Effects of Dissolved Oxygen on Water Imbibition in Gas Shales

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Effects of Dissolved Oxygen on Water Imbibition in Gas Shales Mingxiang Xu, Mojtaba Binazadeh, Ashkan Zolfaghari, and Hassan Dehghanpour Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03955 • Publication Date (Web): 12 Mar 2018 Downloaded from http://pubs.acs.org on March 26, 2018

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Energy & Fuels

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Effects of Dissolved Oxygen on Water Imbibition in Gas Shales

2

Mingxiang Xu1, Mojtaba Binazadeh1,2, Ashkan Zolfaghari1, and Hassan Dehghanpour1

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1

4

University of Alberta, Edmonton, Alberta, T6G 2W2, Canada.

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2

6

Shiraz, Fars, Iran

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Abstract

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Understanding the water uptake of gas shales is critical for designing and optimizing hydraulic

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fracturing operations during which a large volume of fracturing water containing dissolved oxygen is

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injected into tight reservoirs. Recent studies show that the dissolved oxygen may promote oxidation

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reactions which can affect salinity and pH value of flowback water; however, the effects of dissolved

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oxygen and oxidation reactions on water imbibition into the shale matrix and on the concentration of

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individual ions in flowback water are still poorly understood. In this study, we conduct water

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imbibition experiments under degassed and oxic conditions, and measure the imbibed water volume

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and concentrations of different ions in water. The results show that the initial rate and final amount of

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water imbibition are higher under degassed conditions compared with that under oxic conditions.

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These differences are mainly due to the enhanced dissolution of air in the shale pore network into the

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imbibing water under degassed conditions and the consequent increase in relative permeability of

19

water. The results also suggest that oxidation of pyrite by dissolved oxygen produces sulfate and iron

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ions, as well as iron-compound precipitations. Pyrite oxidation is also supported by the abundance of

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pores in the vicinity of pyrite minerals observed in the SEM/EDS images.

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1. Introduction

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Recent advances in horizontal drilling and hydraulic fracturing have unlocked the vast unconventional

Department of Civil and Environmental Engineering, School of Mining and Petroleum Engineering,

Department of Chemical Engineering, School of Chemical and Petroleum Engineering, Shiraz University,

1

ACS Paragon Plus Environment

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hydrocarbon resources such as shale and tight formations. Hydraulic fracturing operations include

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injection of approximately 10-20 million liters of water (cross-linked gel, slickwater, energized and

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hybrids) into the shale/tight formations per well to create fractures.1-3 Some wells in some thick shale

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formations (e.g., Horn River Basin) consume up to 75 million liters of slick water for hydraulic

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fracturing operations.4 Then, the wells are sometimes shut-in for a period of time, (i.e., soaking period)

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to increase the early-time oil and gas production rate.5,6 During the hydrocarbon production stage,

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only a fraction (usually less than 30% within 100 days of flowback) of the injected fracturing water

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(i.e., flowback water) can be recovered.7-9

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Laboratory imbibition experiments have been widely used to investigate the effects of shale-water

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interactions. For example, water imbibition in shales may cause dissolution of precipitated salts in

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shale and raise osmosis effect10,11, increase water saturation in the vicinity of water-rock interface

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(water blockage) 12-14 , and reduce hydrocarbon relative permeability. 15-18 It is also reported that

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shale-water interactions may reduce fracture conductivity and effective length 19 , 20 , induce

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micro-fractures in rock matrix due to clay swelling21,22, and finally, alter shale strength and cause

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failures around the wellbore.23-25

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A common challenge for applying laboratory imbibition data to evaluate fluid-rock interactions during

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fracturing and soaking periods is the differences between laboratory and field conditions:

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Temperature. Previous studies indicate that increasing the temperature can accelerate the water

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imbibition by increasing the wetting affinity of rock to water.26,27 Increasing the temperature increases

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the activity of divalent ions such as Mg2+ and SO to pair with the surface anions of carbonate

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minerals, and enhances the affinity of rock toward water.28

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Pressure. Usually rock samples are free to expand during laboratory water imbibition

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experiments.29-31 However, under the field conditions, overburden pressure limits rock expansion.32

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Ghanbari and Dehghanpour22 conducted water imbibition experiments on confined/unconfined shale

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samples and found that confined samples have less induced microfractures and less water imbibition. 2


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Water-Rock Interface Area. The water-rock interface area differs dramatically at laboratory and field

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conditions. Previous studies show that increasing shale-water interface area per unit volume of shale

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significantly increases the rate and final volume of water imbibition.33 Roshan et al.34 showed that

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the water-rock interface area affects the exchangeable cations in flowback water. Bearinger35 showed

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that the salt concentration in flowback water is related to the complexity of fracture network.

