Analysis of interactions between water and shale by water activity

doi:10.1016/j.jngse.2020.103372

Journal of Natural Gas Science and Engineering

Volume 80, August 2020, 103372

https://doi.org/10.1016/j.jngse.2020.103372 Get rights and content

Highlights

•
A comprehensive analysis on shales from the aspect of water activity was provided.
•
The relationship between shale property and water activity was clarified.
•
The relationships between water activity and micro-fracture mechanism and fractal dimension was analyzed.
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The use of low water activity working fluid in shale gas formation was guided.

Abstract

The interaction between shale and water is the key factor of wellbore instability in shale gas reservoirs. In this work, the influence of water on shale is studied from the perspective of water activity. Two kinds of shale samples from Sichuan Basin, Southwest China, were characterized using X-ray diffraction (XRD), total organic carbon (TOC) content tests, swelling ratio tests, field emission scanning electron microscope (FE-SEM), uniaxial compressive strength (UCS) and Nitrogen adsorption experiments. The relationships between mineralogical composition, TOC content, swelling ratio and water activity were studied respectively, and then the effect of water activity on microfractures was discussed. Based on the Frenkel–Halsey–Hill equation, fractal dimensions of shales were obtained by the Nitrogen adsorption experiments. And the relationship between water activity and various factors was analyzed. The results show that the water activity of shale is positively correlated with the total content of clay minerals (TCCM), especially the montmorillonite content. When the water activity of external solution was higher than that of shale, shale samples expanded due to chemical potential difference. The swelling ratio increased with increasing water activity of external solutions and shales. Shale swelling led to the development of microfractures, which in turn caused the decrease of strength. Therefore, under higher water activity conditions, shales were more prone to develop microfractures, and thus had lower strength. The adsorption data indicated that the fractal dimensions of shale samples ranged from 2.75 to 2.82, what was heterogeneous and irregular. And fractal dimension showed a good positive correlation with the water activity of shale. What is more, the study also shows that the use of low water activity working fluid in shale gas formation (especially in the formation with high water activity) can inhibit hydration and maintain formation stability.

Introduction

Shale gas is playing an increasingly important role in the field of world resource generation (Van Oort, 2003; Pan and Connell, 2015; Xue et al., 2016; Zou et al., 2017). China has abundant shale gas resources, about 26 × 10¹² m³ (Zhang et al., 2009). In recent years, both Xujiahe Formation and Longmaxi Formation in Sichuan Basin have been studied as potential hotspot (Dong et al., 2014; Chen et al., 2016; Gai et al., 2016; Xie et al., 2017; Jiang et al., 2018). However, shale usually has large specific surface area and complex pore structure because of abundant clay mineral and wide pore size distribution, which will lead to strong adsorption capacity for water (Labani et al., 2013; Yuan et al., 2014). The interaction between shale and water causes borehole instability, which is one of the most costly and challenging problems during drilling and hydraulic stimulation (Al-Awad et al., 1996; Chen et al., 2003). In order to study the interaction between shale and water, it is still necessary to analyze the basic physical and chemical action of shale after contact with water.

Water activity is the ratio of the fugacity of the water in that system to the fugacity of pure water (Chenevert, 1970a, Chenevert, 1970b). Water activity can reflect the basic properties of complex structure in shale and accurately characterize the reaction between water and shale from the perspective of potential energy (Yew et al., 1992). In terms of macroscopic view, water activity of shale is the result of the overall effect on water adsorption. However, few studies have focused on the analysis of interactions between water and shale from the aspect of water activity. Rafieepour et al., 2015a, Rafieepour et al., 2015b have studied the mathematical modeling works of inclusion of water activity in wellbore stability during drilling through shale formations. Some scholars have observed that when the water activity of external solution was higher than that of shale, swelling would occur and shale would be damaged by hydration (Chenevert, 1970a, Chenevert, 1970b; Lal, 1999; Wen et al., 2015; Donnelly et al., 2016). But the researches mostly have been focused on the experiments of the same shale and the external solution with different water activity, and the relationship between shale properties and water activity is not clear. This present study is to provide a comprehensive analysis on shales with different water activity, and to clarify the relationship between shale property and water activity, which will prepare for the establishment of water activity model of shale formation in the future. Based on the analysis of hydration failure mechanism of shale with different water activity, the use of low water activity working fluid in shale gas field is guided.

