A stochastic model serves as a tool for estimating the probability distributions of potential outcomes by incorporating random variations in one or more inputs over time. This random variation is typically based on fluctuations observed in historical data for a chosen period, using standard time series techniques.
In the estimation of field reserves, depth conversion plays a crucial role in determining the overall depth. The standard Velit Depth Conversion routines and their associated workflows quickly generate accurate velocity models. However, it is essential to assess the uncertainty associated with the depth horizons of interest.
The Analyse tool allows users to estimate depth uncertainty reasonably by generating numerous depth models derived from one or more depth conversion schemes. It also helps identify the key parameters that influence the range of Gross Rock Volume.
An ‘absolute’ solution to the depth conversion challenge is unattainable. The workflow system’s iterative depth conversion processes produce multiple models. The Analyse Module then facilitates the filtering and analysis of results, providing confidence in the likely range of uncertainty in the model, as well as the deterministic parameters that yield credible P50 (most likely) and endpoint P10 and P90 solutions.
P10, P50, P90
The Analyse Module can estimate Proved, Probable, and Possible (P90, P50, P10) Gross Rock Volumes:
- Proved (P90): The lowest estimate, with 90% of the models exceeding this figure.
- Proved and Probable (P50): The most likely estimate (median), expected to be closest to the true reserves.
- Proved, Probable, and Possible (P10): The highest estimate, with 90% of the models falling below this figure.
Velit employs two primary approaches:
- Parametric Variation: This involves depth variation due to uncertainties in the function parameters within a deterministic model.
- Sequential Gaussian Simulation (SGS): This technique is used to assess lateral uncertainties in mapped velocity parameters, such as V0 or depth residuals.