Relation between the dielectric constant of rocks, sampled from the western exploring area of Daqing Oil Field, and water saturation, salinity is developed. Effect of shale and water distribution on the dielectric constant is discussed through experiments on artificial core samples and shale samples. The mixing law that describes the relation between dielectric constant and water saturation is revised and a new one characterized by the area is proposed.
Lithology in the western exploring area of Daqing Field is complex. Ambiguity of resistivity logs makes them difficult to interpret for the log analyst. Accordingly, it is necessary to study the dielectric constant for well logging interpretation application. 90 rock samples taken from 14 wells in the area have been tested, among which 12 samples have been damaged during testing due to high shaliness. The porosity of samples ranges from 5.07% to 25.43%, with most less than 20%; the shaliness ranges from 4.7% to 40%, mostly from 10% to 30%. The shale of most samples is characterized by recrystallization according to core analysis. The lithology is named for shaly fine sand or shaly siltstone. A few of the core samples contain calcium.
A parallel-plate capacitor, which is applicable to the frequency range of 20-250MHz, has been used to measure the dielectric constant of samples with diameter of 38mm and thickness of 6mm. Measurements at 60MHz and 25MHz are given.
Fig. 1 shows relation between the dielectric constant of rock samples and shaliness under different water content ratios (water content ratio= *Sw, porosity=, water saturation=Sw), in which the dielectric constant increases with shaliness because the double layer formed by absorption of cation in rock pore makes stronger polarization. Furthermore, we can conclude that the higher water saturation increases the sensitivity of dielectric constant to shaliness.
The shale mentioned is a filler with size less than 0.01mm in pore. Constituents of shale are mainly classified as clay minerals and small clay clasts. It is too difficult to study with more detailed classification.
The relation between the dielectric constant of rocks and the way in which the shale is distributed has been followed with interest. However, little relevant literature has been available. The difficulty may come from insufficient knowledge of shale distribution in rock. On the other hand, it is difficult to study one factor alone because of the interaction of many factors. Thus, we have conducted experiments on artificial core samples, and proposed some new notions. Artificial core is made by mixing ground grains of pure sand and pure shale in different ratios and different ways. Compacted in sample holder, the artificial core sample is ready to be tested with different amounts of water added. The artificial core samples we used are made of 30% shale grains and 70% sand grains which were mixed in four different ways as shown in Fig. 2. Homogeneous mixtures, in which sand and shale grains are well-distributed, represents dispersed shale; series mixtures, in which sand components and shale components serially connect in the direction of the electric field, represent structural shale; parallel mixtures, in which sand and shale components parallelly connect with 2 or 4 layers in the direction of electric field, represent laminar shale. All are shown in Fig. 2.
Dark grey shale sampled from the western exploring area in Daqing Oil Field consists of kaolinite, illite, montmorillonite and a small amount of chlorite. The dielectric constant and resistivity measurements of the shale at two frequencies are listed in Table 1.
The sample in Table 1 is saturated with NaHCO3 brine (salinity=6000ppm, resistivity=1.2 m at 25 C). The dielectric constant increases with water saturation. The frequency dependence of dielectric constant is stronger than that of resistivity.
Fig. 3 shows a dielectric constant-water content plot for four artificial core samples. The similarity of dielectric constant for the four samples without water indicates that the dielectric constant of dry sample is independent of shale distribution; In contrast, when the samples are saturated with brine, differences of dielectric constant measurements appear, and the differences become larger and larger with saturation less than approximately 70%; beyond this saturation threshold, the differences reduce because the effect of water overwhelms that of shale distribution.