P 10 Case Part 14 Aug 2021 12:11
Uncertainty-Based Philosophy
(Source - https://www.spe.org/industry/docs/Guidelines-Evaluation-Reserves-Resources-2001.pdf )
The probabilistic evaluation method is, by its very nature, an uncertainty-based approach. However,it can be (and is) also applied using deterministic methods; in such cases, a more appropriate term is the scenario approach because it is based on three specific scenarios for the accumulation rather than on a single deterministic estimate. Project maturity is handled completely separately (see Section 2.6). In this approach, the objective is, first, to make a “best estimate” of the recoverable volume, which represents the estimate considered to be the closest to the quantity that actually will be recovered from the accumulation. Next, an estimate of an upside and a downside case of recoverable volumes is made, which provides a measure of the range of uncertainty in the best estimate. With the scenario approach, therefore, three different scenarios are established for the accumulation that are consistent with the Proved category (downside case), the Proved plus Probable category (the best estimate), and the Proved plus Probable plus Possible category (upside case). Each scenario is chosen to represent a realistic combination of parameters, consistent with the relevant definitions and taking into account potential dependencies between parameters. Thus, the downside case is not simply a matter of combining the low estimate of every input parameter; if two parameters are unlikely to occur together, that must be taken into account (dependencies are discussed further in Chapter 4). The scenarios may incorporate both technical and commercial uncertainty.
This approach is directly comparable to the probabilistic method. However, instead of trying to estimate the complete range of possible outcomes (which includes making assumptions with regard to probability distributions and levels of dependency/correlation), the required three reserve values are estimated by selecting three representative scenarios. Although not as mathematically rigorous as the probabilistic method, this approach has the advantage that each assumption can be identified specifically (e.g., this particular value of average porosity was used for the proved scenario); hence,reserve auditing is facilitated. Perhaps the most important aspect in this regard is the relationship between the development plan (and hence recovery factor) and the geological model (and hence the petroleum-initially-in-place). This potential dependency is frequently overlooked when applying the probabilistic method and may be easier to accommodate with the scenario approach.
Continued Part 2