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Development and analysis of logging saturation interpretation models

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Abstract

In order to understand the research situation of saturation interpretation models, and to offer some basic information for the well logging reservoir evaluation, this paper summarizes saturation interpretation models frequently used throughout the world, discusses the characteristics and development tendency of the saturation interpretation model research, and offers some advice for the future research. Based on theoretical background, the saturation interpretation models are divided into four types: classical Archie formula, saturation interpretation models considering shale effects, saturation interpretation models considering the effects of conductive mineral and multi-porosity, general saturation interpretation models based on the network conduction theory in heterogeneous rock. Except for the classical Archie formula, there arc a variety of saturation models in the other types. The saturation interpretation models considering shale effects are subdivided into three kinds based on argillaceous equivalent volume, argillaceous cationic exchange capacity, and effective medium theory.

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... The Archie formula is the main method for calculating gas saturation systematically [10]. However, that equation has mostly been used for conventional reservoirs and has limitations in the interpretation of tight sandstone gas saturation [11][12][13]. The sealing coring technique is an accurate method to obtain S g , which provides data for the systematic study of tight sand gas-bearing properties. ...
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Gas saturation (Sg) is an important parameter for studying the gas-bearing properties of tight sandstone; however, there has been limited research on gas-bearing properties based on sealing coring. This study determines the controlling factors of the gas-bearing properties of tight sandstone in the Permian He8 Member of the Sulige Gas Field, Ordos Basin, Northern China, based on sealing coring, logging, drilling, gas testing, and laboratory analysis. The He8 Member Sg distribution is 17.9–63.8% (main range: 30–45%) and shows a downward trend from bottom to top. The Ro and hydrocarbon generation intensity of the source rock, reservoir porosity, and permeability tend to control the Sg in stages. When these parameters are less than 1.8%, 17 × 108 m3/km2, 10%, and 0.5 × 10–3 μm2, respectively, Sg increases significantly. When each parameter is greater than the aove boundary value, the change in Sg is not obvious. Four types of gas-bearing patterns of tight sandstone can be observed according to the distribution of reservoir and source rock conditions: “upper and lower constant,” “upper low–lower high,” “upper high–medium low–lower high,” and “upper high–lower low”; these patterns are mainly controlled by high maturity source rocks, reservoir physical properties, reservoir physical properties and structure, and structure, respectively. The corresponding gas test results reveal the existence of pure gas, pure gas and gas–water, gas–water, and gas-bearing water and pure water layers, respectively.
... Over the years, many methods [1][2][3][4][5] represented by Archie formula for calculating reservoir water saturation have been formed, and some models for evaluating reservoir resistivity have been formed, such as the three-water model [6,7]. However, the research on the conductive mechanism of sandstone resistivity and the derivation of the calculation formula are quite complicated, the theoretical derivation and proof logic are not rigorous, and the practical application is often inconsistent. ...
... Unlike cementation exponent, for which a substantial understanding has been obtained through various investigation methods, including theoretical and numerical modeling approaches as well as fractal theories (e.g., Sen et al. 1981;Glover 2009;Han et al. 2015;Tang et al. 2015;Wei et al. 2015;Cai et al. 2017;Yue 2019), there is still no well-accepted physical interpretation for the saturation exponent (Yue et al. 2004(Yue et al. , 2009Glover 2017). The saturation exponent is commonly interpreted qualitatively as some measure of the efficiency for the electrical flow to take place within the water occupying a partially saturated rock (e.g., Sun and Chu 1994;Sun 2008;Li et al. 2012Li et al. , 2013Chen et al. 2016). Recently, based on the generalized Archie's law, Glover (2017) provided a new theoretical interpretation for the saturation exponent in terms of the rate of change of the fractional connectedness with saturation and connectivity within the reference phase. ...
