Nanopores of Tight Sand

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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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Energy & Fuels

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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

ACS Paragon Plus Environment

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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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5909–5916.

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Nano Letters.2004. 4(5) , 989–993.


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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,

311

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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315

Page 16 of 22

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

321 ACS Paragon Plus Environment

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Energy & Fuels

C

D

322 323

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

324

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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Energy & Fuels

A

B

335 D

C

336 337

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

338

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

339

and EDS data from Well Q238.

340


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Page 20 of 22

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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Energy & Fuels

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


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 349 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 350 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

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


Nanopores of Tight Sand

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