Creating an observation file for use with enkf

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General overview

When using ERT to condition on dynamic data, it is necessary to specify which data to condition on. In particular, for a given piece of data to condition on, the enkf application needs to know:


To provide this observation to the ERT, an observation file must be created. The observation file is a plain text file, and is in essence built around for different classes of observations and has an associated keyword for each class:

The HISTORY_OBSERVATION keyword

The keyword HISTORY_OBSERVATION is used to condition on observations from the WCONHIST and WCONINJH keywords in schedule file provided to the enkf project (or alternatively an ECLIPSE summary file if you have changed the HISTORY_SOURCE keyword in the enkf project). The keyword is typically used to condition on production and injection rates for groups and wells, as well as bottom hole and tubing head pressures. An observation entered with the HISTORY_OBSERVATION keyword will be active at all report steps where data for the observation can be found.

In it's simplest form, a history observation is created as follows:

 HISTORY_OBSERVATION WOPR:P1;

This will condition on WOPR in well P1 using a default observation error. The default observation error is a relative error of 10% to the measurement with a minimum error of 0.10. See below on how explicitly set the error.

In general, to condition on variable VAR in well or group WGNAME, one uses:

 HISTORY_OBSERVATION VAR:WGNAME;

Note that there must be a colon ":" between VAR and WGNAME and that the statement shall end with a semi-colon ";". Thus, to condition on WOPR, WWCT and WGOR in well C-17, and for the GOPR for the whole field, one would add the following to the observation configuration:

 HISTORY_OBSERVATION WOPR:C-17;
 HISTORY_OBSERVATION WWCT:C-17;
 HISTORY_OBSERVATION WGOR:C-17;
 
 HISTORY_OBSERVATION GOPR:FIELD;

By default, the observation error is set to 10% of the observed value, with a minimum of 0.10. It can be changed as follows:

 HISTORY_OBSERVATION GOPR:FIELD
 {
    ERROR       = 1000;
    ERROR_MODE  = ABS;
 };

This will set the observation error to 1000 for all observations of GOPR:FIELD. Note that both the items ERROR and ERROR_MODE as well as the whole definition shall end with a semi-colon.

The item ERROR_MODE can take three different values: ABS, REL or RELMIN. If set to REL, all observation errors will be set to the observed values multiplied by ERROR. Thus, the following will condition on water injection rate for the whole field with 20% observation uncertainity:

 HISTORY_OBSERVATION GWIR:FIELD
 {
    ERROR       = 0.20;
    ERROR_MODE  = REL;
 };

If you do not want the observation error to drop below a given threshold, say 100, you can use RELMIN and the keyword ERROR_MIN:

 HISTORY_OBSERVATION GWIR:FIELD
 {
    ERROR       = 0.20;
    ERROR_MODE  = RELMIN;
    ERROR_MIN   = 100;
 };

Note that the configuration parser does not threat carriage return different from space. Thus, the following statement is equivalent to the previous:

 HISTORY_OBSERVATION GWIR:FIELD { ERROR = 0.20; ERROR_MODE = RELMIN; ERROR_MIN = 100; };


Also note that the special keyword include can be used to read an external file. This can be very useful if you want to change the standard configuration for a lot of observations in one go. For example, consider the following code:

 HISTORY_OBSERVATION WOPR:P1 { include "hist_obs_wells.txt"; };
 HISTORY_OBSERVATION WOPR:P2 { include "hist_obs_wells.txt"; };
 HISTORY_OBSERVATION WOPR:P3 { include "hist_obs_wells.txt"; };
 HISTORY_OBSERVATION WOPR:P4 { include "hist_obs_wells.txt"; };
 HISTORY_OBSERVATION WOPR:P5 { include "hist_obs_wells.txt"; };

Where the contents of the file hist_obs_wells.txt may be something like:

 ERROR_MODE  = RELMIN;
 ERROR       = 0.25;
 ERROR_MIN   = 100;

In this case, changing the file hist_obs_wells.txt will affect all of the observations.

