Oil And Gas Reserve Classification And Estimation On The Basis Of Uncertainty Analysis

Reserves should be discovered, recoverable, economic and remaining. The classification and definition of oil and gas resources and reserves recommended by the SPE/WPC/AAPG/SPEE is the most influential standard in the world amongst others. Based on comparison and analysis, it is recogonized that the Proved Developed Remaining Economic Reserves in Chinese system is equivalent to the Proved Reserves of SPE. Howerver, there are distinct dissimilarities between the two system such as the understanding of reserves, standpoint of definitions, phases to the exploration, parallelism between in-place volume and recoverable, dependence of different categories, economic connotation and qualification to the reserves, etc.The primary reason for taking diverse way to classify reserves is because of different understanding on inherent uncertainties implied in oil and gas reserves. From the point of view of errors, there are four error sources of reserves: acquisition error, process error, conversion error and personal error. And there are four kinds of uncertainties as well: randomness, fuzziness, greyness and unacertainty. At least in China, there have relevant applied mathematics to handle the four uncertainties: probability theory, fuzzy mathematics, grey system theory, unascertained mathematics.Theoretically, the errors of in-place volume derived from parameter errors can be deduced on the basis of volumetric equation, and the range of result with error can be determined by inputting a set of parameters which imply errors.Probability approach is an effective way to handle parameter's randomicity and get a range of values. The classical probability approach requires to establish a statistic distribution or preset a probability model in advance. So the application of classical probability mathematics is limited in the reserve estimation. On the contract, the Warren method, which improved the probability theory, shows the lognormal distribution characterizes generally the distribution of multi-parameter product. And the probability approach can be simplified by resolving a set of equation. The use of Warren method then promotes and extends the application of probability theory.Fuzzy synthetical judgment is a way to evaluate the reliability of parameters and results, by subclassifing all the possible fuzzy factors into different levels, and then synthetizing a final judgment vector with fuzzy mathematics. It is very suitable to evaluate the reasonability of geo-parameters.In the practice, knowledges, experiences, rules and attitudes are the four trumps to reduce risks and uncertainties in the reserve estimation. To handle reserve uncertainties, there are six waysto follow with: a) observe regulations in acquiring data, b) prove understanding by data, c) conclude reasonably on experience, d) choose volumetric models with reason, e) avoid confusion of three kinds of limits, and f) weaken risks relying on rules. Here, the volumetric models are volumetric analysis models based on isopaches, including cone volume model, ladder volume model, annulus volume model, and for stratum reservoir, Simpson numerical approach model. And the three limits are limits of traps, reservoirs, and reserves. In the practices, limits of structural traps and fluid contacts are the maximum limits of 3P reserves. The lowest known hydrocarbon would be the lowest limit of proved reserves. And in fact, within the maximum limits on lateral and on vertical, the well drainage controls the proved limits.By application of probability approach and study on Guang'an gas field, it is recognized that the probability limits 90% for proved reserves is too conservative, that means it's too conservative to require there should be at least a 90% probability that the quantities actually recovered will equal or exceed the estimate, If probabilistic methods are used. On the contrast, an 80% probability is used in the Chinese new reserve regulation which is relatively reasonable.