**3. Research method**

Fundamentally, we adopt a qualitative and quantitative approach based on a combination of literature review and empirical analysis of patent filings. As a matter of fact, adopting a pure quantitative approach is not feasible, as the extant literature does not offer clear conventions for analyzing patent information related to upstream oil and gas technologies, such as the widely accepted and well-known rules of algebra through which the validity of mathematical deductions is known [47]. A fundamental characteristic of a qualitative approach, in turn, is that researchers may have less a priori knowledge of what the variables of interest will be and how they will be measured [48]. Accordingly, qualitative researchers are sometimes disposed toward causal determination of events but more often tend to perceive events not simply and singly caused (Stake [50]). Consequently, the combination of qualitative and quantitative approaches can be very synergistic [49].

In practice, we collected data representing the patent applications in Brazil for the period between 2001 and 2012. The choice for this period was motivated by the research objective of this study, which is aimed at improving understanding of the major technological trends associated with the upstream oil and gas industry in Brazil. Additionally, given the fact that the Brazilian patent legislation establishes that a patent application must be published 18 months after the filing date in combination with administrative delays to classify patent applications, patent data concerning the years 2013, 2014, and 2015 were not completely available during of the data collection phase of this study.

The figures regarding Brazilian patent applications were retrieved by executing queries within a system named "Sistema Integrado da Propriedade Industrial" (SINPI). SINPI regards an internal information system that processes administrative information for "Instituto Nacional da Propriedade Industrial" (INPI), which is the Brazilian national patent office. The queries concerned patent applications with filling dates between January 1, 2001, and December 31, 2012. These queries specified the patent applications classified with the International Patent Classification (IPC) scheme in the technical area of E21B, as this particular area covers a wide portion of technologies associated with upstream oil and gas exploration, such as earth or rock drilling obtaining oil, gas, water, soluble or meltable materials, or a slurry of minerals from wells.

Accordingly, E21B was selected for being a highly representative IPC subclass for the upstream oil and gas industry. The collection of patent information on patents describing technologies used in the upstream oil and industry, which are classified as E21B, was analyzed based on patent classification listed in the documents. More specifically, we have examined the total number of documents further classified in terms of the groups that belong to the E21B subclass. This decision enabled us to identify the most active technical fields within the E21B subclass.
