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Holistic Uncertainty Analysis for the Evaluation of Hydrocarbon Pore Volume from Petrophysical Data

Downer, Edward *1
(1) Petrophysics Group, Senergy Oil and gas, Banchory, United Kingdom.

The evaluation of the hydrocarbon pore volume (HPVOL) from petrophysical well log analysis is the starting point for calculating hydrocarbon in place (HIP), or in mature fields, the remaining hydrocarbon in place (RHIP). The understanding of these numbers and the associated uncertainty is critical to economic evaluation. HPVOL is normally calculated using a deterministic petrophysical workflow.

Sometimes a single value for HPVOL is provided for geological modelling. More commonly an attempt is made at characterising the uncertainty in this estimate using one of the following methods:

1. Sensitivity analysis of input parameters.

2. Error Analysis using partial derivative analysis.

3. Monte-Carlo simulation.

Recently Monte-Carlo simulation has grown in popularity. Monte-Carlo involves the repeated calculation of the calculated output parameters through multiple simulation runs. For each simulation the input parameters are randomly sampled from their expected distribution that encompasses the uncertainty. However, Net reservoir and Net Pay calculation are rarely subjected to Monte-Carlo simulation, and simple sensitivity to cut-off remains the most popular approach.

It is now possible to run Monte-Carlo simulations that encompass the complete deterministic workflow, including simulation of the selection of Net-Reservoir and Net-Pay cut-offs. Co-dependencies between parameters can be honoured.

This holistic approach allows the calculation of the uncertainty distribution for HPVOL, including P10, P50 and P90 values. This method can be extended to novel workflows and permeability estimates. Sensitivity analysis can be used to produce a tornado chart identifying key uncertainties allowing the development of a data collection program that can potentially de-risk the HIP estimate.


AAPG Search and Discovery Article #90141©2012, GEO-2012, 10th Middle East Geosciences Conference and Exhibition, 4-7 March 2012, Manama, Bahrain