Hydrogeochemical and Isotopic Indicators of Hydraulic Fracturing

May 9, 2017 - Hydrogeochemical and isotopic data for HFFF from the Dameigou shale formations (Cl/Br ratio (1.81 × 10–4–6.52 × 10–4), Ba/Sr (>0...
0 downloads 15 Views 1MB Size
Subscriber access provided by Eastern Michigan University | Bruce T. Halle Library

Article

Hydrogeochemical and Isotopic Indicators of Hydraulic Fracturing Flowback Fluids in Shallow Groundwater and Stream Water, derived from Dameigou Shale Gas Extraction in the Northern Qaidam Basin Zhaoxian Zheng, Hongda Zhang, Zongyu Chen, Xufeng Li, Pucheng Zhu, and Xiaoshun Cui Environ. Sci. Technol., Just Accepted Manuscript • Publication Date (Web): 09 May 2017 Downloaded from http://pubs.acs.org on May 9, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Environmental Science & Technology is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 34

Environmental Science & Technology

Hydrogeochemical and Isotopic Indicators of Hydraulic Fracturing Flowback Fluids in Shallow Groundwater and Stream Water, derived from Dameigou Shale Gas Extraction in the Northern Qaidam Basin Zhaoxian Zheng,1,2 Hongda Zhang,3 Zongyu Chen,1* Xufeng Li,4 Pucheng Zhu,1 Xiaoshun Cui1 1. Institute of Hydrogeology and Environmental Geology, Chinese Academy of Geological Sciences, Shijiazhuang, 050061, Hebei, P.R. China 2. China University of Geosciences (Beijing), Beijing, 100083, P.R. China 3. Oil & Gas Survey, China Geology Survey, Beijing, 100029, P.R. China 4. Center for Hydrogeology and Environmental Geology, China Geological Suvery, Baoding Hebei, 071051, P.R. China Correspondence to: *Zongyu Chen: Institute of Hydrogeology and Environmental Geology, Chinese Academy of Geological Sciences, No. 268 Zhonghuabei Street, Shijiazhuang, 050061,

ACS Paragon Plus Environment

1

Environmental Science & Technology

Page 2 of 34

Hebei, P.R. China; Phone: +86-311-67598558; Fax: +86-311-67598661; E-mail: [email protected]. TOC Art

ACS Paragon Plus Environment

2

Page 3 of 34

Environmental Science & Technology

1

Abstract: Most of the shale gas production in northwest China is from continental shale.

2

Identifying hydrogeochemical and isotopic indicators of toxic hydraulic fracturing

3

flowback fluids (HFFF) has great significance in assessing the safety of drinking water

4

from shallow groundwater and stream water. Hydrogeochemical and isotopic data for

5

HFFF from the Dameigou shale formations (Cl/Br ratio (1.81×10−4–6.52×10−4), Ba/Sr

6

(>0.2), δ11B (-10–1‰) and εSWSr (56–65, where εSWSr is the deviation of the

7

ratio from that of seawater in parts per 104)) were distinct from data for the background

8

saline shallow groundwater and stream water before fracturing. Mixing models indicated

9

that inorganic elemental signatures (Br/Cl, Ba/Sr) and isotopic fingerprints (δ11B, εSWSr)

10

can be used to distinguish between HFFF and conventional oil-field brine in shallow

11

groundwater and stream water. These diagnostic indicators were applied to identify

12

potential releases of HFFF into shallow groundwater and stream water prior to fracturing

13

and flowback. The monitored time series data for shallow groundwater and stream water

14

exhibit no clear trends along mixing curves towards the HFFF end member, indicating

15

that there is no detectable release occurring at present.

16

1. Introduction

17

87

Sr/86Sr

Energy demand is continually rising in China as a result of rapid economic

18

development. Natural gas consumption in China has increased from 110.5 billion m3 in

19

2010 to 185.5 billion m3 in 2014, and is predicted to increase to ~400 billion m3 in 2030.

