## 2 Theory of Pre-Operational Testing  
One of the main problems during the drilling process is the excess of hardness and abrasiveness found in pre-salt carbonates. These characteristics can lead to early drill bit teeth-cutter breakage/dullness/wear [43]. All these events affect the drilling efficiency in pre-salt operations. The drilling efficiency is directly proportional to the total non-productive time (NPT) [46]. Usually, the NPT represents one-third of the total rig operational expenditure [47]. A statistical analysis in the Gulf of Mexico showed that in 10 years, more than 12% of the NPT was due to a loss of circulation and about 18% was a result of kicks and wellbore instabilities [48]. A similar analysis showed that lost circulation can increase drilling costs by USD 70 to 100 per foot [49]. In this context, there is still room for improvements in the solutions based on computational tools and more automated activities, which may be applicable not just to pre-salt fields but also to the industry overall, from a global perspective, and for the exploration/exploitation of other natural resources [43]. Thus, considering the actual global scenario of the O-G industry, there is a need to search for ways to increase the efficiency through the development, implementation, and deployment of novel and innovative processes and products [43]. To overcome these obstacles, a methodology to achieve the ideal drilling parameters has to be proposed. The selection of the ideal drilling parameters can lead to the most desirable ROP, allowing for a reduction in the energy consumption and NPT. The drillability of the formations can be estimated through pre-operational tests, such as the drill rate test (DRT) and drill-off test (DOT). The DRT and DOT methodology were developed in the 1950s to reduce the drilling time by determining the ideal parameters (WOB and RPM) for drilling a specific formation. In addition, the methodology allows for simulations and tendency estimations, enabling a better understanding of the possible parameters and design combinations for operation optimization and efficiency enhancement [50]. Through the DRT, the following questions can be addressed: (a) how large of an increase in the WOB value would be effective and efficient? (b) how much of an increase would lead to unnecessary machinery overload? The WOB and RPM are critical parameters that must be monitored and optimized in real-time during the drilling process [46]. Other parameters such as the torque, stand-pipe pressure (SPP), mud weight, and mud rheological properties also play a critical role in achieving a better ROP. However, these parameters cannot be manipulated in real-time, since they are a function of the formation type, lithology, and temperature, among others [51]. The pre-operational test is realized at the beginning of a new phase of the drilling process or formation at a short depth interval, and consists in continuously increasing the WOB, maintaining approximately fixed RPM values (which always vary for another test run with a specific given, fixed RPM) [42]. It is basically a step-by-step process of varying the drilling parameters to maximize the ROP and determine the \"founder point\", also known as the \"foundering point\". The founder point is defined as the point at which the ROP stops responding positively to an increase in the WOB and/or RPM [46]. The ROP versus WOB graph is shown for the values obtained in Figure 1.  
The curve has three well-defined regions:  
* Region I: When starting the drilling process, it is necessary to increase the WOB value, i.e., there is only drill bit penetration after a determined WOB value. This way, an increase in the ROP is the result of a disproportionate increase in the WOB;
* Region II: This is defined by a constant slope line with the highest possible efficiency, where an increase in the WOB results in a proportional increase in the ROP;
* Region III: This is defined as the region in which the ROP stops responding linearly with an increase in the WOB.