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Low oxygen-content under reservoir conditions should also be considered in imbibition experiments.

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Shale reservoirs usually have no (or limited) oxygen content (anoxic conditions) before fracturing

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process. Anoxic reservoir conditions can be due to oxygen consumption by organic matter and lack of

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oxygen supply in the reservoir.36 Oxygen is introduced by fracturing water into shale reservoirs,

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creating oxic conditions. Shales usually contain chemically-reactive compounds such as organic

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matter and pyrite, which may participate in oxidation reactions. 37 - 41 Recent studies show that

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oxidation of pyrite may change the pore structure and increase the pore connectivity.42,43 Rowan et

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al.44 observed a positive correlation between oxygen and chloride content in flowback water, which

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suggests that the oxygen dissolved in injected water may affect the geochemical reactions. Zolfaghari

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et al.33 found that the concentration of dissolved oxygen in flowback water may be lower than that in

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deionized water. They concluded that the dissolved oxygen may cause oxidation reactions, which

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could affect salt dissolution, salt precipitation and pH change in flowback water. Thus, understanding

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the effects of dissolved oxygen on water imbibition and ion dissolution is important for more accurate

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interpretation of flowback water chemistry.

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This article extends the previous studies33 to 1) investigate the role of dissolved oxygen on water

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imbibition rate/volume and 2) evaluate the effects of dissolved oxygen on concentration of major ions

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(i.e. potassium and sodium ions) and redox-sensitive ions (i.e. iron and sulfate ions) in flowback water.

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We perform comparative imbibition experiments using air-saturated de-ionized water (oxic water) and

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degassed de-ionized water (degassed water). The rest of the paper is organized as follows: Section 2

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describes the materials and methods. Section 3 shows the imbibition profiles and concentration

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profiles of different ions for the oxic and degassed water. Section 4 describes the conclusions of this 3


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study.

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2. Materials and Methods

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2.1 Shale Samples

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Three pairs of shale samples from three different depths of the Evie Formation of the Horn River

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Basin are labeled as EV-1, EV-2, and EV-3 and used for imbibition experiments. To eliminate the

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effect of sample size, all samples are cut into 35×35×25 mm cubes with approximate mass of 100 g.

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Table 1 provides the average depth, porosity, permeability, and mineralogy of the samples. The

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porosity, permeability, and mineralogy are measured by helium porosimeter, pulse decay permeameter,

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and x-ray diffraction (XRD) respectively using different samples from similar depth.

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Table 1. Approximate depth, porosity, permeability and average rock composition for EV-1, EV-2,

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and EV-3 samples. Sample

EV-1

EV-2

EV-3

Depth, m

2672.5

2681.2

2688.2

Porosity, %

6.4

5.0

4.6

Permeability, nD

575

384

372

Non-Clay Content (wt. %) Quartz

78.1

51.9

65.4

K-feldspar

5.2

4.9

3.4

Plagioclase

1.3

5.7

5.0

Calcite

3.1

13.3

8.4

Dolomite

0.7

2.6

1.1

Pyrite

1.8

3.0

3.0

Total Non-Clay

90.5

81.4

86.5

Clay Content (wt. %)

4


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Illite/Smectite

2.9

6.6

5.2

Illite+Mica

6.7

12.0

8.3

Total Clay

9.6

18.6

13.5

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2.2 Imbibition Experiments

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In order to investigate the effect of dissolved oxygen on water imbibition in the shale samples, for

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each pair of samples, we immerse one sample in the air-saturated de-ionized (DI) water (oxic water)

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and the other sample in the degassed DI water (degassed water). We consider the DI water which was

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in contact with air (air-saturated) for 1 week as oxic water. The dissolved oxygen content of oxic water

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is about 8.3 mg/L, which is measured by rugged dissolved oxygen (RDO) optical electrode (Thermo

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ScientificTM OrionTM). To prepare the degassed water, we place 500 mL of DI water in a desiccator

