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Error calculation and use of first order error propagation as an integral part of petrophysical evaluation

Evening Lecture

Error calculation and use of first order error propagation as an integral part of petrophysical evaluation

Presenter – Stein Ottar Stalheim, Statoil

This presentation is based on the view that the calculation of error is an essential part of measurements and scientific calculations, and that it should be an integral part of petrophysical calculations. Presentation of errors in petrophysical results are rarely applied in formation evaluation and reservoir characterization. Calculation of petrophysical error is commonly executed for input to geostatistics, but often only after the petrophysical calculation is completed. Sometimes the calculation of petrophysical error is excluded altogether. Petrophysical results should include traceable and quantifiable errors.

The motivation of this presentation is to show that errors can easily be integrated to the petrophysical results by including the first order error propagation (FOEP) method as a part of the computer script that gives the petrophysical results. Errors in the petrophysical results are related to the models used and errors in model input. Calculation of petrophysical errors involves understanding how the input errors propagate through the functions to the end product. A commonly used method is Monte Carlo, while FOEP is less used. Different views exist with respect to needs, pros and cons for the various methods, but there also are some doubts regarding limitations around the use of FOEP. The FOEP solution is the chosen method in this paper because it is an analytical and more practical solution related to implementation into the script that computes the petrophysical results.

This presentation includes an introduction to the theory of FOEP in matrix form and contains examples that illustrate the application of petrophysical functions. The mathematics shows how dependencies between variables and asymmetrical distributions are included in calculation of errors. The mathematics, graphical user interfaces (GUI), and plot functionalities are scripted with the use of Python. An example of a GUI for petrophysical input, and example plots including presentation of the errors and error propagation, are also presented.

Bio: Stein Ottar Stalheim is a specialist in petrophysics with Statoil ASA, Norway. Stein Ottar joined Statoil in 1993. His main interest is EM physics, multivariate analysis and scientific computing. The main role for Stein Ottar in Statoil is related to Geo-electrical modeling, dynamic petrophysics and well integrity. Stein Ottar received his M.Sc. and Ph.D. degree in physics from the University in Bergen.

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Stein Ottar Stalheim

Wednesday, 10th May
6:30pm, The Geological Society

Wine & Savouries included


The Geological Society
Burlington House
London W1J 0BG

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