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Saturation exponent is an important parameter in Archie's equations; however, there has been no well-accepted physical interpretation for the saturation exponent. We have theoretically derived Archie's equations from the Maxwell-Wagner theory on the assumption of homogeneous fluid distribution in the pore space of clay-free porous rocks. Further theoretical derivations showed that the saturation exponent is in essence the cementation exponent for the water-air mixture and is quantitatively and explicitly related to the aspect ratio of the air bubbles in the pores. The results have provided a theoretical backup for the empirically obtained Archie's equations and have offered a more physical and quantitative understanding of the saturation exponent.
... Unlike cementation exponent, for which a substantial understanding has been obtained through various investigation methods, including theoretical and numerical modeling approaches as well as fractal theories (e.g., Sen et al. 1981;Glover 2009;Han et al. 2015;Tang et al. 2015;Wei et al. 2015;Cai et al. 2017;Yue 2019), there is still no well-accepted physical interpretation for the saturation exponent (Yue et al. 2004(Yue et al. , 2009Glover 2017). The saturation exponent is commonly interpreted qualitatively as some measure of the efficiency for the electrical flow to take place within the water occupying a partially saturated rock (e.g., Sun and Chu 1994;Sun 2008;Li et al. 2012Li et al. , 2013Chen et al. 2016). Recently, based on the generalized Archie's law, Glover (2017) provided a new theoretical interpretation for the saturation exponent in terms of the rate of change of the fractional connectedness with saturation and connectivity within the reference phase. ...
Article
Full-text available
Saturation exponent is an important parameter in Archie's equations; however, there has been no well-accepted physical interpretation for the saturation exponent. We have theoretically derived Archie's equations from the Maxwell-Wagner theory on the assumption of homogeneous fluid distribution in the pore space of clay-free porous rocks. Further theoretical derivations showed that the saturation exponent is in essence the cementation exponent for the water-air mixture and is quantitatively and explicitly related to the aspect ratio of the air bubbles in the pores. The results have provided a theoretical backup for the empirically obtained Archie's equations and have offered a more physical and quantitative understanding of the saturation exponent.
... Similar to the calculation of porosity distribution, for a given image frame, distribution of apparent formation water resistivity can be calculated. According to the method given below, in deep window for a given length, all pixels can be used to work out an apparent formation water resistivity value which can be counted by frequency histogram to generate frequency distribution curve [4,5]. According to their distribution, we can understand distribution of apparent formation water resistivity size in correspondence with the window. ...
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... The water saturation is an important index to evaluate the oil-bearing reservoir and one of the essential parameters for quantitative interpretation of well logging, it is also important to the evaluation of low resistivity reservoir [1,2]. Due to the diversity of complex sandstone, the pore structure and wettability of it are different from those of pure sandstone and its lithology, electrical property, oil-bearing property also shows 'atypical Archie' phenomenon [3,4]. ...
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According to core data, this paper studies variation of resistivity in different pore structures and wettability conditions. The results show that with the increase of pore structure index m, the resistivity will increase significantly when the saturation is constant. Similarly, with increasing saturation index n, the resistivity will also increase even with the same saturation. With fixed m and n, the calculated formation water saturation will be very high, resulting in hydrocarbon reservoir being ignored. This variation characteristic is significant for the identification of hidden reservoir with atypical Archie formula.
... In view of the characteristics of conglomerate reservoirs, i.e. complex lithology and severe heterogeneity, and based on detailed analysis of the meaning of the parameters and their determination in various models for calculation of water saturation (such as the Archie equation, revised Archie equation, Simandoux equation and multiple linear regression equation) (Sun et al, 2008;Shen et al, 2008), and using the coring data of sealed coring wells, the parameters value for model calculation of characteristic conglomerate reservoir are eventually determined. The accuracy of various models in calculation of the water saturation of a conglomerate reservoir was analyzed (see Fig. 3) and a multiple linear regression equation was selected as the model for calculation of water saturation of the Kexia Group conglomerate reservoir. ...
