Reserves Follow-up Using an Integrated Deterministic-Probabilistic Approach

Abstract

Abstract Currently, the deterministic methods for reserves estimation are the preferred method for both, companies and the U.S. Securities and Exchange Commission SEC. Last year, the SEC published the modernization rules for reporting oil and gas reserves where both, the deterministic and probabilistic methods, were accepted as valid methods for reserves estimation. In addition to the compulsory reporting of the oil and gas proved reserves, the probable and possible reserves were recognized to be disclosed in SEC filings for the first time, although it is optional for these two categories. In this paper the integrated deterministic-probabilistic approach is briefly reviewed, the cumulative probabilistic lognormal plots are used instead of a full Monte_Carlo simulation, and a procedure is proposed using these plots to follow up the reserves evolution in time. Plotting the deterministic reserves using probabilistic scale often reveals deviations with regards to the expected lognormal cumulative probability distribution behaviour, shown by real case examples. The corrections to obtain a better fit for the probabilistic distribution suggest to either changing the probable or the possible reserves or both. Furthermore, the slope of the probabilistic plots appears to be related to the uncertainty level of the probable and possible reserves. This property was used to follow up the reserves behaviour in time, finding consistency with the idea that as the fields are developed the level of uncertainty should be progressively reduced. Examples of the reserves follow up are also presented. The contribution of this paper is oriented to a practical use of an integrated deterministic-probabilistic approach as a complementary tool for reserves estimation for consistency with the probabilistic behaviour. The follow-up procedure of the reserves resulted to be quick and simple but powerful as indicator of the reserves evolution.