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(vaccum chamber) as shown in Figure 1. The desiccator is then sealed and connected to a vacuum of 1

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kPa for 1 week. The dissolved oxygen content of degassed water is about 0.7 mg/L. Generally, the

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pressure difference can potentially impact the imbibition results. However, the pressure difference

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between oxic and degassed experiments is relatively low (100 KPa) compared with that between

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laboratory and field conditions (usually more than 10,000 KPa). For example, the reservoir pressure in

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the Horn River Basin can be as high as 53,000 KPa.45 In this study, compared with the significant

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pressure difference in laboratory and subsurface, we assume that the relatively-small pressure

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difference between the oxic and degassed experiments has a negligible impact on the rock pore

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volume and imbibed mass. The imbibition experiments are conducted according to the following

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steps:

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1.

104 105

samples. 2.

106 107

All the samples are dried in the oven at 100 ℃ for 24 hours to remove the moisture in the

For each pair of samples, one sample is placed in an imbibition cell filled with 500 mL oxic water. The other sample is placed in an imbibition cell filled with 500 mL of degassed water.

3.

All the imbibition cells filled with degassed water are placed in a vacuum chamber connected to 5


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a vacuum pump at 1 kPa to ensure degassed conditions. It is assumed that vacuum pressure

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during degassed conditions has a negligible effect on the pore volume and imbibed mass. The

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schematic of the vacuum set-up is shown in Figure 1.

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4.

At each time step, the shale samples are taken out of the imbibition cell to measure the samples’

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mass. We reported the normalized imbibed mass by dividing the mass gain by the initial mass of

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the dried sample. Sartorius™ Entris™ Toploading Balance with the accuracy of ±0.01 g is used

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to measure the samples’ mass. The dissolved oxygen concentrations of the oxic and degassed

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water are measured using RDO optical electrode.

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5.

2 mL of the oxic and degassed water are periodically collected for measuring iron concentrations

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using inductively coupled plasma mass spectrometry (ICP-MS) and ion chromatography (IC).

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During the measurements in steps 4 and 5, the shale and water samples are exposed to air. This may

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cause oxygen adsorption on the shale and water samples. We maintained vacuum pressure at 1 kPa

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during the experiments to reduce potential oxygen adsorption on the shale samples during the

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measurements. The 2 mL collected degassed water is sealed in 2 mL air-tight vials to prevent oxygen

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contamination.

123 124

Figure 1. The schematic of the vacuum set-up used for the imbibition tests under degassed conditions.

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2.3 SEM-EDS Imaging

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Scanning

electron

microscope

(SEM),

scanning

helium

6


ion

microscope

(SHIM),

and

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energy-dispersive x-ray spectroscopy (EDS) are used to visualize the pores and obtain the elemental

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maps in EV-2 sample. One end piece from EV-2 sample is prepared for SEM-EDS imaging. We first

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cut the end piece into 1×1×0.5 cm cubes. The top surface of the sample is then mechanically polished

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using 600-, 1000-, and 2000-grit polishing pads. The sample surface is further polished using

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argon-ion milling to minimize the influence of roughness and artifacts of the sample surface on the

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SEM images. All SEM/SHIM images are obtained using ~15 Kv beam energy.

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3. Results and Discussion

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In this section, we first compare the imbibition profiles of the oxic and degassed imbibition

135

experiments. Then, we discuss the observed difference in ion concentration profiles of the oxic and

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degassed water samples.

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3.1 Imbibition Profiles

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Figure 2 shows the water imbibition profiles versus time. The imbibed water mass is normalized

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through dividing it by the mass of the dry shale sample. The imbibition profiles increase over time and

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reach to a plateau for both oxic and degassed experiments. Figure 2 shows that 1) the imbibition rate

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and the final normalized mas of imbibed water are higher for degassed experiments compared with

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those for oxic experiments, and 2) EV-1 samples show the lowest gap between oxic and degassed

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water imbibition profiles while EV-3 samples show the largest gap. We explain these two observations

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by investigating the dissolved oxygen in Section 3.2 and ion concentration profiles for both oxic and

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degassed water samples in Section 3.3. The measured shale sample mass is also added in Table A in

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the Appendix.

7


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Figure 2. Mass of imbibed oxic and degassed water for (a) EV-1, (b) EV-2, and (c) EV-3 samples.

