A new method to determine wettability of tight sandstone: water

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A new method to determine wettability of tight sandstone: water imbibition evaporation rate ratio measurements Xuejuan Song, Yong Qin, Hao Ma, Kristian Waters, Ziwei Wang, and Guozhang Li Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b04184 • Publication Date (Web): 01 Feb 2019 Downloaded from http://pubs.acs.org on February 3, 2019

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Energy & Fuels A new method to determine wettability of tight sandstone: water imbibition evaporation rate ratio measurements Song et al.

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A new method to determine wettability of tight sandstone: water imbibition

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evaporation rate ratio measurements

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Xuejuan Songa,b,c*, Yong Qina,b, Hao Mac, Kristian Edmund Watersc, Ziwei Wanga,b,

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Guozhang Lia,b

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a. Key Laboratory of Coalbed Methane Resources and Reservoir Formation Process, Ministry

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of Education, China University of Mining and Technology, Xuzhou, Jiangsu 221008, China

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b. School of Resources and Geoscience, China University of Mining and Technology, Xuzhou,

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Jiangsu 221116, China

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c. Department of Mining and Materials Engineering, McGill University, 3610 University,

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Montreal, Quebec, Canada, H3A 0C5

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Abstract:

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With the development of gas exploration techniques, tight sandstone gas reservoirs have

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become the main source of new natural gas deposits and production in recent years. The

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wettability of tight sandstone is a very important parameter, affecting the gas-water distribution

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and movement in the sandstone, which is vital to gas recovery. Generally, the porosity and

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permeability of tight sandstone is low, with a non-homogeneous pore structure. Thus, the

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traditional testing methods of wettability on sandstone may not be suitable to treat tight

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sandstone. This paper proposes a new method of determining the wettability of tight sandstone

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by measuring the water imbibition evaporation rate ratio. Experimental results showed that the

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new proposed method could achieve the same results of tight sandstone wettability as the

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Amott-Harvey method, with a much shorter test duration, cheaper equipment and improved

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ease of operation. The results of the new method showed a strong correlation with sandstone

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A new method to determine wettability of tight sandstone: water imbibition evaporation rate ratio measurements Song et al.

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composition and structure; and it was more sensitive to small changes in wettability, with higher

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resolution and accuracy.

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Keywords: Wettability; gas recovery; tight sandstone; Amott-Harvey; water imbibition; water

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evaporation

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1. Introduction

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Gas reservoirs with an estimated in-situ gas permeability of 0.1 md or less are officially

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recognized by the U.S. Federal Energy Regulatory Commission (FERC) as “tight gas reservoirs”

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[1], with this criterion being adopted widely [2, 3]. Since the 1970s, continuous increases in natural

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gas prices, along with advances in evaluation, completion and stimulation technology, have led

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to substantial developments of low quality tight gas reservoirs globally. Since 2006, the

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exploration and development of tight gas reservoirs in China has grown rapidly. Tight gas

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sandstone reservoirs have been discovered in many basins including the Ordos, Sichuan, Tuha,

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Songliao, Junggar, Tarim, Chuxiong and East China Sea basins, which are mostly associated

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with coal strata [3, 4]. The production of tight gas in China was 33 billion cubic metres in 2016,

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accounting for 26.8% of China’s total natural gas production

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reach 80 billion - 1/3 of total gas production. The tight sandstone gas is gradually replacing the

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conventional natural gas resources; thus, these deposits will be key components of China’s

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future oil and gas industry [6].

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Currently, the main problem with tight sandstone gas reservoirs is that the gas cannot be

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extracted economically unless they are pre-treated. Generally, large hydraulic fracture

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treatments must be used to stimulate gas flow and increase the gas recovery from the reservoir

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[7].

[5].

In 2020, this is predicted to

Wettability is a major factor controlling the location, flow and distribution of fluids in a


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reservoir rock, having a significant effect on the development of oil & gas field and the gas

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recovery [8]. Normally, wettability refers to the adhesion tendency of one fluid towards a solid

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surface when two immiscible fluids co-exist [9]. It has been noted that when the wettability of

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porous media changed from liquid-wet to gas-wet, gas recovery improved significantly [10-14].

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Thus, the study of wettability is very important for tight gas reservoirs.

