pay formation, pressure build-up curve, simulation model, statistical method, the Bourdet derivative method. It provides examples of the well test data processing for pay formations from some oil, gas and condensate fields in the southeast of the Dnieper Donets Depression. It shows that usage of the optimality criteria on the basis of properties of the covariance matrix of formation evaluation improves their accuracy quite significantly. It offers a review of the comparative results of pressure buildup curve processing with the proposed techniques and Method and PT4.0 software packages and specifies errors in evaluation of pay formation properties. The evaluations of statistical characteristics of pay formation filtration properties (hydroconductivity, permeability and skin) have been modeled for the likelihood function criteria and properties of the covariance matrix. The study shows that use of optimality criteria on the basis of properties of the covariance matrix improves the accuracy of formation evaluation essentially.
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