Characterization of the Fluid Distribution in Petroleum Reservoirs

It has been observed many times that the distribution of hydrocarbons in petroleum reservoirs may be non-uniform. This variation may be especially high in tight reservoirs, where the equilibration between the different parts of a reservoir may be slow and difficult. The different parts of a reservoir may contain oil or gas of different compositions; the content of methane, ethane, intermediate and heavy hydrocarbons may vary from one point to another. Another complication comes from the inaccurate determination of the composition of the reservoir fluids caused by recombination problems or contamination of the fluid samples.

Valdemar field is an example of a petroleum reservoir over which the fluid composition varies, and there remains an uncertainty in the fluid properties. The observed lateral variation is puzzling. It could be formed by non-equilibrium effects, which contribution is unknown. A pressure-volume-temperature (PVT) model is required to capture the variations seen in the composition of reservoir fluid samples. The purpose of this project is to reduce uncertainties in the reservoir fluid characterization, reservoir connectivity, and the lateral and vertical compositional grading observed in Valdemar and other tight reservoirs of the Danish sector of the North Sea.

Gravity and temperature variances contribute to differences in petroleum composition.Thermodiffusion is the governing phenomenon determining the contribution of the geothermal gradient. Transport coefficients like diffusion and thermodiffusionare the key parameters in non-isothermal compositional grading. The present work proposes a unified thermodynamic theory for the diffusion and thermodiffusion coefficients.The model is developed and verified by comparison with the experimentaldiffusion and thermodiffusion data for some binary mixtures. Moreover, the effectof non-equilibrium thermodynamic models is applied to calculate the compositional gradients under the varying temperature. To determine the variations in pressureand composition with depth, we have developed a model based on the principlesof irreversible thermodynamics. The distribution of hydrocarbons in a petroleumreservoir is described based on the Onsager relationships where pressure, chemical potentials, and thermal gradient are linked. A computational algorithm accounting for the non-ideality of the mixture, characterization, and phase transitions has been developed. The model and the computational procedure have been validated by comparison with different case data. It has been shown that the model can predict the fluid distributions with depth with no or a minimum of adjustable parameters.

High quality and accurate PVT data reduce uncertainty in fluid modelling, fluid properties measurement, and construction of thermodynamic model for representative reservoir fluid. To find representative fluid samples and study depth gradient and lateral variations in the Lower Cretaceous reservoir, the consistency analysis of the reservoir fluid sample compositions is carried out by different methods. Based on the representative samples, it is analyzed whether observed compositional variations between samples can be explained by compositional gradients originating from gravitational and/or thermal gradient effects. This work illustrates how a depth gradient analysis can help in understanding fluid communication between the different points in the reservoir.