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3.2 Dissolved Oxygen Profiles

149 150

Figure 3. The surface of EV-1 shale samples in a. oxic water and b. degassed water 2 hours after

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starting the imbibition experiments. The blue areas in the figure are reference areas used for

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determining the sample depth.

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As can be seen in Figure 3, gas bubbles are observed on the shale samples’ surface when oxic water

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imbibes into the samples 2 hours after starting the imbibition experiments. We did not observe gas

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bubbles for degassed experiments which could be due to the rapid dissolution of gas existing in the

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shale sample pore space into degassed water. The low pressure in vacuum chamber not only reduces

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the concentration of the dissolved oxygen in degassed water, but also reduces the concentration of

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other dissolved gases such as carbon dioxide and nitrogen.46

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The oxic water imbibing into the shale sample displaces the gas out of the pore system. The gas 8


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bubbles expelled out by the imbibed oxic water were also observed in previous studies.34,47 The gas

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bubbles in Figure 3(a) are probably the air initially present in the pores of the shale sample. In addition,

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the gas bubbles observed in Figure 3(a) may also contain the gas produced by reaction of rock

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minerals with water. For example, carbon dioxide may be produced by reaction between pyrite,

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dolomite, water and oxygen48,49:

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() +

() + ( ) + → ()() + + + 2 + 2

166

(1)

167

The possibility of pyrite oxidation reaction will be discussed in section 3.3.

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Under degassed conditions, the gas existing in the pore space dissolves into water, while under

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degassed conditions, the gas cannot completely dissolve in the imbibing water. For the oxic water

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imbibition, some of the gas in pore space may be trapped and reduce water saturation and relative

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permeability.12 The trapped gas during oxic water imbibition may also increase the resistance against

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the capillary force for water imbibition which can be referred to as relative permeability effect.50

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According to Figure 2, the imbibition rate of degassed water is higher than that of oxic water. For

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example, for the EV-1 sample, the degassed water imbibition rate for the initial 18.5 hours is about

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0.07 g/hr which is higher than that for the oxic water imbibition rate (0.05 g/hr).

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When the gravity effect is ignored, the imbibed mass is proportional to the square root of time:51 = !"

177 178 179

#$ %%&' ∅)* + ,* -

. × √1

(2)

Here, is the mass of imbibed water, 2 and 3 are viscosity and density of the imbibed water respectively, is the water saturation of shale samples at time 1, 45 is the capillary pressure at the

180

saturation of , 678 is water relative permeability, 6 and ∅ are absolute permeability and porosity

181

of the shale samples, respectively.

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Figure 4 shows the normalized mass of imbibed water versus square root of time. Compared with the

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degassed tests, the lower slope for oxic water imbibition can be due to the potential trapped gas in the 9


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pores of the rock. More specifically, if the gas traps in pore space, krw reduces due to the declined

185

water saturation. The potential decline of krw can be responsible for the lower imbibition rates during

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oxic water imbibition compared with that during degassed water imbibition.

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Figure 4. Mass of imbibed oxic and degassed water versus square root of time for (a) EV-1, (b) EV-2,

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and (c) EV-3 samples. The trendlines show the initial imbibition rate.

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During degassed water imbibition, dissolution of gas into the water may provide additional pore space

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for water imbibition, which explains the difference in imbibed mass observed in Figure 4. On the other

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hand, the gas trapped in pore space during oxic water imbibition may partially block the path for water

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imbibition. In Figure 4c, the equilibrated imbibed mass of degassed water is up to 2 times higher than

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that of oxic water in EV-3 samples which suggests that up to 50% of the water-accessible pore volume

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is occupied by trapped gas during oxic water imbibition.

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3.3 Ion Concentration Profiles

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In this section, ion concentrations in oxic and degassed water during the imbibition process are

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presented. We measure concentration of major ions such as potassium and sodium and redox sensitive

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ions such as iron and sulfate to investigate the chemical reactions promoted by the presence of

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dissolved oxygen in water.

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Energy & Fuels

3.3.1 Potassium and Sodium Ions

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203 204

Figure 5. Potassium (a, c, e) and sodium (b, d, f) concentration of degassed/oxic water in contact with

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EV-1 (a, b), EV-2 (c, d), and EV-3 (e, f) shale samples measured by ICP-MS.

