# Extended-Reach Drilling (ERD)--The Main Problems and Current Achievements
## 4 Analysis of the Presented Case History Data
### Well Trajectory  
Although the first statement defined extended-reach wells as wells with a ratio between the horizontal reach (departure) and true vertical depth larger than 2, today, those wells can be categorized as horizontal wells. Figure 5 presents the cumulative data for 26 analyzed wells from Table 2, categorized by authors as extended-reach wells. Because there is a lack of data about horizontal reach (departure) for the analyzed wells, the measured depth (MD) and true vertical depth (TVD) were put in a relationship and compared. It is important to emphasize that some authors use exactly this ratio for the classification of extended-reach wells.  
It is evident from the analyzed data shown in Figure 5 that 24 wells have a ratio between the measured depth (MD) and true vertical depth larger than 2, and that this ratio is larger than 5 for 4 of them. According to the last news, Abu Dhabi National Oil Company (ADNOC) this year drilled the longest extended-reach well at its Upper Zakum Concession, with a measured depth of 15,240 m (50,000 ft) [91], overcoming the world record achieved in 2017 in the Chaivo field in the Sea of Okhotsk [92]. Unfortunately, there is no additional information about those two wells, especially detailed information about the well construction, which makes it impossible to compare these boreholes with previously made ones. Furthermore, today, operators use the directional difficulty index (DDI) as the measure for assessing how difficult it is to drill a certain extended-reach well rather than a simple ratio between the horizontal reach (departure) and true vertical depth. Calculating the directional difficulty index information about the total depth of the well, along-hole displacement and total vertical dept is necessary, as well as information about the tortuosity.  
The cumulative information about the true vertical depth (TVD) of the analyzed wells from Table 2 are presented in Figure 6.  
Figure 5: The ratio between measured depth (MD) and true vertical depth (TVD) of 26 analyzed wells.  
It is evident from the presented data that 12 wells from the analyzed 28 wells with available data have a true vertical depth (TVD) between 1000 m (3280.84 ft) and 2000 m (6561.68 ft), and 12 of them have a TVD between 2000 m (6561.68 ft) and 3000 m (9842.52 ft). Only 4 wells have a TVD larger than 3000 m (9842.52 ft) but no more than 4000 m (13,123.36 ft). These data are in accordance with the main task of drilling an extended-reach well, drilling the well at a relatively shallow depth with a large horizontal departure.  
The measured depth (MD) is the only element of the well trajectory available for all 30 analyzed wells (Figure 7).  
From the data shown in the picture, it can be concluded that half of the analyzed extended-reach wells have a measured depth (MD) larger than 8000 m (26,246.72 ft), and almost a third of them have a measured depth (MD) larger than 11,000 m (36,089.239 ft). In addition, it is important to note that almost two thirds of the analyzed extended-reach wells have a kick-off point (KOP) at a depth of less than 650 m (2132.55 ft).