**4. Conclusions**

This study attempted to shed light on technological development of the US shale petroleum industry. Here, this study focused on developments and convergences of critical technologies such as directional drilling, increasing permeability, and smart system. To analyze the technological progress, this study measured association strength as relatedness of technological components, described technologies as network of technological components, and measured betweenness centrality as priority of technological components. The results can be stylized as follows: First, the technological developments have been intensively conducted since 2012. Second, the development of DD technologies has been closely related to SS from 2012 to 2016. Except for CDS, the development of converged technologies is lower, considering their intensity and variety, compared to unconverged technologies. Third, the IP technologies are less converged with the other technological domains of direction drilling and smart system. However, IP technologies have intensively developed with higher complexity, more components, and number of inventions than other technological domains. Fourth, some technologies are more significant in the form of converged technology. Horizontal drilling (DD046) is significant in the form of CDI. Tools locking sections of underground apparatus (DD067) and underground automatic control systems (SS005) are significant in the form of CDS.

This study has limitations as its focus is restricted to only some technologies of the shale petroleum industry. However, this study suggests specific information for the respective technologies by using the most specific level of data (full digit of TI). Thus, further investigation is required that analyzes other key technologies of the shale petroleum industry by using the most specific level of data.

**Author Contributions:** Conceptualization, J.-H.K. and Y.-G.L.; methodology, J.-H.K.; software, J.-H.K.; validation, J.-H.K. and Y.-G.L.; formal analysis, J.-H.K.; investigation, Y.-G.L.; resources, Y.-G.L.; data curation, J.-H.K.; writing—original draft preparation, J.-H.K.; writing—review and editing, Y.-G.L.; visualization, J.-H.K.; supervision, Y.-G.L.; project administration, Y.-G.L.; funding acquisition, Y.-G.L. All authors have read and agreed to the published version of the manuscript.

**Funding:** This paper was funded by Inha University and National Research Foundation (NRF-2018S1A3A2075175).

**Conflicts of Interest:** The authors declare no conflict of interest.
