COHIBA 5.5 - Surface Modelling and Depth Conversion
COHIBA 5.5 - Surface Modelling and Depth Conversion
COHIBA Version 5.5 was released September 8th 2017. Important improvements are:
The handling of well data uncertainty has been significantly improved. There is now a clear distinction between surface uncertainty, well point pick uncertainty, and well path positioning uncertainty. Surfaces and uncertain well paths will now intersect at well points if the well point pick uncertainty is zero. The vertical positioning uncertainty in the well is called well path TVD uncertainty and the uncertainty in the interpretaion of zone log transitions along the well path is called well point MD pick uncertainty. The vertical projection of the pick uncertainty is called well point TVD pick uncertainty. The specification of well uncertainty has been simplified as a consequence of this.
- Several elements have changed names for two reasons: To reflect a more consistent terminology and to reflect a simpler specification of well uncertainty. Legacy element names will still work but the user is encouraged to update the element names.
- Many output files have been given new names. Also the output file structure has been reorganized so that the important files are simpler to find among the numerous output files.
- Many small errors have been corrected.
For a complete list of all modifications please consult the release notes in the user manual.
COHIBA is a fast and accurate tool for making deterministic and stochastic surfaces. COHIBA can use information from:
- Surface observations in wells (well points)
- Horizontal well paths with zone logs
- Seismic travel time maps
- Interval velocity maps and models
- Isochore maps and models
- Spill point depth
COHIBA uses the available data in a consistent manner to minimize the uncertainty. The accuracy is further improved by linking together all surfaces in a multi-layered model.
COHIBA provides two ways of evaluating uncertainty:
- A local depth uncertainty at every surface location can be calculated
- Simulated (Monte Carlo) surface realizations can be generated. A set of these spans the uncertainty range
The main advantages of COHIBA are:
- Ability to condition to horizontal wells
- Handles many surfaces and explicitly takes into account their interdependencies
- Handles erosions, pinch-outs, thin layers, and on-lapping surfaces
- QC and robustness. COHIBA analyze all input data, filter away erroneous data, and reports possible problems
- Can handle well path TVD uncertainty in multilateral horizontal wells and modify the well depth to obtain consistency with surfaces and other wells
For details and examples please have a look at the COHIBA user manual:
Conditioning to well points versus conditioning to well paths
Below are two cross sections showing the improvements obtained by conditioning the surfaces to well paths in addition to the well points. The left figure is obtained using only well points while the right picture is obtained using both well points and well paths. Note how all surfaces are modified to obtain consistent and realistic zonation.
Below is a second example. Again we see how COHIBA modifies all surfaces to make a consistent and realistic zonation.
The following picture shows the result of conditioning to well paths. We clearly see how the surfaces are accurately determined along the well paths.
The left-hand pictures show the results from using well points whereas the right-hand pictures show the results from using both well points and the well paths. The uncertainty is significantly reduced along the well paths because the well trajectories are confined to very thin zones similar to the situations in the cross sections above.
The animation below simulates the use of distance data acquired from deep directional resistivity (DDR) logs during a drilling process, and shows a vertical cross section along a planned well trajectory. The reservoir consists of a top and a base surface with the reservoir zone in red. The planned trajectory is shown as a continuous line entering the top of the anticline and passing through the reservoir. The actually drilled trajectory starts to deviate from the planned well as the true distances to the top and base reservoir are updated. The dashed lines represent the surface uncertainty envelopes and shrinks as more data becomes available.
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