Analysis of Black Oil Correlations for PVT Properties Estimation

Abstract

Over the last 30 years, considerable research into the assessment of PVT-properties of hydrocarbon formation has been carried out. Based on laboratorial and field research of oil and gas samples, two basic approaches of PVT-properties assessment were developed. They are correlation approaches, based on statistical analysis of the result of research and compositional simulation approach based on the equation of state (EOS) solution. These approaches have both advantages and disadvantages. It is clear that both of them are essentially dependent on the quality of the initial data obtained from the analysis of reservoir and surface samples of hydrocarbons. Therefore, it would be wrong to contrast two approaches in terms of priority. Compositional simulation is indispensable for the assessing of reservoir system behavior near the critical point for volatile oil and gas condensate systems, which have no correlation models, or give results with poor accuracy. The basis of compositional simulation is the composition of the formation system, which consists of individual components to C6 inclusively, and fractions of C7+ pseudocomponents. Correlation models are successfully used in assessing of the behavior of black oil. Basic parameters of PVT model correlation are Rsb - gas-oil ratio at bubble point, γo - oil specific density, γg - gas specific density, Pb - bubble point pressure, Tres - reservoir temperature. It should be noted these parameters can be obtained in different experiments of degassing, namely, single or differential, as well as within the framework of standard or commercial separation. Moreover, each PVT correlation usually tuned to its own regional data bank. As a result, different correlations may use different statistics on overlapping intervals. Recently, several articles have been published, which analyzed the correlation approaches of PVT properties assessment [1-3]. However, the study provides an analysis of correlations without recommendations for their use. The main conclusion of these studies is a statement of fact that the choice of correlations should be conducted with respect to basic oil properties. Usually, it recommends to use new coefficients for the standard correlation model.