A new approach to study the friction-reduction characteristics of viscous/conventional slickwater in simulated pipelines and fractures
Introduction
Shale oil and gas are among the most important energy resources in the world (EIA, 2016). Due to the low permeability of shales, treatments like horizontal well drilling and hydraulic fracturing are often taken to reach commercial production. Different methods are used to fracture unconventional and conventional reservoirs: complex networks composed of fractures with relatively low permeability are usually created in the former type of reservoirs, while long and highly permeable fractures are usually created in the latter (Guo et al., 2018; Song et al., 2015; Zhang et al., 2019a, 2019b). Numerous indoor and wellsite operations have shown that, compared with the fluids used in traditional hydraulic fracturing, those used in slickwater fracturing facilitate the formation of complex fracture networks in unconventional oil/gas reservoirs (Warpinski et al., 2008; Scanlon et al., 2014; Gallegos et al., 2015). Slickwater is a water-based fracturing fluid that contains a friction reducer (typically, a high molecular weight polypropylene or its copolymer). The friction reducer usually exists in the form of an oil-in-water emulsion, containing polymers in both their water and oil phases. When a friction reducer is added to water, the emulsion quickly reverses and releases polymers into the water, forcing its further hydration. Other types of additives (e.g., flowback aids, clay stabilizers, biocides, and scale inhibitors) can also be added to slickwater (Liang et al., 2017; Zhao et al., 2018).
The friction reducer's concentration of a conventional slickwater can vary between 0.05 wt% and 0.20 wt%, while the viscosity of the base liquid oscillates between 1 mPa s and 7 mPa s; these characteristics show as a 70–80% reduction of the friction during the pumping process (Zhao et al., 2018; Li et al., 2018). Compared to guar gum fracturing fluids, slickwater implicate a lower residue damage but lesds to a lower sand-carrying capacity. The sand-carrying capacity can be improved by increasing the pump speed and the following turbulent flow; this process, however, also implies a higher friction loss (Sun et al., 2014, 2017). To overcome the shortcomings of conventional fracturing fluids, a new viscous slickwater to be injected at a relatively low pressure was proposed; this slickwater implies a low friction reduction and good sand-carrying properties as well. Shales fractured in this way can be broken easily (without leaving solid residues) and get a particularly high fracture conductivity (Zhao et al., 2018).
Until present, relatively few studies have focused on viscous slickwater. In recent years, some authors have summarized the applications of viscous slickwater in unconventional reservoirs and statistics on their concentrations, viscosities, and temperature ranges (Zhao et al., 2018; Johnson et al., 2018; Brannon and Bell et al., 2011; Van Domelen et al., 2017). The viscoelasticity, static sand-carrying performance, and friction-reduction properties of viscous slickwater have also been studied, while their dynamic sand-carrying properties were not fully considered (Aften, 2018; Motiee et al., 2016; Quintero et al., 2019). Additionally, other authors have studied the viscoelasticity, dynamic sand-carrying visualization process, and friction-reduction performance of viscous slickwater (Ba Geri et al., 2019; Hu et al., 2018 & 2019; Dahlgren et al., 2018).
In this paper, the friction-reduction performances of a conventional friction reducer (FR-800) and a highly viscous friction reducer (HVFR-900) in a pipeline and fractures were tested through independent R &D friction loop and visual crack modules. First, we calculated the friction factor (λ) based on experiments; then, we fitted the friction factor equations for FR-800 and HVFR-900. Finally, we observed the corresponding dynamic sand-carrying properties by means of a visual crack module.
Section snippets
Friction reducers
For this study, we selected a conventional friction reducer (FR-800) and a highly viscous friction reducer (HVFR-900), both consisting of polyacrylamide or polyacrylamide copolymers. The friction reducers (0.05–0.60 wt%) were both prepared with fresh water in the laboratory under room temperature. We then measured their shear stress at different concentrations using a ZNN-D6 type six-speed rotating viscometer (Fig. 1, Fig. 2).
The rheological curves of FR-800 and HVFR-900 at different
Relationship between pressure drop and linear flow rate in the pipe
The frictional pressure drops of FR-800 and HVFR-900 measured in the 8-mm tube are shown in Fig. 7, Fig. 8. Under the same linear flow rate and an increasing concentration of FR-800, the pressure drop in the pipe gradually increased: the friction-reduction effect gradually weakened for increasing concentrations of friction reducer. The opposite mechanism was observed in the case of HVFR-900, indicating that the friction-reduction effect was enhanced for increasing concentration of this friction
Conclusions
In this paper, the friction-reduction performances of a conventional friction reducer (FR-800) and a highly viscous friction reducer (HVFR-900) in pipes and fractures were tested by using independent R &D friction loop and visual crack modules. First, we calculated the values of λ based on experiments; then, we fitted the corresponding equations for FR-800 and HVFR-900 in the pipe and fracture; finally, we analyzed the dynamic sand-carrying properties of FR-800 and HVFR-900 through the visual
Credit author statement
Special gratitude is owed to Professor Zhou Fujian and Hao Bai. Professor Zhou Fujian, as the corresponding author of this paper, provided research ideas, funding and laboratory; Wang Jie is the data analysist and first author; Hao Bai, Jilong Ma, Pengcheng Yang, Feng Zhang, Lishan Yuan and Dongya Wei are the key accomplishers of the laboratory experiment.
Declaration of competing interest
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
This work is financially supported by the National Science Foundation of China (Grant No. 51804033), the National Science and Technology Major Projects of China (Grant Nos. 2016ZX05051, 2016ZX05014-005, and 2017ZX05030).
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