Note that the keyword include can be used anywhere in the configuration file. However, nested inclusion (use of include in a file that has already been included with include) is not allowed.

By default, an observation entered with the HISTORY_OBSERVATION keyword will get the observed values, i.e. the 'true' values, from the WCONHIST and WCONINJH keywords in the schedule file provided to the ERT project. However it also possible to get the observed values from a reference case. In that case you must set set HISTORY_SOURCE variable in the ERT configuration file, see Creating a configuration file for ERT.

To change the observation error for a HISTORY_OBSERVATION for one or more segments of the historic period, you can use the SEGMENT keyword. For example:

  HISTORY_OBSERVATION GWIR:FIELD
  {
     ERROR       = 0.20;
     ERROR_MODE  = RELMIN;
     ERROR_MIN   = 100;

     SEGMENT FIRST_YEAR
     {
        START = 0;
        STOP  = 10;
        ERROR = 0.50;
        ERROR_MODE = REL;
     };

     SEGMENT SECOND_YEAR
     {
        START      = 11;
        STOP       = 20;
        ERRROR     = 1000;
        ERROR_MODE = ABS;
     };
  };

The items START and STOP sets the start and stop of the segment in terms of ECLIPSE restart steps. The keywords ERROR, ERROR_MODE and ERROR_MIN behaves like before. If the segments overlap, they are computed in alphabetical order.

Error covariance for "merged" updates

When merging the historical observations from several report steps together in one update the different steps are not independent, and it is beneficial to use a error covariance matrix, by using the keywords AUTO_CORRF and AUTO_CORRF_PARAM ERT will automatically estimate a error-covariance matrix based on the auto correlation function specified by the AUTO_CORRF keyword, with the parameter given by the AUTO_CORRF_PARAM parameter (i.e. the auto correlation length). The currently available auto correlation functions are

 EXP   ~ exp(-x)
 GAUSS ~ exp(-x*x/2)

where the parameter x is given as:

  x = (t2 - t1) / AUTO_CORRF_PARAM

The SUMMARY_OBSERVATION keyword

The keyword SUMMARY_OBSERVATION can be used to condition on any observation whos simulated value is written to the ECLIPSE summary file, e.g. well rates, region properties, group and field rates etc. A quite typical usage of SUMMARY_OBSERVATION is to condition on the results of a separator test.

Note: Although it is possible to condition on well and group rates with SUMMARY_OBSERVATION, it is usually easier to use HISTORY_OBSERVATION for this.

In order to create a summary observation, four pieces of information are needed: The observed value, the observation error, the time of observation and a summary key. A typical summary observation is created as follows:

 SUMMARY_OBSERVATION SEP_TEST_2005
 {
    VALUE = 100;
    ERROR =   5;
    DATE  = 21/08/2005;
    KEY   = GOPR:BRENT;
 };

This will create an observation of group oil production for the brent group on 21th of august 2005. The observed value was 100 with a standard deviation of 5. The name SEP_TEST_2005 will be used as a label for the observation within the ERT and must be unique.

Similarly to the name of a HISTORY_OBSERVATION, the item KEY in a SUMMARY_OBSERVATION is used to look up the simulated value from the summary file. And again, as when declaring a HISTORY_OBSERVATION, to condition on VAR in well, group or region WGRNAME, one uses:

 KEY = VAR:WGRNAME;

For example, to condition on RPPW in region 8, one uses:

 KEY = RPPW:8;

It is also possible to give the observation time as a restart number using the RESTART item or as time in days from simulation start using the DAYS item. Here are two examples:

 -- Giving the observation time in terms of restart number.
 SUMMARY_OBSERVATION SEP_TEST_2005
 {
    VALUE    = 100;
    ERROR    =   5;
    RESTART  =  42;
    KEY      = GOPR:BRENT;
 };
 
 
 
