# Extended-Reach Drilling (ERD)--The Main Problems and Current Achievements
###### Abstract
Figure 1: Different well types (Reprinted with permission from Ref. [2]. 2023, Croatian Academy of Engineering). With developments and improvements in horizontal drilling and completion technology, there is a constant tendency for operators to drill ever longer lateral wells to increase hydrocarbon production [9]. Drilling a horizontal extended-reach well (ERW) with the shortest time is of great significance for drilling safety, reducing or eliminating reservoir damage, efficiency and economic benefits [10]. This technology enables access to a new energy source that was previously technologically challenging to exploit, such as natural gas hydrate [11]. The open-hole extended-reach limit (the maximum measured depth of the horizontal ERW) is mainly dependent on the annular pressure drop and the fracture pressure of the drilled formation, and the related equivalent circulation density (ECD) of the used drilling fluid. The success of extended-reach well drilling is not only delivering excellent results in maximizing the length of the wellbore and hydrocarbon production, but also in minimizing environmental effects and surface disturbance because fewer wells are needed and surface sites have a smaller footprint [12]. In addition, there is the possibility of drilling more extended-reach/multilateral wells and developing the field from one drilling pad location, significantly reducing the environmental impact of the exploration and production activities. Additionally, the use of special design systems for directional drilling, such as rotation steerable systems (RSSs), results in a more in-gauge hole than drilling with other systems, as well as a precise control of the well trajectory. Thus, this results in lower volumes of drilled cuttings waste and a lower drilling fluid loss [12]. In order to be in line with the project goals, especially during the drilling of the designer well with a complex trajectory, it is necessary to ensure continuously bidirectional communication between the surface location and the equipment on the bottom of the well. The need to transfer the large quantity of information from the bottom hole measuring equipment up to the surface, as well as the need to send information to the steering mechanism in the opposite direction, push forward the development of new data transfer systems. Although mud pulse telemetry is still the main data transfer mechanism in directional drilling operations because of its low data transmission capacity, engineers are trying to further improve it and develop a new data transmission system to meet the new requirements related to the bidirectional transfer of the large amounts of data [13; 14; 15; 16]. Since the  
Figure 2: The ratio of measured depth vs. the true vertical depth and reach for wells such as BD-04, M-16, OP-11 and Z-44 and other wells presented in Table 2.  
maximum capacity for data transfer with mud pulse telemetry has been overcome, further development is in the direction of using wireline drilling tools (now second generation) as well as electromagnetic and acoustic waves. The recent downhole measure-while-drilling technology also allows for the storage of a large quantity of data and subsequent data retrieval once the bottom hole assembly (BHA) is pulled out of the hole. However, well-bore trajectory management during the drilling of the extended-reach wells includes the use of the measure-while-drilling and logging-while-drilling units, which require the application of real-time communication between the BHA and the surface for transferring large amounts of data. Real-time communication is also important for fully automating and optimizing the drilling process, significantly reducing drilling problems and overall costs [17].