Nanopores of Tight Sand

Mar 26, 2018 - ... of environmental scanning electron microscopy (ESEM) and energy-dispersive spectrometry (EDS) is proposed in this paper on the basi...
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Determining the Occurrence of Oil in Micro-nano Pores of Tight Sand: A New Approach Using Environmental Scanning Electron Microscopy Combined with Energy Dispersive Spectrometry Yanjie Gong, Keyu Liu, and Shaobo Liu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b00174 • Publication Date (Web): 26 Mar 2018 Downloaded from http://pubs.acs.org on March 27, 2018

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Determining the Occurrence of Oil in Micro-nano Pores of Tight Sand: A

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New Approach Using Environmental Scanning Electron Microscopy

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Combined with Energy Dispersive Spectrometry

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Yanjie Gonga,b,*; Keyu Liuc; Shaobo Liua,b

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a

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Xueyuan Road, Haidian District, Beijing 100083, China

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b

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District, Beijing 100083, China

Research Institute of Petroleum Exploration & Development, PetroChina (RIPED), No. 20

Key Laboratory of Petroleum Accumulation of CNPC, No. 20 Xueyuan Road, Haidian

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c

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Australia

Department of Applied Geology, Curtin University, GPO Box U1987, Perth, WA 6845,

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*Corresponding author. E-mail address: [email protected], telephone

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number: 8601083597514, and fax number: 8601083597480

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ABSTRACT

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The use of fluorescence slices and other regular methods tend to be limited in resolution

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because of optical microscopy issues when they are applied to characterize oil occurrences in

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tight sand micro-nano pores. To address this, an experimental method that combines the use of

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environmental scanning electron microscopy (ESEM) and energy dispersive spectrometry

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(EDS) is proposed in this paper on the basis of a large number of experiments; oil was

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observed during these tests using ESEM and the carbon (C) content was qualitatively

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evaluated using EDS. Data show that the most appropriate experimental parameters were a

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sample room pressure of 10 Pa, a working distance of 5 mm, a working voltage of 15 kV, and

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an electronic beam spot size of 4.5 nm. The experimental analysis of six samples from five

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wells within the Songliao and Sichuan basins, China, reveals that oil mainly occurs in the form

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of oil films and oil droplets within micro-intergranular seams, micro-nano intergranular pores,

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and nano-intragranular pores. Observations aslo show that oil films are found mainly within

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micro-intergranular seams and micro-intergranular pores, while oil droplets, in contrast, are

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mainly found within nano-intragranular pores. Data show that the occurrence space occupied

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by oil films is relatively larger, having planar dimensions between 200 nm and 10µm by

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between 1µm and 10µm. The oil films adhere to the pores just like bonding with the pores.

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And the content of the C element in the oil films is 50%-90%. In contrast, the occurrence

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space occupied by oil droplets is relatively smaller, with intragranular pore plane dimensions

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predominantly between 200 nm and 1,000 nm by between 200 nm and 1,000 nm. Data also

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show that oil droplets are controlled by the shape of intragranular pores, and occur at scales

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between 200 nm and 1,000nm by 200 nm and 1, 000 nm. Thus, although the occurrence space

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occupied by oil droplets is smaller than that of oil films, the development of numerous

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intragranular dissolved pores provides the necessary room for oil droplets to occur. The EDS

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data of these droplets show that the content of the C element of the oil droplets in intragranular

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pores are relatively small, mainly concentrated between 15% and 30%.

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KEYWORDS: tight oil; micro-nano pores; environmental scanning electron microscopy;

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energy dispersive spectrometry

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1. INTRODUCTION

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The exploration of unconventional oil and gas has recently developed into a key research topic ACS Paragon Plus Environment

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in unconventional oil and gas studies, 1-4 especially as these resources have become ever more

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important reservoirs for production. Reservoir rocks of unconventional oil and gas are

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characterized by low-porosity and low-air permeability5-7, and by the fact that oil and gas

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mainly occurs in micro-nano pores and throats. 6-9 Because the pore space occupied by crude

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oil in tight sands is very small, mainly micro-nano scale, the effective characterization of

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occurrence of crude oil in tight sands tends to be complicated. But this work is very important

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to answer the following questions: What types of pores are crude oil stored in; Is it possible

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that crude oil can be stored in all the micro-nano pores, if not, what is the pore size range?

