HETEROGENEITY OF FRACTURED GRANITE: EFFECTS ON PETROPHYSICAL PROPERTIES AND WATER SATURATION OF PRESERVED CORES

摘要

Heterogeneity of oil-bearing fractured granite of the Bach Ho oil field has been studied through petrophysical measurement.Full-size and plug core data of permeability and porosity were compared to show the scale of macroscopic heterogeneity of the rocks.An attempt to evaluate the content of connate water was made on preserved cores(taken by using water-based mud).Excessive water(invaded into the macropores)was removed by capillary extraction using chalk powder.The regime of this process has been calibrated with porous plate technique.The percentage of invaded water was found close to the partition coefficient of rnacrofractures.Electrical properties have been studied on preserved,”preserved afier chalk”and cleaned cores.Scatter in resistivity index vs.water saturation cross-plots for these states was observed whereas a satisfactory relationship of formation factor vs.porosity was obtained.Rock matrix was found to obey Archie's law with certain cementation and saturation exponents.For samples with dual porosity(containing macrofractures)the slope”n”of RI-Sw curves changed from high a value of water saturation to low values.Electrical anisotropy was also investigated by measurements in perpendicular directions.The results show that electrical anisotropy exists due to the orientation of macrofractures while RI-Sw curves have the same characteristics.Explanations of these irregularities of electrical properties can be found by reconstructing the theoretical model of fractured media with dual porosity.Fair agreement between the characteristics of electrical parameters of actual cores and reconstructed model shows the essential role of macrofractures in the behavior of fractured granite.