Uncertainty Analysis Workflow to Estimate Oil and Gas Reserves as A Guideline for Field

Abstract

It is common practice that estimating resources and reserves for exploration activities is different from the development stage, since has high uncertainty, lack of available data and information, so the method that is often used to estimate resources is  pobabilistic. However, in development or exploitation period since it is presumed it had more data, the evident of oil & gas discovery, and the presence of commercial hydrocarbons that can be produced, the deterministic method is preferred to be used.

In this paper, the discussion will focus on deterministic applications for field development. Geoscientists assume that the uncertainty at this stage is very low and even tends to be ignored so that the reserve figure used is only a single number. While in this period, uncertainty still exists covering technical aspects, technology application, and commercial valuation with different levels depending on subsurface variables to estimate hydrocarbon reserves. Since 2010, all reserves booked for any field  development proposal have always used the deterministic method, however starting in 2018 SKK Migas released the 2nd Revised Pedoman Tata Kerja (PTK) No. 37 on the Plan of Development (POD) and one of its points contains that all subsurface  valuations require to obtain uncertainty analysis & risk management and as well as use multi-scenario methods in calculating hydrocarbons in place. Multi-scenario is a incorporation of probabilistic and deterministic methods for the use of uncertainty analysis of multiple subsurface parameters such as seismic interpretation, petrophysical evaluation, conceptual geology and static modelling. The expected results, for example, are several low case, base case, and high case geological models and are able to provide geological maps for each case.

As a derivative of the PTK-POD implementation, an uncertainty analysis workflow is required to support the use of a multi-scenario method in order to capture all variations of subsurface parameters that have the potential to have an effect on project maturation. The deterministic method is still used as an initial reference in identifying the uncertainty of geological and reservoir parameters through the Tornado Chart. This step is able to reveal which subjects are most sensitive to the Hydrocarbon In-Place value. Once the parameter is selected, the probabilistic calculation will provide a normal distribution curve and show the stability of the parameter distribution in the geostatistical model. In conclusion, the results of the multi-scenario model of low case, base case, and high case in the calculation of reserves are a reflection of the possible risk-outcome that should be considered for decision making in oil and gas projects.