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What is the form of the occurrence of crude oil in the pores? The solution of the above

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questions is of great significance to the tight oil resources estimation. The experimental

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methods that are commonly applied to characterize conventional oil sample include the

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fluorescence observation of thin sections with optical microscopy,10 and the use of confocal

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laser scanning microscope (CLSM).11,12 These approaches are restricted, however, to just

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dozens of microns by the maximum resolution that can be attained using optical

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microscopes;12 although CLSM resolution is higher than fluorescence, pores less than 1 µm in

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size cannot readily be observed.11,12 The available resolution of these approaches is not enough

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to be used to characterize the occurrence of tight oils in nano-scale to micro-scale pores and

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throats. Although the macroscopic distribution of the remaining oil has been studied in the

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context of hydrocarbon development using electromagnets,13 geophysical approaches,14

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logging,15 and numerical simulations,16 the microscopic occurrence of crude oil in pores has

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rarely been addressed. This is in part because oil-bearing cores tend to cause serious crude oil

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volatilization because of their long storage times; this means that microscopic occurrences of

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oil are often destroyed, and so are difficult to study. As a result, the use of systematic methods

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to characterize the occurrence of tight oils in micro-nano pores and throats have rarely been

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discussed.

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Although the SEM has a high resolution of several nm, it can't be used to observe the fluid.

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Compared to SEM, the use of ESEM utilizes a multiple pressure-limiting diaphragm to form a

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multi-stage vacuum system, and high degree of vacuum is not required 17-22. The degree of

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vacuum within the sample room, its optical path, and that of the electronic gun room differs

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between several orders of magnitude, and was 10, 10-2~10-5, 10-7 Pa respectively; the

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electronic gun and lens cone maintain a high vacuum state to ensure the normal working state

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of this system (Figure 1). When the sample room is in a low-vacuum state at pressures

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between 500 and 3,000 Pa (the ESEM model), fluid-bearing samples can be observed directly.

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The scanning imaging principle of ESEM is that positive voltage is applied to an electronic

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panel above the sample, forming an electric field between its surface and the polar plate; this

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means that a new electron and a positive ion are generated by the secondary electron

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motivated by the incident beam. 19-24 This electron is accelerated to sufficient energy by the

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electric field and so additional gas molecules are ionized to generate more electrons. These are

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then received by the polar plate, amplified by a video, and subsequently treated in several

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ways to form a secondary electronic image.

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From the mid-1960s onwards, the use of ESEM has been employed to application

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technologies related to reservoir rocks and fluids, such as reservoir sensitivity, 25 material

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analysis26-28, and the pore size and structure. 29 The greatest advantage of ESEM is to be able

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to observe the samples with fluid. Scientists have used ESEM to observe the samples with

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water or other fluids, such as radiation damage of water, 30 hygroscopic behavior of individual

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aerosol particles, 31imaging wet and insulating materials, 32 water in carbon Nano pipes, 33ice

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nucleation capability of individual atmospheric aerosol particles.34

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This method has not, however, been applied to address the occurrence characteristics of oil in

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micro-nano pores. So the ESEM was used to observe the occurrence of oil in micro-nano

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pores of tight sand in this paper.

