عنوان مقاله [English]
Determining the petrophysical properties of a reservoir is of great importance. The most important of these properties are the porosity and permeability of the reservoir sections. The petrophysical properties of a reservoir are usually obtained by methods including core analysis and well testing techniques which require spending a very high time and cost. Furthermore, the results of determining these parameters by the mentioned methods are not of good quality due to the lack of sufficient cores, lithological changes and heterogeneity of reservoir rock. Therefore, the optimal method is to use advanced evaluations of petrophysical diagrams in advanced software environments such as Geolog. The results of petrophysical assessments can be compared and calibrated with the results obtained from the core.
Usually, porosity is estimated based on the one of the neutron logs, density logs, sound logs or combination of two logs in-site. Moreover, it can be calculated by laboratory methods which give different evaluations of porosity, of course. Experience has also shown that estimation of porosity using one type of log is not accurate due to the response of each log to certain parameters of a formation.
Another method to estimate the porosity is to use longitudinal and transverse wave velocities and the ratio of these velocities. This method uses surface wave data and Vertical Seismic Profiling (VSP) surveys. Using the empirical relations between the porosity and seismic velocities, a proper estimation of porosity could be provided. In this research, porosity has been estimated using the VSP data and also some other petrophysical logs in a well in the Persian Gulf and the results have been compared with other methods.
The results show that the porosity values obtained from VSP data are closer to the core analysis than acoustic log results. One of the advantages of porosity estimation using VSP data is the insignificant effectiveness of shale contents on this method.
The well wall caving, especially at the bottom of the well, causes error in the reading of some logs and affects the amount of porosity calculated by petrophysical logs. However, well caving has little effect on VSP data readings and results in the estimated porosity values to be more accurate than the Vp/Vs velocity ratios in these areas.
The porosity obtained from the dynamic method shows only the initial porosity of the rock and is not able to calculate the total porosity. Furthermore, the studies conducted in this research showed that the porosity achieved by acoustic method does not conform well to the helium gas in neutron log and density log methods (as a reliable method), and also does not have the necessary validity. For these reasons, in this study, a method was proposed to determine the porosity by combining the ratio of the longitudinal wave velocity to shear wave velocity (Vp/Vs) and rock density. Comparison of the results presented in this study with other available methods showed a good consistency and suggests that the proposed empirical equation provides a more realistic calculation for rock porosity.
As a recommendation, we suggest that VSP data be acquired for the whole well column because extracting the compressional and shear wave velocities in all surveyed depths and calculating the ratio of them, will make it possible to estimate the amount of different porosities in all existing formations.