containing the regular expression 'ecl'
ctest -E ecl      # Run all tests NOT containing the regular expression 'ecl'

Using the cmake set_property() function it is possible to assign labels to the test, and the -L and -LE options to ctest can be used to limit which tests to run. A test can only have one label; in the current ctest setup different labels are combined into one composite label with a ":" separator, e.g.

set_property( TEST test_xxx PROPERTY LABELS StatoilData:Python)

will set the 'StatoilData' and 'Python' properties on test_xxx. The labels currently available in the libecl test setup are:

StatoilData: This implies that the test makes use of Statoil internal data. If you are work for the Bergen office of Statoil, you can read the the file test-data/README for instructions on how to make this data available.

  If you are not for Statoil in Bergen, you must pass the option:
  "-EL StatoilData" to ctest to skip all the tests which require
  Statoil internal data.

StatoilBuild: There is one python test which makes use of Statoil internal configuration data, this test is labeled with StatoilBuild. If you want to run this test, you must set the cmake option ECL_LOCAL_TARGET to point to a file which contains these local configuration settings, e.g. where the ECLIPSE binary is installed.

Python: This label is used to indicate that the test uses Python.

LSF: This labels indicates that the test needs a working LSF environment to run.

ctest -L Statoil         # Run all tests labeled with Statoil - both
                         # StatoilData and StatoilBuild

ctest -EL "Statoil|LSF"  # Exclude all tests labeled with Statoil or LSF.