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2. METHODS and MATERIALS

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2.1 Methodogy. A method that combines both ESEM and EDS is proposed in this study on

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the basis of a large number of experimental attempts and analyses. The use of a low-vacuum

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ESEM model enabled the shape of micro-nano pores and oil occurrences to be evaluated

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within these pores; 17, 29 the acquisition of data is straightforward and reliable. 18 The first of

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these methods enables secondary electron imaging of hydrated, wet samples, via

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non-conductive, uncoated materials that have a maximum resolution of 3.5 nm at 30 kV and a

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water vapor pressure of 1 kpa.19-22 In addition, no special sample preparation is needed for the

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application of this approach, while the use of EDS enables the effective measurement of

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elemental compositions,26,35,36 shows that the content of C element is much higher than those

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of the corresponding calcium (Ca) as well as other elements and demonstrates that the

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observed oil film comprises crude oil rather than carbonate minerals. Thus, in general, when

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excitation voltage was 10 kV, the probe was able to detect the content of C element within

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pores of 1 µm and 2 µm.19-22 The measurement apparatus used in this study comprised a

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Quanta 400 field emission ESEM equipped with EDS; all experiments were conducted in an

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environmental low-vacuum model at room temperature, while the moving range of the sample

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desk was 100 mm.

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Samples with a higher oil-bearing grade such as oil immersion were used in experiments to

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observe whether, or not, oil is present inside, or on the surface of, micro-nano pores before the

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occurrence space and form of crude oil was judged via the real-time detection of hydrocarbon

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content using EDS. The specific procedures used in this study included breaking a fresh rock

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specimen from the core that is 0.5 cm in diameter and immediately placing it within the ESEM

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observation chamber to prevent hydrocarbon loss. Magnification times were then gradually

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increased to observe micro-nano pores in samples, while the existence of oil was confirmed

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either on the basis of its occurrence in intergranular pores or micro-seams via impregnation

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and adhesion. At the same time, oil films within the observation area can be deformed, moved,

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shrunken, or even disappeared as observation time passes as it tends to volatilize. The element

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content within observed oil films were then also measured using EDS as these had higher

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contents of C between 50% and 90%. The contents of C were much higher than those of other

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elements; data confirm that observed oil films comprised crude oil rather than carbonate

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minerals.

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2.2. Method advantages. Experimental results show that the use of a scanning electron

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microscope (SEM) has limited application compared to ESEM because of the vacuum

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requirements of the former; at the same time, because core samples taken from oil fields need

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to be washed, the oil and water distribution in the rock cannot be observed using ESM.

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However, the environmental chamber of the SEM means that experimental conditions can be

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adjusted according to the characteristics of the actual sample to meet observation demand.20,21

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The combined use of ESEM and EDS therefore meets the unique requirement of being able to

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observe micro-nano pores and fluids at the same time to reveal and classify the microscopic

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state of occurrence of the oil that remains. This approach to the classification and objective

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evaluation of both oil microscopic state and space of occurrence can be used to guide

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exploration and production.

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2.3 Experimental parameters. The most important parameters related to secondary ESEM

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electronic imaging include working distance, sample room pressure, working voltage (i.e.

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electron beam acceleration voltage), and the nature of the electronic beam spot. Thus, the

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longer the working distance, the larger the space between the secondary electron and the probe,

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and so there will also be a concomitant loss of sample information. In contrast, the higher the

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sample room pressure, the higher the air molecular concentration in the sample room will be

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as well as the interaction between these molecules and the electron beam. In particular, this

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will also influence the energy or movement track of secondary electrons motivated from the

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sample surface, and therefore imaging quality. The higher the acceleration voltage of the

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electronic beam, the stronger the energy of the electronic beam, the better the imaging quality,

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but the more severe the irradiation damage of the organic sample. Finally, a low sample room

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pressure, a small working distance, a high working voltage and a high electronic beam spot

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size will lead to a higher resolution.