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Article
A laboratory study was made on the relation of wettability and wetting equilibrium to electrical resistivity, particularly under dynamic conditions. Teflon cores and synthetic fluids as well as reservoir cores and fluids wereused. Resistivities were measured by a four-electrode system. Under static conditions and with the wetting equilibrium prevailing, exponents as high as 9 were calculated from Archie's Equation when the interstitial conductive liquid was the non-wetting phase. When it was the wetting phase, saturation exponents ranged from 2 to 3 for the same core. This means that in laboratory measurements of Archie's saturation exponents on reservoir cores, the corewettability must represent the wettability of the reservoir. Otherwise, thecalculated connate water saturations will be in error. Under dynamic conditions, wetting equilibrium prevails only for very low displacement rates. At high flow rates the rate has an important bearing on the measured resistivity, indicating non-equilibrium fluid distribution in some experiments, the porous medium was saturated with the non-wetting liquid first. It was shown that, in these tests, the wetting equilibrium is either notobtained at all or it is obtained extremely slowly. These laboratory resultsare explained in terms of fluid distribution in porous media. A qualitative discussion is given on the implications of this work in electrical loginterpretation and the recovery of crude oil by chemical waterfloods. INTRODUCTION HYDROCARBON-BEARING FORMATIONS are generally identified through themeasurement and interpretation of their electrical properties by well logging. The empirical equation used in the estimation of the hydrocarbon saturations from resistivity data is: {See equation in full paper}, In this equation, S, is connate water saturation, R. is the resistivity ofthe 100 per cent water-saturated formation, Rt is the resistivity of the formation containing both water and hydrocarbon, and n is the saturation exponent. The true resistivity, R, of the un-invade zone and R. are obtained from the logs; n is found from laboratory measurements on reservoir cores. Implicit in the above approach are the assumptions that:the saturation-resistivity relationship is unique,laboratory-derived n valuesapply to the reservoirn is constant for a given porous medium andall of the water contributes to the flow of the electric current. Numerous studies on the electrical resistivities of cores partly saturated with sodium chloride solution have shown that the above assumptions are notnecessarily invalid. For the same saturations, the resistivity of cores hasbeen found to vary significantly with wetting conditions, clay content, textureand salinity. To find the effect of wettability on resistivity, some studieshave been performed on natural or synthetic cores in which the wettability waschanged through a chemical reaction, i.e. hot solvent extraction, oxidation or coating the surfaces with a chemical. There are some complicating factors insuch an approach; namely, the resulting core wettability is not uniform, and cannot be described or measured, and exchangeable ions from natural mineral scan change the resistivity of the electrolyte filling the pores. Also, the porosity of the core, particularly the effective porosity, can change.
Article
Simple qualitative methods are explained for identifying those shaly sands in a well that are most likely to contain oil. A need for more precise measurement of the variables that enter shaly sand analysis is indicated. Field examples are given to illustrate the methods. A theoretical discussion of the quantitative interpretation of shaly sands is given as a basis for discussion and as a guide for the future. While not generally capable of practical application at the present time because of the lack of sufficient accuracy of the electric log data, these methods may become more feasible in the future as the result of the improved logging methods now being introduced. Introduction Experience has shown that for porous formations containing only a negligible amount of clayey material, reliable information on the fluid saturation and porosity of the reservoir rocks can usually be derived from the electrical logs. The interpretation is based on empirical formulae relating the true resistivity of a porous formation to its lithologic character, to the resistivity of the interstitial water, and to the proportions of water and hydrocarbons in the pores. If the resistivity of the interstitial water is not known, its approximate value can be derived from the SP curve. The porosity can usually be determined to a good approximation from a MicroLog, or a MicroLaterolog. When the reservoir rocks contain an appreciable percentage of clayey material, an additional factor is introduced into the analysis. In a clean formation, the matrix is an electrical insulator, so that the ability of the formation to conduct current is due only to the conductivity of the electrolytes in the pores; in a shaly formation, the shale constitutes a part of the rock matrix able to conduct current, and influences the resistivity of the formation.