This paper takes the water activity of shale as the research object, and macroscopically reflects the water control ability of different shales. The relationships between mineralogical composition, TOC content, swelling ratio and water activity were characterized. Then the effects of water activity on the mechanism of micro-damage and mechanical property change of shale were revealed. According to the calculation and analysis of fractal dimension of shale, the impact of fractal dimension on water activity was also discussed. Furthermore, the relationship between water activity and various factors was analyzed, and the working fluid with low water activity was recommended to be used in shale gas formation.

Section snippets

Materials and experiments

Eight samples were selected from both Xujiahe Formation and Longmaxi Formation in Sichuan Basin (Fig. 1). Samples SX-1, SX-2, SX-3, SX-4 were obtained from Xujiahe Formation and SL-1, SL-2, SL-3, SL-4 from Longmaxi Formation. More detailed information about these two formations can be obtained in Refs (Yang et al., 2014, 2020; Tang et al., 2015; Xie et al., 2017; Jiang et al., 2018).

The samples were evaluated experimentally by water activity tests, XRD analysis, TOC content tests, swelling

Compositional analysis and TOC

The results of XRD analysis results are presented in Table 1. The test samples from both formations were rich in clay minerals and quartz, similar to previous descriptions (Yang et al., 2014; Chen et al., 2017). For Xujiahe Formation, the average quartz content was 42.2% (28.3%–55.9%), carbonate mineral (calcite and dolomite) 7.68% (0%–25.4%), and clay mineral 30.9% (26.3%–36.2%). By contrast, Longmaxi Formation had a lower average content of quartz (24.3%) but higher in carbonate (30.7%), and

Relationships between water activity and the composition and TOC content of shales

The TCCM and TOC content of shales generally have high values, which differ from those of conventional reservoir rocks (Jarvie et al., 2007; Loucks and Ruppel, 2007). The relationships between water activity and TCCM, montmorillonite content, TOC content are illustrated in Fig. 7. The water activity is positively correlated with both TCCM and montmorillonite content. Shale sample with higher TCCM has a higher water activity. The relationship between water activity and montmorillonite content of

Conclusions

In this paper, the water activity of different shales was obtained to reflect the ability of controlling water. The relationships between mineralogical composition, TOC content, swelling ratio and water activity was investigated. Then the effect of external solution water activity on the microfracture formation and strength variation of the same shale was discussed, which was also analyzed for shales with different activity. Furthermore, based on the FHH theory, the correlation between fractal

CRediT authorship contribution statement

Yubin Zhang: Conceptualization, Methodology, Writing - original draft. Zhengsong Qiu: Funding acquisition, Supervision. Gongrang Li: Investigation. Hanyi Zhong: Visualization, Validation. Xin Zhao: Writing - review & editing.

Declaration of interest statement

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The authors would like to acknowledge the financial support of the Major State Basic Research Development Program of China (973 Program) (2015CB251205), Major National Science and Technology Projects (2016ZX05021-004), Project of the National Science & Technology Major Project (2017ZX05032-004-005) and National Natural Science Foundation of China (No.51704322).

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Analysis of interactions between water and shale by water activity

Highlights

Abstract

Introduction

Section snippets

Materials and experiments

Compositional analysis and TOC

Relationships between water activity and the composition and TOC content of shales

Conclusions

CRediT authorship contribution statement

Declaration of interest statement

Acknowledgments

JKSUES

Fuel

Appl. Clay Sci.

J. Nat. Gas Sci. Eng.

J. Petrol. Sci. Eng.

Energy

J. Nat. Gas Sci. Eng.

J. Nat. Gas Sci. Eng.

Fuel

J. Petrol. Sci. Eng.

J. Nat. Gas Sci. Eng.

Fuel

J. Nat. Gas Sci. Eng.

Fuel

Fuel

J. Nat. Gas Sci. Eng.

J. Petrol. Sci. Eng.

J. Nat. Gas Sci. Eng.

Appl. Clay Sci.

J. Nat. Gas Sci. Eng.

Fuel

Int. J. Coal Geol.