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Emphasizing the four key experimental parameters discussed above, a series of

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comparative analyses were performed in this study by varying one parameter and keeping the

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others constant. Thus, imaging quality at sample room pressures of 10, 20, 50, and 100 Pa

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were compared (Figure 2); image comparisons show that the best sample room pressure is 10

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Pa. Indeed, when other experimental parameters were held the same, the working distance was

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varied between 5, to 10, 15, and 20 mm (Figure 3); Comparisons show that there is a positive

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correlation between working distance and image quality. The smaller the distance, the higher

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the image quality. And the best working distance was 5 mm. At the same time, when all other

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experimental parameters were held the same, the working voltage changed from 5 to 10 kV,

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and then 15 to 20 kV (Figure 4). Image comparison results show that the image qualities under

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working voltage of 5 kV, 10 kV, are worse than that under working voltage of 15 kV and 20

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kV, so a higher but not the highest working voltage (15 kV) was chosen. The experiments at

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different electron beam spots (1.5 nm, 3.5 nm, 4.5 nm, and 5.5 nm) show that when all other

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experimental parameters were held the same (Figure 5), the best spot size was 4.5 nm. The

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image at electron beam spots of 1.5 nm is the worst. However, the images at electron beam

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spots of 3.5 nm and 5.5 nm are better and that at electron beam spots of 5.5 nm is the best.

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2.4. Experimental samples. A series of Jurassic system oil samples were selected from two

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wells (G104 and G30) within the Sichuan Basin, alongside samples from the Cretaceous

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system that derive from three wells (Q238, Z59, and C45) within the South Songliao Basin,

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China. All samples were then subject to tight oil characterization experiments (Figure 6).

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Observation show that at depths of around 2,500 m depth, samples from the Jurassic system

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within the Sichuan Basin were off-white fine sandstones with porosity less than 6% and

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permeability less than 0.1 × 10–3 µm2. Data also show that samples of Cretaceous age from the

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South Songliao Basin that were collected from burial depths between 1,600 m and 2,200 m

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were grayish brown fine sandstones with porosity less than 10% and permeability less than 1 ×

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10–3 µm2. Core samples were classified into two oil-bearing grades, oil immersion and oil trace;

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three samples were of the better oil immersion grade while two samples were of the poorer oil

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trace grade (Table 1).

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3. RESULTS AND DISCUSSION

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Experiments show that oil mostly occurs within micro-scale intergranular seams, micro-nano

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scale intergranular micropores, and nano-scale intragranular micropores in the form of oil

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films and droplets. Data also show that oil film and droplet occurrence spaces differs: on the

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one hand, oil films mainly occur in micro-scale intergranular pores and micro intergranular

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seams, while droplets mainly occur in intragranular micropores.

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ESEM pictures (Figure 7A-D) show that a sample taken from a depth of 1605.2 m within

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Well Q238 contained abundant intergranular micropores; in this case, the planer size of

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intergranular micropores mainly ranges between 1 and 10 µm by 1 and 10 µm. The pictures

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are profiles of the intergranular pores, and the oil films are mainly attached to the intergranular

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pores. During the observation process, the oil films gradually deformed due to the change of

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pressure. At the same time, the planer size of intergranular micro seams was 0.2 µm by 1 and 2

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µm (Figure 8A-D). These oil films in Figure7 and Figure8 are irregular in shape, while their

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size, controlled by intergranular micropore and intergranular micro-seam shapes, was mainly

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within a range between 0.2 µm and 10 µm by 1 µm and 10 µm. Data show that oil films occur

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in intergranular micropores or intergranular micro-seams largely in impregnation and adhesion

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states, while oil is apt to volatilize under low vacuum state; this means that the oil film within

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a vision field would gradually deform, move, shrink and even disappear as observations

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continue. For example, the areas of the oil films in the Figure 7D and Figure 8A are smaller

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than those in the figures. Data show that the content of C element of most oil films was also

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very high; the EDS at the oil film position (Figure 8A, Figure 8D) show that the content of C

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element in these cases, ranging between 55% and 90%, is much higher than that of other

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elements (Table 2).

