Study of the properties of nongas dielectric capa

Authors: HongQi Liu Yan Jun YouMing Deng

Publish Date: 2015/02/01

Volume: 12, Issue: 1, Pages: 104-113

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Abstract

The size of pores and throats is at the nanometer scale in tight oil and shale gas zones and the resistivity of these reservoirs is very high so the reservoirs show more dielectric properties than conductivity properties The conductive and dielectric characteristics of a parallel plate capacitor full of fresh water NaCl solutions and solid dielectrics for example sands are investigated in this paper and the capacitance data of the nongas capacitor are measured at different salinities and frequencies by a spectrum analyzer The experimental results illustrate that the capacitance of this kind of capacitor is directly proportional to the salinity of the solutions and inversely proportional to the measuring frequency the same as a vacuum parallel plate capacitor The remarkable phenomenon however is that the capacitance is inversely proportional to the square of the distance between two plates The specific characteristic of this capacitor is different from the conventional parallel plate capacitor In order to explain this phenomenon the paper proposed a new concept named “single micro ion capacitor” and established a novel model to describe the characteristics of this particular capacitor Based on this new model the theoretical capacitance value of the single micro ion capacitor is calculated and its polarization and relaxation mechanisms are analyzedAs we know rocks have both conductive and dielectric properties The conductivity is completely determined by various anions and cations such as Na+ Mg2+ Ca2+ K+ Cl− OH− HCO3 − SO4 2− and CO3 2− in water solution in the intergranular pores of rocks Which property of the rocks will play a predominant role depends on the salinity porosity permeability and the geometric structure of the pores In fact almost all substances in nature are dielectrics and only a few have conductivity σ Havriliak and Negami 1966 The conductive paths of these charges are usually continuous in relatively high porosity and high permeability reservoirs but in most occasions they are discontinuous in tight oil and shale gas zones or in the low porosity and low permeability reservoirs Gasparrini et al 2014 Ghanizadeh et al 2014 Therefore the electric properties of rocks include two major aspects one is the conductive capability of free positive and negative charges in the water solution through the paths formed by pores and throats which connected with each other the other is the dielectric capability of the bound charges of the nonconductive substances and particles including rock matrix particles oil or gas molecules and pure water molecules Freedman and Vogiatzis 1979 Jonscher 1983 Endres and Bertrand 2006 Nevertheless whether conductive or dielectric the current paths in the formation are influenced by the geometric pore structure of rocks The relationship between the conductive and dielectric properties of rocks and the pore structure has been widely discussed in detail in the literature Toumelin and TorresVerdin 2009 so it is not described here Prior to 1940 studies on the electrical properties of rocks mainly focused on conductivity In 1941 K S Cole and R H Cole established the Cole–Cole model for dielectric constants Cole and Cole 1941 later many scholars studied ionic conduction and rock polarization processes analyzing the dielectric constant property of nonhomogeneous multipore materials Hilfer et al 1995 1999 Nover et al 2000 Ruffett et al 1991 Davidson and Cole 1950 1951With regard to the electrical logging technology in the petroleum industry almost all research has focused on the detection and interpretation of formation resistivity However more and more complex and unconventional reservoirs have been discovered and the traditional simple geophysical conductive model cannot solve the problems of strongly heterogeneous reservoirs such as low porosity and low permeability oil reservoirs tight oil zones and shale gas zones Although scholars have proposed various solutions and models for some important questions to describe the electrical conductivity mechanism in rocks Chelidze and Gueguen 1999 Chelidze et al 1999 Asami 2002 it is increasingly difficult to accurately determine fluid types and calculate the saturation of hydrocarbon In 1988 Clark et al proposed an electromagnetic propagation tool EPT to measure the dielectric constant of the formation at frequencies of 2 MHz–11 GHz Glover et al 1994a b 1996 Clark et al 1990 This method can identify fluid types and calculate the fluid saturation in high porosity and high permeability reservoirs Dong and Wang 2009 studied the dielectric constant of several common minerals including quartz calcite and dolomite within a frequency range from Hz to GHz level and they identified pore structures and the distribution of formation water using the dielectric constant In 1985 Lockner and Byerlee 1985 studied the complex resistivity of rocks later other scholars also studied the complex resistivity of rocks However due to the extraordinarily complex heterogeneity of porous rocks there are still many disputes over the conductive and dielectric mechanisms in rocks Clavier et al 1984 Zemanek 1989 Hamada and AlAwad 1998 Scholars have proposed many conductive models for rocks of which the most representative is the dualwater argillaceous sandstone conductive model proposed by Waxman and Smits in 1968 Waxman and Smits 1968 Knight and Nur 1987 Hassoun et al 1997

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