20

However, natural gas production in China has only increased from 99.0 billion m3 in

21

2010 to 134.5 billion m3 in 2014, and is predicted to be ~250 billion m3 in 2020.1, 2

22

Because of the large shortfall between supply and demand, more attention is being given

ACS Paragon Plus Environment

3

Environmental Science & Technology

Page 4 of 34

23

to the large reserves of organic-rich shale gas in China. At present, large-scale

24

commercial shale gas development is still in its infancy in China. Nonetheless, the

25

extraction of gas from low-permeability organic-rich shale formations through hydraulic

26

fracturing has dramatically increased with the rapid development of shale gas

27

industrialization and commercialization.3 Hydraulic fracturing flowback fluids (HFFF),

28

which are produced by ubiquitous high-volume hydraulic fracturing, comprise injected

29

and formation water released adjacent to the shale formations.4 HFFF are typically highly

30

saline and contain human-made additives and naturally occurring chemicals at toxic

31

concentrations.5, 6

32

The toxic substances potentially associated with HFFF have led to strong public

33

concerns about their impact on groundwater and surface water during fracturing, storage,

34

transport, and disposal.5 The occurrence of HFFF release can be identified if specific

35

fracturing fluids that are rare in the area and/or artificial chemical tracers added to the

36

fracturing fluid are found in the water under scrutiny. However, patent protection and/or

37

fracturing efficiency is constraining the application of these identification methods.7

38

Therefore, a robust methodology for identifying of HFFF spills is currently a key

39

research topic given the global interest in shale gas extraction. Many studies have focused

40

on the characterization of HFFF from the world’s largest unconventional natural gas

41

play—the Marcellus Formation in the Appalachian Basin, USA. Data from the USGS

42

produced water database reveal that the formation water is typically hypersaline and

43

characterized by a Natrium-chloride composition with high Br/Cl ratios that reflect

44

different degrees of seawater evaporation and water–rock interactions8. Chapman stated

45

that strontium isotopic ratios from the Marcellus Formation HFFF fall within a narrow set

ACS Paragon Plus Environment

4

Page 5 of 34

Environmental Science & Technology

46

of values (87Sr/86Sr = 0.710148 to 0.712119), and observed that this isotopic range is

47

distinct from most oil and gas brines in the Upper Devonian Venango Group associated

48

with western Pennsylvania acid mine drainage9. Warner suggested that trace element

49

ratios (B/Cl >0.001, Li/Cl >0.002) and isotopic fingerprints ( δ11B = 25 to 31‰, δ7Li = 6

50

to 10‰) for HFFF from the Marcellus and Fayetteville black shale formations were

51

distinct in most cases from produced water sampled from conventional oil and gas

52

wells.10 We observed that most shale gas formations in the United States were deposited

53

in an oxygen-deficient marine environment 11 and that in previous studies the major

54

signatures of HFFF are closely linked to the depositional environment and the origin of

55

shale formation water.

56

However, a large number of shale gas plays in northwest China (e.g., the Qaidam,

57

Ordos, Tuha, and Tarim Basins) were deposited in a continental environment,12 and the

58

origin of formation water is different from most shale gas plays in the United States (e.g.,

59

the Marcellus formation water is of ancient seawater origin10). The continental origins of

60

the shale gas plays in northwest China has given the HFFF distinctive hydrogeochemical

61

and isotopic characteristics compared with HFFF produced from the Marcellus marine

62

shales in which the values of 87Sr/86Sr, δ11B and Cl/Br are strongly affected by formation

63

water that originated from evaporated seawater10, 13. Furthermore, the shallow

64

groundwater in arid areas of northwest China is highly saline because of intense

65

evaporation.14 This salinity has masked the presence of HFFF when it has migrated to

66

shallow groundwater and stream water. Finding unique indicators for HFFF is, therefore,

67

critical for distinguishing HFFF salts from autochthonous salts in shallow groundwater

68

and surface water in shale plays in northwest China. These indicators could be used for

ACS Paragon Plus Environment

5

Environmental Science & Technology

69

establishing a diagnostic index system for identifying accidental releases and potential

70

migration pathways during shale gas development in the Qaidam Basin.