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The data presented in this paper show that the occurrence space of oil droplets falls within

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samples; ESEM pictures (Figure 9 A-D) reveal that oil droplets mainly occur in intragranular

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nano-scale pores sized between 200 nm and 1,000 nm by 200 nm and 1,000 nm, which means

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that the available space for oil droplet’s occurrence is also larger. These pores are mostly

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dissolved in quartz and feldspar grains, spheroid, flat, or wedge-shaped, and distributed in

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linear, banded, or irregular forms. The development of oil droplets is also controlled by

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intragranular pore shapes; so the planer size of oil droplets mainly falls within the range

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between 200 nm and 1,000 nm and 200 nm and 1,000 nm. Although the occurrence space of

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oil droplets was smaller than that of oil films, the intragranular pores were denser (Figure 9C);

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in particularly, the massive growth of quartz and feldspar dissolution pores (Figure 9C) can

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provide more space for the occurrence of oil droplets. As they occur in smaller intragranular

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pores, oil droplets were smaller in volume, and more apt to volatilize than oil films; in a

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general state, data also show that it is very hard to see a complete form of oil droplets, and that

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these are judged mainly by data across the EDS. EDS data (Table 2) reveal that the contents of

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C element of oil droplets within intragranular micropores are relatively small, mainly

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concentrated between 20% and 50%.

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It should be pointed out that the method proposed in this paper also has a number of

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limitations including that oil can volatilize which changes it patterns of occurrence. This

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means that optimal parameters must be adjusted over the course of the experiment when the

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occurrence of oil in micro- to nano-scale pores. The next stage of this research will be to

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evaluate this method using tight oil samples taken from the Williston, Ordos, Junggar, and

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other basins. This approach will enable us to identify differences in tight oil occurrences

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between different basins.

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4. CONCLUSIONS (1)An experimental method combining ESEM and EDS is presented in this study to enable

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the observation of oil in micro-pores and nano-pores in tight oil reservoirs.

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(2)This study shows that a low-vacuum ESEM model can be used to determine the shape of

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micro- and nano-pores as well as oil occurrences within them, and if the content of C element

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are much higher than those of other elements like Ca, then the observed oil film can be

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confirmed as crude oil rather than carbonate.

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(3)This method is practical and adaptable across a wide range of conditions. These new

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experimental data can be used to reveal oil occurrence form and space in tight reservoirs, a

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direct basis for the evaluation of these reservoirs.

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AUTHOR INFORMATION

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Corresponding Author

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*Tel.: +86 10 83597514. Fax: +86 10 83597480. E-mail: [email protected].

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ORCID

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Yanjie Gong: 0000-0003-0448-7912

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Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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We thank Caineng Zou from the Research Institute of Petroleum Exploration & Development,

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PetroChina, for data and support. This work has been financially supported by National Key

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Projects of China (No:2016ZX05003-002), the 13th Five-year Program of Petrochina

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(No:2016B-0502).

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Nano Letters.2004. 4(5) , 989–993.

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Environment 2007. 41(37), 8219–8227. (35)

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183–191. (36)

Akkaş, E.; Akin, L.; Çubukçu, H. E.; Artuner, H. Comput. Geosci. 2015, 80, 38–48

305 306

Figure 1. The ESEM working principle.12

307 A

B

308

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C

D

309 310

Figure 2. Comparison of SEM images at different sample room pressure intensities (A: 10 Pa,

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B: 20 Pa, C: 50 Pa, D: 100 Pa). A

B

C

D

312

313 314

Figure 3. Comparison of SEM images at different working distances (A: 5 mm, B: 10 mm, C:

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15 mm, D: 20 mm).

316 A

B

C

D

317

318 319

Figure 4. Comparison of SEM images at different working voltages (A: 5 kV, B: 10 kV, C: 15

320

kV, D: 20 kV). A

B

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C

D

322 323

Figure 5. Comparison SEM images at different electron beam spots (A: 1.5 nm, B: 3.5 nm, C:

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4.5 nm, D: 5.5 nm).

