A Complex Core Study To Chinese Sandstone Formation With SIP Measurements And Other Petrophysical Experiments

A complex core study to a set of Chinese sandstone samples with IP measurements and other petrophysical experiments are carried out to fulfill our research.The general aim of the work here is to investigate the pore space properties and related question-to predict permeability, also to investigate the behavior of the IP signal under the condition of a high brine salinity.The determination of permeability is one of the most important and challenging subjects in the field of geophysics.Pore space properties are important for the description and characterization of fluid storage and transport in reservoir rocks.The internal structure of reservoirs may be heterogeneous, porous, or even empty.In 1980s, the famous French mathematician Mandelbrot established a new branch, the Fractal Geometry, which provides a simple and effective way to investigate the inhomogeneous and irregular structures in nature.The fractal structure commonly exists in the pore of porous rocks, which can be measured by various methods. Fractal structure provides a simple and flexible way to derive the permeability of rock reservoir from porosity, effective radius, or specific internal surface area. These parameters are always related to each other, but the values of different rock samples are not the same, for their fractal dimensions are diverse, which also means that their internal pore structures are different. Empirically, we can build a relatively suitable model for certain group of samples. But there are still many problems such as the accuracy needed to improve, and more information and techniques are still to be explored.The geometry of pore structure of reservoir rock is described by the shape, size, distribution and connection of pores and pore throats of rocks.We used the fractal concept to describe the geometrical structure of the pores of 24 samples from an Eocene sandstone formation in Zhongyuan oil field of east China. The fractal behavior of pore volume distribution was investigated by capillary pressure curves and nuclear magnetic resonance (NMR). Additionally, the fractal dimension of the pore surface was determined based on data of the specific surface area per unit pore volume (Spor). The comparison of fractal dimensions determined by three different methods indicates a clear differentiation into’surface dimension’and’volume dimension’. The fractal dimension resulting from longer transverse NMR relaxation times and lower capillary pressure reflects the volume dimension of larger pores.The fractal dimension derived from short NMR relaxation times is similar to the fractal dimension of the internal surface. The surface dimension increases with rising Spor. The average value of surface dimension was determined to be 2.30 for the set of Eocene sandstones.This value of fractal dimension was successfully applied in a model of permeability prediction that is based on formation factor and specific surface area (Spor).Complex resistivity instruments measure the resistivity magnitude and phase. When these measurements are made as a function of frequency, they are called spectral induced polarization (SIP) or electrical impedance spectroscopy (EIS) measurements.Discovered by Schlumberger in 100 years ago, this phenomenon still needs detailed explanation for the mechanism.An additional study should show whether complex conductivity spectra can be used to determine the fractal dimension of the pore space. Complex resistivity magnitude and phase measurements in a low frequency range were acquired on a wide variety of 24 sandstone samples. A fractal dimension can be determined on the basis of an IP relaxation time distribution. Further investigations will compare the fractal dimension derived from other methods.All working groups agree that frequency dependent electrical behavior is a complex function of pore solution chemistry, surface chemical properties, and sample micro-geometry.Numerous samples from multiple databases indicate a polarization maximum at higher fluid salinity. A further salinity increase results in a decrease in the imaginary part of electrical conductivity. This behavior is not in agreement with the previously proposed double layer polarization models.We examined the complex conductivity spectra over a wide range of pore fluid conductivity for four Eocene sandstone samples. All samples confirm a strong decrease of the imaginary conductivity for fluid conductivity exceeding 3 S/m (or fluid resistivity less than 0.33 Ohm-m).The Stern layer polarization model is extended by an exponential term describing the polarization decrease at high fluid salinity. The new mathematical model with four free parameters is also adapted to sandstone samples of other databases. The dependence of the four parameters on other physical quantities is presented. The parameter of the decaying term shows only a weak variation and seems not to be related to petrophysical quantities. The assumption is supported that the polarization decrease results from a decrease in the mobility of the counterions in the electrical double layer at high ionic concentrations.Innovations are made in respects of:1) We first proposed a calculating equation of fractal dimension from the data of specific internal surface;2) We first proposed a quantitative description for the IP behavior under the condition of high salinity of the saturating brine in the rock;3) We first established a relationship between fractal concept and the induced polarization.