71

Page 6 of 34

This study focused on the hydrogeochemical and isotopic characteristics of shale

72

formation water that might strongly influence the composition of HFFF. The variations of

73

inorganic elements such as Br, Ba, Sr, and B in the HFFF that can be affected by the

74

hydraulic fracturing process were also investigated. The injection of fresh water into the

75

shale formations causes the release of elements from exchange sites on the mineral

76

surfaces.10 The injection of fresh water could also alter the isotopic compositions of

77

elements (e.g. Sr and B) in flowback fluids during mixing and water-rock interactions.15

78

The overall aim of the current study is to distinguish the altered HFFF from the shallow

79

groundwater and stream water, and to provides key information for forensic evaluations

80

of HFFF accidental releases. Thus, this study is addressed by three objectives: (1) study

81

the origin of Dameigou shale formation water to characterize its chemical and isotopic

82

compositions; (2) identify the unique hydrogeochemical signatures of HFFF that could be

83

used as environmental tracers differentiating it from conventional oil-field brine (COB)

84

in shallow groundwater and streams; and (3) apply the hydrogeochemical and isotopic

85

tracers as environmental indicators at field sites to distinguish the contamination of

86

shallow groundwater and streams by HFFF, and evaluate the sensitivity of HFFF

87

indicators. Our study investigated the characteristic indexes of HFFF associated with

88

hydraulic fracturing of continental shale for the first time. Our hydrogeochemical and

89

isotopic indicators, including Br/Cl, Ba/Sr, δ11B, and 87Sr/86Sr, are consistent in HFFF

90

found across the Qaidam Basin shale gas play in which the Dameigou shale constitutes

91

the most favorable reservoir for exploitation. When combined with mixing models, these

ACS Paragon Plus Environment

6

Page 7 of 34

Environmental Science & Technology

92

indicators may be applied elsewhere in northwest China to identify shallow groundwater

93

and stream contamination by HFFF in extensive continental shale gas plays.

94

2. Study area

95

The study area lies within the northeastern margins of the oil-bearing and gas-rich

96

Qaidam Basin (Figure 1a). The study area has an arid continental climate with cool

97

summers and frigid winters, with daily highs in the coldest/warmest months (January/July)

98

of −28.0 °C and 28.1 °C, respectively16. Precipitation ranges from 8.4 to 87.7 mm/year

99

across the region, while evapotranspiration varies from 2065 to 3040 mm/year16. The

100

terrain of the study area slopes downwards from the northwest to the southeast. Tectonic

101

erosional mountains and hills with medium to low relief, and alluvial and proluvial plains

102

and valleys are well developed in the study area. The surficial geology is dominated by

103

unconsolidated gravel, coarse sand, and fine sand of the Upper Pleistocene, which ranges

104

from a few meters at the piedmont to ~40 m in the valleys.

105

A few Neogene and Eogene sedimentary strata are exposed downstream of the Naoer

106

River. Chai Ye 1 (CY1) well, the first shale gas exploration well in terrestrial Jurassic

107

sediments in northwest China, was hydraulically fractured at the study area in August

108

2014. The Dameigou Formation (comprising interbedded fractured shale, coal, and

109

siltstone) is part of the Middle Jurassic stratigraphic sequence that was deposited in an

110

oxygen-deficient semi-deep lake environment.17 The Dameigou Formation dips to the

111

south and lies 1920 m to 2150 m below the ground surface in the study area. A typical

112

synthesized columnar section of Qaidam Basin can be seen in Figure S1. The area

113

contains multiple faults and lineaments as mapped in Figure 1b. Confined shallow

ACS Paragon Plus Environment

7

Environmental Science & Technology

Page 8 of 34

114

groundwater in the semi-consolidated Neogene strata is dominated by pore flow through

115

primary intergranular porosity from the topographic highs to the lowland discharge areas.