206

According to Figure 5, potassium and sodium ion concentrations increase during both oxic and

207

degassed water imbibition experiments and no considerable difference is observed in the concentration 11


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profile of these ions. The source of potassium and sodium can be 1) dissolution of precipitated salts

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(such as NaCl and KCl) in shale samples; 2) dissolution of potassium or sodium-bearing components

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in the shale samples such as halite52 and 3) leaching of exchangeable potassium and sodium ions from

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interlayer of clays.53,54 The similarity of ion concentration profiles between oxic and degassed water

212

imbibition suggests that the presence of oxygen in the imbibition water has negligible impact on the

213

concentrations of produced Na+ and K+ ions.

214

To investigate the potential sources of produced ions during the imbibition experiments, we plot the

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molar ratio of K+/Cl- and Na+/Cl- in Figure 6. Both K+/Cl- and Na+/Cl- molar ratios are more than one.

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Leaching of exchangeable cations from the interlayer of clay minerals is a possible reason for the

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excess produced Na+ and K+.55 Both K+/Cl- and Na+/Cl- molar ratios show an early increase for oxic

218

water. This can be due to the fact that chloride can only be oxidized (not reduced).56 Thus, in an oxic

219

environment, the leached K+ and Na+ from the interlayer of clay minerals may be responsible for the

220

early increase in K+/Cl- and Na+/Cl- molar ratios. Moreover, chloride ion may oxidize and decrease the

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concentration of Cl- in the solution. Thus, at early times, both K+/Cl- and Na+/Cl- molar ratios increase

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in the oxic environment. At later times, the potential dissolution of chloride-bearing minerals (such as

223

NaCl and KCl) can be responsible for the declining trends of K+/Cl- and Na+/Cl- molar ratios.

224

At early times, both K+/Cl- and Na+/Cl- molar ratios are higher for degassed water compared with that

225

for oxic water. On the other hand, the imbibed volume is higher for the degassed conditions compared

226

with that for the oxic conditions (Figure 2). Enhanced imbibition can potentially enhance the

227

accessibility to the clay-rich zones, facilitating the ion-exchange reactions. Thus, the probability of

228

leaching exchangeable cations (i.e., K+ and Na+) from the interlayer of clay minerals is higher in the

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degassed conditions compared with that in the oxic conditions. Similarly, the possible dissolution of

230

chloride-bearing minerals reduces K+/Cl- and Na+/Cl- molar ratios at later times.

12


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232

233 234

Figure 6. The K+/Cl- (a, c, e) and Na+/Cl- (b, d, f) molar ratio of degassed/oxic water in contact with

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EV-1 (a, b), EV-2 (c, d), and EV-3 (e, f) shale samples.

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3.3.2 Iron and Sulfate Ions

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Previous studies have shown that the concentration of redox sensitive elements (such as Fe and S) can

238

be influenced by the presence of oxygen.33,57 Figure 7 shows concentration profiles of iron and sulfate 13


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ions in oxic and degassed imbibition experiments. The concentration of iron ion increases over time

240

during the oxic water imbibition while the concentration of iron is almost zero during the degassed

241

water imbibition. This observation indicates that pyrite oxidation reactions is inhibited in the absence

242

of oxygen. According to Table 1, pyrite is the only source of iron in the shale samples. It should be

243

noted that XRD may not detect other possible sources of iron such as goethite, hematite, or magnetite

244

in our shale samples due to their low concentration. The pyrite reactions with water and oxygen may

245

be a possible reason for the increasing trend of iron concentration profile during oxic water imbibition

246

in Figure 7. SEM images in section 3.4 shows pores in the size of 5 to 200 nm near pyrite minerals.

247

These pores provide an interface at which imbibed water, dissolved oxygen, and pyrite co-exist and

248

react.

249

250

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Figure 7. Iron (a, c, e) and sulfate (b, d, f) concentration of degassed and oxic imbibing water in

253

contact with EV-1 (a, b), EV-2 (c, d), and EV-3 (e, f) shale samples measured by ICP-MS and IC.

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In pyrite oxidation reactions, the oxygen atoms from dissolved oxygen in water may react with the

255

sulfur atoms in pyrite molecules at the rock-water interface 58 and produce ferrous ions (Fe2+)

256

according to the following aqueous reactions:59-62 9

257

() + () + → + 2 + 2

258

2 + 2 + () → 2 +

(3)

(4)

259

The ferrous ions (Fe2+) can react with dissolved oxygen (Eq. 4) to produce ferric ions (Fe3+ in Eq. 4).