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Wettability measurements are usually conducted in the laboratories and wellholes. There are

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many different measuring methods including contact angle method

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method [16], USBM (U.S. Bureau of Mines) method [17], spontaneous imbibition rate method [9,

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18],

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method

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heterogeneous structure, therefore, the contact angle method is usually less reproducible with

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high uncertainty, thus, this method is not very suitable. Currently, most researchers use the

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Amott-Harvey and USBM methods to measure the wettability of sandstone. Regarding the

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Amott-Harvey method, although it covers a wide range of sandstone types from strong water-

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wet to strong oil-wet ones, it is not sensitive to the neutral-wet condition and the process is very

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complicated

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sandstone, it can take more than one week to perform one Amott-Harvey test; another

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disadvantage associated with the Amott-Harvey method is that the results obtained from the

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test are very low in value, causing bias in accuracy. For example, the water displacement during

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oil spontaneous imbibition test can be as low as 0.01 to 0.001 ml, while the current volume

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spectrometer usually has an accuracy of 0.01 ml. The USBM method is less complicated,

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usually having a shorter test period while it is sensitive to neutral-wet sandstone; however, it

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requires an ultra-high-rate centrifuge specially designed for rock core. In addition, during the

Nuclear Magnetic Resonance (NMR) relaxation method [20, 21],

[19],

well logging evaluation method and others

[16].

[15],

the Amott-Harvey

relative permeability curve

[22].

Sandstone usually has a

In addition, due to the extremely low porosity and permeability of tight


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centrifuge process, the original microporous structure of core plug may be altered, thus

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distorting the results [9]. It also involves a higher operating cost. Thus, based on this information,

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these two methods (Amott-Harvey and UBSM) might not be the most suitable techniques to

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measure the wettability of tight sandstone.

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In order to reduce the cost and improve the operating efficiency, a new method is proposed:

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water imbibition evaporation rate ratio (abbreviated as WIERR) method, this method not only

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can be used in the field or the laboratory, but also has an easy operation and lower time-

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consumption. The results from the new method were compared with the Amott-Harvey method

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in order to determine the applicability of the new method.

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2. Materials and Experimental Methods

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2.1 Sandstone Sampling Information

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Twenty-nine tight sandstone samples were collected from the Linxing-Shenmu Area located in

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the northeast of the Ordos Basin, China. The wells where the samples were located were

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distributed from north to south over the entire research area shown in Figure 1(a), and the

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sampling layers were concentrated within the Benxi, Taiyuan and Shanxi Formations, which

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can be seen in Figure 1(b).


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Figure 1 Sandstone sampling information

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The layers have a depth of 1700-2100 metres, with the samples being representative of the

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whole sandstone reservoir. Detailed information regarding the samples is given in Table 1.

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Table 1 The geological information of the samples Sample Well Depth No. (m) No.

Formation

B-16

Lower Shanxi

1 2 3 4 5 6 7 8 9 10

B-16 B-16 B-16 B-17 B-17 B-17

1984.1 1989.2

Upper Taiyuan

Sample Well Depth No. (m) No. 16 17

2025.8 Lower Taiyuan 2085.3 Lower Benxi 1717.6 Upper Shanxi 1724.1 Upper Shanxi Lower Shanxi 1816.5

18 19 20 21

B-17 1820.2 Lower Shanxi B-17 1854.2 Upper Taiyuan B-17 1897.2 Lower Taiyuan

23 24 25

22

B-36 A-7 A-7 A-9 A-9 A-9 A-9

1837.9 1931.8 2007.7 1795.0 1797.1 1823.7 1847.2

A-9 1902.7 A-9 1905.5 A-17 1987.2


Formation Lower Taiyuan Lower Taiyuan Upper Benxi Lower Shanxi Lower Shanxi Upper Taiyuan Lower Taiyuan Upper Benxi Upper Benxi Upper Shanxi

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A new method to determine wettability of tight sandstone: water imbibition evaporation rate ratio measurements Song et al. 11 12 13 14 15

B-20 1825.9 Lower Benxi B-20 Upper Taiyuan 1739.2 B-36

1743.5

Lower Shanxi

B-36 1752.5 Lower Shanxi B-36 1825.7 Lower Taiyuan

26 27 28 29

A-17 2014.1 Lower Shanxi A-17 Lower 2048.1 Taiyuan A-17 Lower 2070.9 Taiyuan A-17 2102.8 Upper Benxi

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2.2 XRD Analysis

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X-ray powder diffraction analysis was carried out on all sandstone samples using a Rigaku

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D/max-2600 X-ray diffractometer with Cu-Kα radiation (wavelength: 0.154nm). Diffraction

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patterns were recorded with variable slit values over the 2θ range of 5° to 40° with a step size

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of 0.02°, the results were then converted to fixed variable values through Rietveld analysis to

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measure the mineralogy of the samples [23].