325 326

Figure 6. Map to show the distribution of study wells within the Sichuan and Songliao basins,

327

China

328

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A

Page 18 of 22

B Oil film Intergr anular microp ore Intergr anular microp ore

Oil film

329 C

D

Oil film

Oil film

Intergranular micropore

Intergranular micropore

330 331

Figure 7. Well Q238 at 1,605.2 m within the Songliao Basin. This figure shows the

332

occurrence spaces of oil films include intergranular micropores between particles in these

333

samples.

334

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A

B

335 D

C

336 337

Figure 8. A and B, The occurrence of oil films in intergranular micro-seams and EDS data

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from Well Q238; C and D, the occurrence of sample oil films in intergranular micro-seams

339

and EDS data from Well Q238.

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A

B

C

D

341

342 343

Figure 9. A and B, The occurrence of sample oil droplets in intragranular pores and energy

344

spectrum data from Well G104; C and D, the occurrence of sample oil droplets intragranular

345

pores and energy spectrum data from Well G30.

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Table 1. Sample statistics from the tight oil occurrence characterization experiment. Strata No.

Porosity

Basin

Well

Depth/m

series

Comprehensive R50/nm

3

/%

1

Permeability/10– Lithology

µm2

Core oil-bearing grade explanation of logging

G104

2,529.1

5.9

0.08

310

G30

2,547.9

4.8

0.01

370

Off-white fine sandstone

Oil layer

Oil immersion

Oil layer

Oil trace

Oil-water layer

Oil immersion

Jurassic Sichuan

Grayish brown fine system

2

sandstone Grayish brown powder Q238

3

1,605.2

10.0

0.70

338 sandstone Grayish brown fine

Z59

4

2,121.0

8.5

0.20

Sealed coring sample, Oil-water layer

sandstone

Cetaceous

oil immersion

Songliao system

Grayish brown fine Z59

5

2,121.5

6.6

0.11

Sealed coring sample, Oil-water layer

sandstone

oil immersion

Grayish brown fine C45

6

2,105.4

7.0

0.12

291

Oil-water layer

Oil trace

sandstone

Table 2. EDS data of the samples. Q238, 1,605.2 m

Z59, 2,121.0 m

Z59, 2,121.5 m

Mean value

Occurrence

Mean Element

Mass

Atomicity

Mass

Atomicity

Mass

Atomicity

Occurrence space

Mean Mass

form

atomicity percentage/%

percent/%

percent/%

percentage/%

percentage/%

percentage/%

percentage/% percentage/%

C

62.26

67.94

56.08

65.89

77.51

86.59

65.28

73.47

Intragranular micro

O

34.87

35.40

31.48

27.77

9.25

7.75

25.20

23.64

pores and

Oil film

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Si

4.73

2.34

Na

1.39

1.23

Al

2.93

2.05

G104, 2,529.1 m

4.02

2.10

8.43

4.23

C45, 2,105.4 m

Page 22 of 22

6.81

3.25

5.19

2.56

0.61

0.36

1.00

0.80

2.51

1.25

4.62

2.51

G30, 2,547.9 m

micro-seams

Mean value

Occurrence

Mean Element

Mass

Atomic

Mass

Atomicity

Mass

Atomicity

Occurrence space

Mean Mass

form

atomicity percentage/%

percentage/%

percentage/%

percentage/%

percentage/%

percentage/%

percentage/% percentage/%

C

54.60

66.89

18.11

31.44

9.76

16.99

27.49

38.44

O

23.33

21.46

17.55

22.88

39.24

51.29

26.71

31.88

6.56

5.95

6.56

5.95

Na Mg

1.23

1.06

1.23

1.06

Intragranular nano-pores

Oil droplet Al

4.21

2.29

12.99

10.04

8.82

6.83

8.67

6.39

Si

17.87

9.36

38.09

28.28

22.16

16.5

26.04

18.05

K

16.91

6.33

16.91

6.33

Ca

1.00

0.52

1.00

0.52

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