 -- Giving the observation time in terms of days
 -- from simulation start.
 SUMMARY_OBSERVATION SEP_TEST_2008
 {
    VALUE    = 213;
    ERROR    =  10;
    DAYS     = 911;
    KEY      = GOPR:NESS;
 };

The BLOCK_OBSERVATION keyword

This is observations of variables in grid blocks/cells. The observations can be of arbitrary ECLIPSE fields like PRESSURE (typically for an RFT), PORO or PERM. A block observation is entered with the BLOCK_OBSERVATION keyword. Here is an example of a typical block observation:

 BLOCK_OBSERVATION RFT_2006
 {
    FIELD = PRESSURE;
    DATE  = 22/10/2006;
 
    OBS P1 { I = 1;  J = 1;  K = 1;   VALUE = 100;  ERROR = 5; };
    OBS P2 { I = 2;  J = 2;  K = 1;   VALUE = 101;  ERROR = 5; };
    OBS P3 { I = 2;  J = 3;  K = 1;   VALUE = 102;  ERROR = 5; };
 };

This will condition on observations of the pressure in grid blocks (1,1,1), (2,2,1) and (2,3,1) on the 22/10/2006.

By default the BLOCK_OBSERVATION requires that the specific field which has been observed (e.g. PRESSURE in the example above) must have been specified in main ERT configuration file using the FIELD keyword, and ECLIPSE must be configured to produce a restart file for this particular time. Alternatively it is possible to tell ERT to use the summary vector as source of the data:

  BLOCK_OBSERVATION RFT_2006
  {
     FIELD = PRESSURE;
     DATE  = 22/10/2006;
     SOURCE = SUMMARY;  

     OBS P1 { I = 1;  J = 1;  K = 1;   VALUE = 100;  ERROR = 5; };
     OBS P2 { I = 2;  J = 2;  K = 1;   VALUE = 101;  ERROR = 5; };
     OBS P3 { I = 2;  J = 3;  K = 1;   VALUE = 102;  ERROR = 5; };
  };

In this case the data will be loaded from the BPR vectors in the summary file.

Note the use of the sub class OBS to specify the actUal observed values, the observation errors and their grid location. Each OBS shall have a unique key within the BLOCK_OBSERVATION instance, and is required to have the items I, J, K, VALUE and ERROR. These are the grid i,j and k indicies for the observation point, the observed value and it's standard deviation.

As with a SUMMARY_OBSERVATION, the observation time can be given as either a date, days since simulation start or restart number. The respective keys for setting giving it as date, days or restart number are DATE, DAYS and RESTART. Note that each BLOCK_OBSERVATION instance must have an unique global name (RFT_2006 in the example above).

Block observations can often be quite long. Thus, it is often a good idea to use the special keyword include in order to store the OBS structures in a different file. This is done as follows:

 BLOCK_OBSERVATION RFT_2006
 {
    FIELD   = PRESSURE;
    RESTART = 20;
    
    include 'RFT_2006_OBS_DATA.txt';  
 };

Where the file RFT_2006_OBS_DATA.txt contains the OBS instances:

   OBS P1 { I = 1;  J = 1;  K = 1;   VALUE = 100;  ERROR = 5; };
   OBS P2 { I = 2;  J = 2;  K = 1;   VALUE = 101;  ERROR = 5; };
   OBS P3 { I = 2;  J = 3;  K = 1;   VALUE = 112;  ERROR = 5; };
   OBS P4 { I = 3;  J = 3;  K = 1;   VALUE = 122;  ERROR = 5; };
   OBS P5 { I = 4;  J = 3;  K = 1;   VALUE = 112;  ERROR = 5; };
   OBS P6 { I = 5;  J = 3;  K = 1;   VALUE = 122;  ERROR = 5; };

The GENERAL_OBSERVATION keyword

The GENERAL_OBSERVATION keyword is used together with the GEN_DATA and GEN_PARAM type. This pair of observation and data types are typically used when you want to update something special which does not fit into any of the predefined enkf types. The ERT application just treats GENERAL_OBSERVATION (and also GEN_DATA) as a range of number with no particular structure, this is very flexible, but of course also a bit more complex to use.