260

Pyrite oxidation rate generally increases with increasing pH and dissolved oxygen concentration.63-65

261

In the imbibition experiments, the pH range for degassed and oxic imbibing water is between 8.0 to

262

9.0 and 7.5 to 8.5, respectively. The higher pH of degassed water can be due to the removal of some

263

dissolved carbon dioxide in degassed water by vacuum.66 The overall pyrite oxidation reaction in

264

alkaline solution is as follows:67,68

265

() +

() + 4 → ()() + + 2

(5)

266

According to Eq. 5, oxidation of one mole of pyrite produces two moles of sulfate ion. It explains why

267

iron and sulfate concentrations in oxic water are higher than that in degassed water, as observed in

268

Figure 7. The rate of pyrite oxidation (Rpyrite) can be expressed by the rate of sulfate concentration 15


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269 270

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difference:69 R ?@A =

BCDE7FGHI BG

=

B(J+KL*MNOPQ$ J+KL*M NRSOPQ$ )

(6)

BG

271

However, the difference in sulfate concentration of oxic and degassed imbibition is around 2 orders of

272

magnitude larger than the corresponding difference in Fe2+/Fe3+ concentration. The reacted pyrite will

273

produce either soluble Fe3+ (Eq. 4) or insoluble Fe3+ hydroxide precipitate (Eq. 5). Based on Pourbaix

274

diagram of iron, 70 Fe3+-bearing compounds start to precipitate at low pH values. For example,

275

Fe3+-hydroxide starts to precipitate at pH values of about 1.71 Since the pH range in our experiments

276

is 7.5-9.0, the released iron during pyrite oxidation may ultimately form insoluble Fe3+-bearing

277

compounds which cannot be identified by ICP-MS tests. According to the Pourbaix diagram of iron,70

278

at pH values of 7.5 to 9.0, the iron oxidation products could be Fe2+, Fe(OH)2, Fe2O3·nH2O, Fe3O4,

279

which are listed in Eqs. 3, 7, 8, 10, 11. Highly reactive Fe2+/Fe3+ ion may participate in production of

280

water insoluble iron hydroxide, goethite, hematite or magnetite according to Eqs. 7 to 11:54

281

+ 3 → ()() + 3

(7)

282

+ 2 → ()()

(8)

283

+ 3 → () +

(9)

284

2 () → () +

(10)

285

()() + 2 () → () +

(11)

286

It is worth mentioning that the imbibition experiments are conducted at room temperature,

287

atmospheric pressure and nearly neutral pH, which are different compared with the reservoir

288

conditions. For example, Horn River reservoir gas contains about 9-14% of CO2,72 and the reaction of

289

CO2 with water can potentially produce carbonic acid. It must be noted that although carbonic acid

290

production can reduce the pH, natural buffer systems such as dolomite and calcite may neutralize the

291

pH under reservoir conditions.33 Furthermore, the reservoir temperature in the Horn River Basin is in

292

the range of 135 to 175 ℃ and the reservoir pressure is between 20,000 to 53,000 KPa.45 The 16


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Energy & Fuels

293

different in laboratory and reservoir conditions may lead to difference in “speciation and complexation”

294

of ions, which can alter the pH.

295

3.4 SEM and EDS Images

296

In Figure 8, SEM images of a focus area and the corresponding elemental map of sulfur and iron show

297

that pyrite is widely spread in shale samples. High density of sulfur and iron elements in Figure 8.c

298

and 8.d suggests a pyrite-rich area. Figure 8.b shows the magnified SEM image of the area surrounded

299

by dashed line in Figure 8.a, where many pores with a throat size of 50 to 200 nm exist in the

300

pyrite-rich area. Figure 9 shows a scanning helium ion microscope (SHIM) image of the pyrite area.

301

Two pyrite framboids with the approximate size of 1 µm are shown in Figure 9 and nanopores are

302

observed between the pyrite particles. These pores provide an interface at which imbibed water,

303

dissolved oxygen, and pyrite chemically interact. The interaction between these three components may

304

result in iron-bearing precipitation on the surface of nanopores which may narrow or block the pores

305

and may ultimately lower the permeability of the shale sample.

