**1. Introduction**

Unconventional petroleum has gathered grea<sup>t</sup> interest in recent times, especially with growing concerns over the depletion of conventional petroleum sources. In a few countries, unconventional petroleum is already being produced economically along with a steady growth in the industry.

In the US, unconventional petroleum, such as shale oil and tight gas, has seen an increase in production amounts as energy resources. In 2008, shale gas and tight oil constituted as much as 16 and 12% of production of natural gas and crude oil in the US, respectively. In 2018, these production amounts reached 70 and 60%, respectively [1]. This economical production of shale petroleum (shale gas and tight oil) can be considered as an achievement of technological advancement in shale petroleum. Despite the economic benefit of shale petroleum, the environmental problem is the most important factor that could pose a threat to shale petroleum production. According to Cooper et al. [2], shale petroleum causes greenhouse gas (GHG) emissions, water overuse, and local issues around the production site.

According to Holditch [3], the mechanisms for producing unconventional petroleum versus conventional petroleum are distinguishable in two ways. First, increasing the permeability of underground formation is a critical mechanism to produce unconventional petroleum such as shale gas and tight oil, which typically are un-permeable formations that contain petroleum.Second, reducing the viscosity of hydrocarbons is a critical mechanism to produce unconventional petroleum such as heavy oil and oil shale. Permeable underground formations contain excessively viscous petroleum, making it difficult for this petroleum to flow into an apparatus or equipment. Therefore, producing unconventional

petroleum of a high viscosity requires a method that makes petroleum flowable by using thermal energy and other methods. For example, steam-assisted gravity drainage (SAGD) supplies thermal energy to petroleum, such as bitumen or heavy oil, by injecting it with high temperature steam [4]. Furthermore, producing unconventional petroleum in tight formation requires a method that makes the formation permeable by connecting the production well with the reservoir. For example, shale gas and tight oil have been economically produced by adopting horizontal drilling and hydraulic fracturing [5].

Geny [5] suggested that the factors responsible for the successful and economical production of shale petroleum are the advanced technologies of hydraulic fracturing and horizontal drilling, and the combination of the two technologies. Construction of horizontal wells began in 2003 in the US. The industrial growth of shale petroleum started approximately 4 years since the start of construction. Furthermore, this technological development, which initiated the industrial growth of shale petroleum, has contributed toward associated results such as diversification in the operation strategies of energy companies [6], and regional growth in income and employment [7].

According to previous studies [8–15], production technologies of shale petroleum have grown since production began. Kim and Lee [8] argued that productivity grew by 1.9%, while cumulative production of shale gas doubled from 2008 to 2016. However, the prices of natural gas and crude oil declined in late 2008, and the price of crude oil declined in late 2014. Moreover, the price of natural gas has settled down under 4 dollars per million British thermal unit [16,17]. Due to the decline in the prices of natural gas and crude oil, questions regarding the economic feasibility of producing shale petroleum have cropped up. Even with the decline in the prices of natural gas and crude oil, the productivity gain by developing technologies seemed stagnant until 2013 [9]. In addition, during the period with low commodity prices, some producers tried to gain productivity by changing the proportion of oil (or gas) in the production portfolio [10], or cutting the service costs [11]. Analyzing data from North Dakota's Bakken shale formation, Covert [12] argued that shale petroleum producers have slowly and insufficiently improved their production skills until 2011. Nevertheless, the shale petroleum industry has continued to increase its annual production. Moreover, some factors, such as improvements in the decline curves and increase in the lateral length of the wellbore, indicate that the development of production technologies has affected productivity from 2013 to 2016. The optimization of the production of a well could be conducted after its completion by refracturing the reservoir [14]. West [15] reviewed the improvement in productivity and the issues in production technologies by analyzing the topics of technical papers on shale petroleum from 2018 to 2019. The study found promising technologies such as enhanced recovery, digitalization of instruments, machine learning, advanced modeling, and method or apparatus for solving parent-child issues.

Furthermore, recent research [18–22] showed the various ways of technological development in the shale industry. Davarpanah [18] tested and suggested rheologically effective formate fluids for shale formation by composition. Davarpanah and Mirshekari [19] suggested enhancing gas recovery for shale formations based on a model with improved prediction accuracy on diffusivity in carbon dioxide and methane akinetic absorption. Hu et al. [20] empirically showed improved oil-recovery enhancement for tight reservoirs from an optimized injection method of foam and brine components. Hu et al. [21] also showed a trade-off between carbon dioxide-injection and oil-recovery for enhancing recovery. Jin and Davarpanah [22] suggested water treatment techniques that reduced water use by at least 70% for the enhanced oil-recovery method.

Owing to the arguments calling for productivity gains in the shale petroleum industry, research on the technological development of shale gas has been of interest and has been carried out primarily via patent analysis. International Science and Economic Development Canada (ISED) [23] called for the need for a technological field of shale petroleum production and for a major developer in the technological field. The author summarized that major petroleum companies in the US specialize mainly in technologies such as well casing drilling, fracturing formation, drilling formation, data detection, and determining data. Wei et al. [24] observed that US petroleum companies hold the majority of the technologies, such as equipment and device for drilling, extraction exploration, feed purification, and technology for digital simulation. Kim and Lee [25] argued that the critical technological aspects of the shale petroleum industry, such as obtaining resources, investigation, and data processing, have been actively developed from 2010 to 2016.

This study describes the development of technologies of directional drilling (DD) and increasing permeability (IP), which are core technologies of the shale industry, in terms of convergence with smart systems (SS). This is considering that the shale industry's technological developments were focused on productivity improvement. This study found the following: First, the intensity of technological development, measured by the incidence of patents, increased significantly from 2012 to 2016. This point is concordant with the results of Sandrea [9] who observed that there was progress in the productivity of shale petroleum technology. Second, since 2012, a high proportion of technological developments of DD have been developed in the form of convergence with SS. Third, IP technology had a low tendency to converge with DD and SS, but both the number of patents developed and the complexity of the technology were the highest. Furthermore, "reinforcing fractures by using prop" appeared to be the most critical field of IP technologies since 2012. This result is consistent with the results of Shah et al. [14] who also found that productivity improvement was achieved through reinforcing in the shale industry.

#### **2. Data and Methodology**

This study investigated the development of technologies of shale petroleum by analyzing patent data. This study utilized patent data related to production technologies of shale petroleum, such as IP, DD, and SS, from 1997 to 2016. For its analysis, this study calculated the association strength and betweenness centrality by utilizing the most finely distinguished scope (full digit) of technological index (TI) of patent, such as international patent classification (IPC) and cooperative patent classification (CPC).

The analysis of this study has some features. This study focuses only on a portion of technologies of the shale petroleum industry from the data collection stage, focusing on only 26 of the approximately 3900 technology indices. Thus, there are limitations to presenting comparative analysis of various technologies, and to presenting new technologies in an exploratory manner. Still, this study has some advantages. It focuses on the critical technologies of the shale petroleum industry and has the advantage of using association strength, instead of cosine similarity, as the similarity measure. According to Eck and Waltman [26], association strength is an unbiased measure compared to other similarity measures, such as cosine similarity. This is because association strength is not substantially correlated to the occurrence of input data. However, cosine similarity is positively correlated to the occurrence of input data. That is, a frequently occurring TI tends to have higher similarity than a less frequently occurring TI. Lastly, this study distinguishes and presents the results by technological domains, and suggests the results in a numerical and visualized form.