GENERAL_OBSERVATION GEN_OBS1{
   DATA     = SOME_FIELD;
   RESTART  = 20;
   OBS_FILE = some_file.txt;
};

This example a minimum GENERAL_OBSERVATION. The keyword DATA points to the GEN_DATA instance this observation is 'observing', RESTART gives the report step when this observation is active. OBS_FILE should be the name of a file with observation values, and the corresponding uncertainties. The file with observations should just be a plain text file with numbers in it, observations and corresponding uncertainties interleaved. An example of an OBS_FILE:

1.46 0.26
25.0 5.0
5.00 1.00

This OBS_FILE has three observations: 1.46 +/- 0.26, 25.0 +/- 5.0 and 5.00 +/- 1.00. In the example above it is assumed that the DATA instance we are observing (i.e. comparing with) has the same number of elements as the observation, i.e. three in this case. By using the keywords INDEX_LIST or INDEX_FILE you can select the elements of the GEN_DATA instance you are interested in. Consider for example:

GENERAL_OBSERVATION GEN_OBS1{
   DATA       = SOME_FIELD;
   INDEX_LIST = 0,3,9; 
   RESTART    = 20;
   OBS_FILE   = some_file.txt;
};

Here we use INDEX_LIST to indicate that we are interested in element 0,3 and 9 of the GEN_DATA instance:

GEN_DATA                     GEN_OBS1
========                     ===========             
1.56 <---------------------> 1.46  0.26
23.0        /--------------> 25.0   5.00  
56.0        |    /---------> 5.00  1.00
27.0 <------/    |           =========== 
 0.2             |
1.56             | 
1.78             |
6.78             |
9.00             | 
4.50 <-----------/
========

In addition to INDEX_LIST it is possible to use INDEX_FILE which should just point at an plain text file with indexes (without any ',' or anything). Finally, if your observation only has one value, you can embed it in the config object with VALUE and ERROR.

Matching GEN_OBS and GEN_DATA

It is important to match up the GEN_OBS observations with the corresponding GEN_DATA simulation data correctly. The GEN_DATA result files must have an embedded '%d' to indicate the report step in them - in the case of smoother based workflows the actual numerical value here is not important. To ensure that GEN_OBS and corresponding GEN_DATA values match up correctly only the RESTART method is allowed for GEN_OBS when specifying the time. So consider a setup like this:

-- Config file:
GEN_DATA RFT_BH67 INPUT_FORMAT:ASCII RESULT_FILE:rft_BH67_%d    REPORT_STEPS:20
...                                                       /|\                /|\ 
...                                                        |                  | 
-- Observation file:                                       |                  |
GENERAL_OBSERVATION GEN_OBS1{                              +------------------/ 
   DATA       = RFT_BH67;                                  | 
   RESTART    = 20;   <------------------------------------/
   OBS_FILE   = some_file.txt;
};

Here we see that the observation is active at report step 20, and we expect the forward model to create a file rft_BH67_20 in each realization directory.

Error covariance

The optional keyword ERROR_COVAR can be used to point to an existing file, containing an error covariance matrix. The file should contain the elements of the matrix as formatted numbers; newline formatting is allowed but not necessary. Since the matrix should by construction be symmetric there is no difference between column-major and row-major order! The covariance matrix

     [ 1      0.75  -0.25]
C =  [ 0.75   1.25  -0.50]  
     [-0.25  -0.50   0.85]

Can be represented by the file:

1
0.75
-0.25
0.75
1.25
-0.50
-0.25
-0.50
0.85

without newlines, or alternatively:

1       0.75  -0.25
0.75    1.25  -0.50 
-0.25  -0.50   0.85

with newlines.

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