COHIBA 3.0 - Surface Modelling and Depth Conversion
Latest version released November 4th 2011.COHIBA is a fast and accurate tool for making 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.
- Geological interpretation of depositional thickness.
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 span the uncertainty range.
- Ability to condition to horizontal wells.
- Handles many surfaces and explicitely takes into account their interdependecies.
- 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.
COHIBA has been developed in cooperation with Statoil.
Conditioning to well points vs 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 relaistic zonation.
Uncertainty reduction
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 uncertianty 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.
Some background literature
The methods used by COHIBA are explained in the following reports and publications:
P. Abrahamsen (1993), "Bayesian Kriging for Seismic Depth Conversion of a Multi-layer Reservoir" in A. Soares (ed.) "Geostatistics Troia '92". Kluwer Academic Publ., Dordrecht, pp. 385-398.
P. Abrahamsen and H. Omre (1994), "Random Functions and Geological Surfaces", in proceedings "ECMOR IV, 4th European Conference on the Mathematics of Oil Recovery, Røros, Norway, 7-10 June 1994.
P. Abrahamsen (1996), "Geostatistics for Seismic Depth Conversion", Report, NR-note SAND/06/1996, 9 pages.
P. Abrahamsen (1999), "FFT algorithm for simulating Gaussian random fields", Report, NR-note SAND/10/99, 28 pages
P. Abrahamsen and F.E. Benth (2001), "Kriging with Inequality Constraints", Math. Geol. Vol. 33 No. 6, pp. 719-744.
P. Abrahamsen (2005), "Combining Methods for Subsurface Prediction", in O. Leuangthong and C.V. Deutsch (eds.) "Geostatistics Banff 2004", Vol. 2, Springer, Dordrecht, pp. 601-610
Petter Abrahamsen, Pål Dahle, Frode Georgsen and Arne Skorstad (2010), "A Consistent Geostatistical Approach for Constraining Multiple Surfaces to Horizontal Wells", poster presented at GEO 2010, 9th Middle East Geoscience Conference and Exhibition, 7 - 10 March 2010, Manama, Bahrain
The word cohiba means tobacco in the language of the Taino Indians who lived in both Cuba and Hispaniola (now the Dominican Republic). COHIBA is by today one of most appreciated Cuban cigars.