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2.3 Standard Amott-Harvey Test

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In the standard Amott-Harvey method, the wettability of sandstone samples was measured as a [8].

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function of the displacement properties of the rock-water-oil system

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operations are carried out during the process: spontaneous displacement of oil by water, forced

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displacement of oil by water using a centrifuge, spontaneous displacement of water by oil, and

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forced displacement of water by oil [24, 25]. Ratios of the spontaneous displacement volumes to

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the total displacement volumes are calculated as wettability indices [8, 16]. The oil used in Amott-

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Harvey test is a synthetic oil with a viscosity of 1.0562 mPa·s and density of 0.7755 g/cm3 at

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25℃. The water used was CaCl2 solution with a concentration of 30 g/L, prepared to mimic the

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salinity of formation water. It should be noted that in general, as the concentration of low-

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concentration brine increases, the hydrophilicity of the rock deteriorates

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performed at 30℃. The results obtained were used as a metrics for evaluating results from the

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WIERR method.

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2.4 Thin Section Analysis


Four displacement

[26-28].

The test was

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The fresh sandstones were cut into thin sections with a thickness of 6 mm by a diamond saw.

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After being filled with blue epoxy-resin, thin sections were sanded, and the final samples were

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stained with a thickness of 0.03 mm. A Leica DM4500P optical microscope was utilized to

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identify the components, structures and pores of sandstones.

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2.5 Water Imbibition Evaporation Rate Ratio (WIERR) Method

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2.5.1 Theories

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This method could be applied to the cemented tight sandstone samples with known porosity

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and permeability, while it is not applicable to the sandstone that will expand upon contact with

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water. The relationship between capillary pressure, interfacial tension and contact angle is

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shown in Equation 1 [29]: pc=2σ*cosθ/r

(1)

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Where pc, σ, θ, r represent capillary pressure, interfacial tension, contact angle and radius of

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pore throat, respectively. When the wetting phase is water, the contact angle between sandstone

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surface and water is less than 90 degrees. In this situation, it can be seen from Equation 1 that

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a lower contact angle will lead to a higher capillary force. Since the capillary force represents

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the driving power for sandstone to imbibe water spontaneously, an increase in capillary force

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will lead to a greater capacity to imbibe water, and to expel oil. As a lower contact angle means

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higher hydrophilicity, the sandstones with higher hydrophilicity will imbibe water faster during

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the imbibition test.

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However, in hydrophilic sandstone, capillary force becomes a resistance to water evaporation

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in water evaporation experiment. As mentioned earlier, a sandstone with higher hydrophilicity

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will have a greater capillary force. Therefore, for a sandstone, the more hydrophilic it is, the

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greater the water evaporation resistance and the slower the water evaporation rate will be. In


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addition, the relationship between evaporation rate and wettability can also be explained by

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Kelvin equation [30]: 𝑝𝑟

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RTln𝑝0 =

2𝛾𝑉𝑚 𝑟𝑚

(2)

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Where R, T, pr, p0, γ, Vm, rm represent the universal gas constant, temperature (absolute), the

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actual vapor pressure, the saturated vapor pressure, the surface tension, the molar volume of the

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liquid and the radius of curvature of the liquid/gas interface, respectively.

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In evaporation test, water evaporates from the pores of sandstone. The higher the hydrophilicity

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of the sandstone, the smaller the contact angle of the water in the pores with the rock surface

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will be, resulting in a smaller radius of curvature of the liquid/gas interface (rm). According to

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Kelvin equation, a smaller rm leads to a higher actual vapor pressure, and this will cause a lower

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water evaporation rate. Thus, the higher the hydrophilicity, the slower the water evaporation

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rate of the sandstone will be.

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Therefore, the water imbibition and evaporation rates are related to the hydrophilicity of

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sandstone. With an increase in its hydrophilicity, the water imbibition rate will also increase

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while the water evaporation rate will be slower. Based on this information, we can determine

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the wettability of sandstone through the ratio of water imbibition rate to evaporation rate.

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2.5.2 Sample Preparation

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In order to prepare the sample, a hydraulic drilling machine was operated in a direction

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perpendicular to the bedding planes of the sandstones at a low drilling speed (less than 400

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r/min), a cylindrical sample with a length of 5 cm and a diameter of 2.5 cm was cut from the

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sandstone. The sample was air-dried at 60℃ for 24 hours, and then stored in a desiccator.