116

Phreatic pore water in the Upper Pleistocene strata drains as multiple springs along the

117

Naoer River. Generally, the shallow groundwater has a high TDS (>10 g/L) and a Na-Cl

118

composition in both aquifers, although isolated areas of low salinity upstream of the

119

phreatic groundwater are found.18

120

3. Materials and Methods

121

3.1 Sampling

122

Five categories of water samples were used in this analysis: (1) a single sample of

123

formation water was obtained before hydraulic fracturing; (2) five samples (CY1-1d to

124

CY1-7d) of HFFF from CY1 were obtained as a time series from the beginning of

125

flowback; (3) ten samples of shallow groundwater (Neogene aquifer) from two

126

monitoring wells (CJ1, CJ2), located downstream and upstream of CY1, were collected

127

as time series commencing the day after fracturing; (4) four samples from a single spring

128

(CJ3) discharged from the Upper Pleistocene were collected as time series f commencing

129

the day after fracturing; (5) five samples from the same sampling location in the Naoer

130

River (R1) were collected as time series commencing the day after fracturing. The

131

specific day of sample collection from fracturing event can be seen in Table S1.

132

Background samples were also collected before fracturing to monitor potential mixing

133

relationships between the HFFF and the shallow groundwater and stream water.

134

Formation water was collected, before fracturing, from perforated holes made through the

135

casing and cement, and extending into the shale formation, using deep penetrating

ACS Paragon Plus Environment

8

Page 9 of 34

Environmental Science & Technology

136

charges. There are two main Neogene shallow aquifers in the study area: the upper

137

aquifer is composed of semi-consolidated siltstone and is located at a depth of 96.2 to

138

105.0m, and is accessed by monitoring well CJ2; and the lower aquifer is composed of

139

semi-consolidated fine sandstone at a depth of 145.0 to 150.5m, and is accessed by CJ1.

140

HFFF samples labeled 1d through 7d indicate the number of days from the hydraulic

141

fracturing event.

142

Samples were field filtered through a 0.45 µm nylon filter into pre-cleaned HDPE

143

bottles with no head space. Samples for cation, Sr, and B isotopes were acidified with

144

ultrapure concentrated HNO3 to a pH of less than 2. All samples were kept on ice while

145

in the field and refrigerated in the lab at 4 °C until the analyses were completed.

146

3.2 Analytical Methods

147

Major anions, except HCO3−, were analyzed by a Thermo Scientific Dionex ICS-4000

148

(precision, ±1%). The HCO3− concentration was determined by phenolphthalein titration,

149

and major cations and minor elements were analyzed by a PerkinElmer Inductively

150

Coupled Plasma Optical Emission Spectrometer (ICP-OES) Model Optima 8300

151

(precision, ±1%) at the National Research Center for Geoanalysis, Chinese Academy of

152

Geological Sciences. The δ18O and δD measurements were completed on a Picarro

153

L2130-i Analyzer (precision, ±0.025‰ for O and ±0.1‰ for H) at the Institute of

154

Hydrogeology and Environmental Geology, Chinese Academy of Geological Sciences.

155

Strontium and Boron isotopes were measured using an Isotopx Phoenix Thermal

156

Ionization Mass Spectrometry (TI-MS) and PerkinElmer ELAN DCR-e Inductively

157

Coupled Plasma Mass Spectrometry (ICP-MS), respectively, at the Center of Analysis,

ACS Paragon Plus Environment

9

Environmental Science & Technology

Page 10 of 34

158

Beijing Research Institute of Uranium Geology. The average 87Sr/86Sr ratio of NIST

159

SRM 987 over the period of these analyses was 0.710244 ±0.000015 (n= 30). The

160

average 11B/10B ratio of NIST SRM 951 during this study was 4.0436 ±0.0016 (n= 25).

161

The long-term standard deviation of δ11B in the standard through replicate measurements

162

was