17


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306 307

Figure 8. (a) SEM image of a focused area on a EV-2 shale sample; (b) magnified SEM image

308

showing pores in pyrite; (c) sulfur elemental map and (d) iron elemental map obtained by EDS of the

309

EV-2 shale sample. Darker area in elemental maps represents higher element density.

310 311

Figure 9. SHIM image of the EV-2 shale sample. Nano pores are observed around pyrite framboid. 18


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312

Finally, pyrite dissolution and oxidation may have a dual effect: 1) pyrite dissolution may create more

313

pore space and may enhance water imbibition and 2) precipitation of iron-bearing compounds formed

314

as a result of pyrite oxidation in the pore space may reduce the flow conductivity for water imbibition.

315

Reduced imbibition rate and lower final imbibed water mass under the oxic conditions in this study

316

suggest that precipitation of iron-bearing compounds formed by pyrite oxidation in the pore space has

317

a more pronounced effect in comparison with the pore space creation by pyrite dissolution.

318

4. Conclusions

319

Imbibition experiments using degassed and oxic water were conducted on samples from Evie

320

Formation, a shale member of the Horn River Basin. Concentration of different ions in imbibing water

321

were measured to investigate the effect of dissolved oxygen on water imbibition and produced ions.

322

Pore network and minerals of the shale samples were investigated using SEM-EDS imaging. The key

323

results of this study are summarized as follows:

324

•

Degassed water can dissolve the gas in the shale pore space, contributing to a higher

325

imbibition rate and final imbibed mass, compared with the oxic water. This observation

326

indicates that the initially-degassed conditions in reservoir may accelerate the rate of water

327

imbibition. Less gas entrapment under degassed conditions promote the water imbibition into

328

shale matrix.

329

•

The oxygen dissolved in water can react with pyrite-bearing components in shale and produce

330

iron and sulfate ions. Interactions of degassed water with shale do not result in iron

331

production. This observation indicates that dissolved oxygen in fracturing fluid can cause

332

pyrite oxidation and dissolution.

333

•

Pores observed by SEM are in the vicinity of both pyrite framboids and pyrite crystals.

334

Although pyrite constitutes only a small portion of shale minerals, the oxygen-water-pyrite

335

interactions cannot be ignored when interpreting the chemistry of water produced during

336

flowback processes. 19


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337

Acknowledgements

338

The authors are grateful to (1) Nexen Energy ULC, INPEX Gas British Columbia Ltd and Natural

339

Sciences and Engineering Research Council (NSERC) for supporting this project, (2) Doug Bearinger

340

for useful discussions, and (3) Todd Kinnee for his help in conducting the experiments.

341

References

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342

Appendix

343

Table A. Measured sample mass of oxic and degassed imbibition experiments.

Time Hours 0.00 1.30 2.58 9.83 18.50 27.50 44.72 72.83 94.83 119.00 147.42 172.33 194.50 218.58 242.67 265.67 361.67 457.67 505.67 553.67 625.67

Measured Sample Mass, g EV-1

EV-2

EV-3

Oxic

Degassed

Oxic

Degassed

Oxic

Degassed

91.90 92.06 92.15 92.29 92.37 92.39 92.44 92.49 92.55 92.55 92.60 92.60 92.61 92.63 92.65 92.65 92.69 92.71 92.72 92.73 92.73

96.87 97.21 97.33 97.46 97.57 97.62 97.66 97.69 97.74 97.76 97.76 97.79 97.81 97.81 97.82 97.84 97.84 97.84 97.85 97.86 97.86

98.85 99.43 99.60 99.82 99.93 100.00 100.02 100.02 100.02 100.02 100.02 100.02 100.02 100.02 100.02 100.02 100.02 100.02 100.02 100.02 100.02

101.61 102.47 102.73 103.08 103.17 103.17 103.18 103.20 103.20 103.20 103.20 103.20 103.20 103.20 103.20 103.20 103.20 103.20 103.20 103.20 103.20

112.14 112.33 112.36 112.39 112.43 112.45 112.48 112.48 112.52 112.54 112.54 112.54 112.55 112.55 112.56 112.56 112.56 112.54 112.56 112.56 112.56

108.40 109.00 109.11 109.16 109.22 109.25 109.26 109.28 109.30 109.33 109.33 109.34 109.35 109.35 109.35 109.35 109.34 109.36 109.34 109.35 109.33

344

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Effects of Dissolved Oxygen on Water Imbibition in Gas Shales

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