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2.5.3 Testing Equipment

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As shown in Figure 2, an electronic balance (Model: JA5003) purchased from Shanghai


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Hengping Scientific Instrument (China) was used to determine the water imbibition and

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evaporation rate of tight sandstone. Before each test, the sample was placed on the weighing

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pan which was immersed in the deionized water, the mass of the sandstone sample can be

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accurately read on the balance. During the test, the tight sandstone will imbibe water and the

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weight will increase as shown on the balance. It can be seen that this method could help avoid

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the potential experimental errors associated with traditional sample weighting process since the

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sample is immersed in water for the whole process.

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Figure 2 The electronic balance, with a precision of 0.001g

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2.5.4 Testing Procedure

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(1) Water Imbibition Test

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The dry tight sandstone sample was immersed in deionized water at 30℃. The sample mass

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was recorded until there was no further change in the reading. Since the water imbibition rate

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refers to the amount of water imbibition in unit time driven by the capillary force. The following

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equations are shown as follows to calculate the water imbibition rate: Vt=(Wt-W0)/ρw

(3)

v1=dVt/t

(4)


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Where W0 represents the initial sample mass (g); Wt represents the sample mass at time t during

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water imbibition test (g); ρw represents the water density at room temperature (g/ml); Vt

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represents the water volume that the sandstone sample has imbibed at time t while v1 represents

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the water imbibition rate (ml/min). The maximum sample mass was recorded as W1, which

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means that no further increase in sample mass can be seen and the sandstone sample reaches

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100% water saturation. Based on Equation 3, The sandstone water saturation can also be

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calculated using Equation 5: Sw1=Vt/Vp×100%

(5)

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Where Vp represents the total pore volume of the sandstone sample (ml); Sw1 represents water

180

saturation of sandstone at time t (%).

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(2) Water Evaporation Test

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After the water imbibition test ends, the sample saturated with water was placed under 30℃

183

and 35% - 40% relative humidity, and the sample mass at different times were recorded. Since

184

the water evaporation rate refers to the amount of water evaporation of water saturated tight

185

sandstone in unit time driven by the resistance of capillary force, the following equations are

186

used to calculate the water evaporation rate: Vt=(W1-Wt)/ρw

(6)

v2=dVt/t

(7)

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Where W1 represents the maximum sample mass (g); Wt represents the sample mass at time t

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during evaporation (g); ρw represents the water density at room temperature (g/ml); Vt represents

189

the water volume that the saturated sandstone sample has evaporated at time t while v2

190

represents the water evaporation rate (ml/min). Based on Equation 6, The sandstone water

191

saturation during evaporation can also be calculated using Equation 8:


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Sw2= (Vp-Vt)/Vp×100%

(8)

192

Where Vp represents the total pore volume of sandstone samples (ml); Sw2 represents water

193

saturation of sandstone at time t during evaporation (%).

194

A classical water imbibition and evaporation curve can be seen in Figure 3. It can be seen that

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in the water imbibition test, with an increase in water saturation, the water imbibition rate is

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decreasing; while in the water evaporation test, the water evaporation rate is decreasing with a

197

decrease in water saturation.

198 199

Figure 3 Sandstone sample (No.13) water imbibition/evaporation curve

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The water imbibition/evaporation rate declines with time (or saturation) and the rate decreases

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much faster in the initial stage than the final stage. There are two reasons: 1. In imbibition test,

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larger pores which are usually well connected are firstly filled with brine. Therefore, in the

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beginning, a large amount of water can easily enter the pores of the rock, and it has a higher

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water imbibition rate. Then the water enters the remaining small pores which are not only small

205

in total volume but also difficult to enter due to poor connectivity, so the water imbibition rate

206

is significantly reduced. Similarly, in the evaporation experiment, in the initial beginning, the

207

water in the large pores with good connectivity evaporates quickly, and then the water in the

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poorly connected small pores finally evaporates slowly. That's why the imbibition/evaporation ACS Paragon Plus Environment

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rate declines with time; 2. It is well known that imbibition rate is proportional to imbibition

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capillary pressure, which is dependent on water saturation

211

capillary force is a function of water saturation. With the increase of water saturation, capillary

212

pressure decreases, which is a non-linear process, and it tends to decrease sharply in the early

213

stage and slowly in the later stage. Thus, the decrease in water imbibition rate is non-linear,

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which is in agreement with other work

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behaves in such a manner as shown in Figure 3.

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Based on past experience on various sandstone samples from Ordos and other gas reservoirs,

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plus the information above, the sandstone wettability can be determined using the following

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criteria: Under 50% water saturation, when v1/v2

A new method to determine wettability of tight sandstone: water

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