## 1 Introduction  
Successful wellbore construction is an exceptionally challenging process and, because a large number of potential problems can occur, drilling processes can be slowed down and in extreme cases the well may be even abandoned [1; 2; 3; 4; 5]. Further, the mentioned problems result in non-productive time, and cause additional cost for companies involved in the drilling process as well. According to some sources, wellbore instability alone causes more than a billion unplanned costs for the oil and gas industry and makes up approximately one-third of all unplanned costs during drilling operations [6; 7; 8; 9]. With the increased depth of drilled wellbores and the increasing number of inclined and horizontal wells, the oil and gas industry faces issues which were not encountered during the drilling of vertical wells [10]. On the other hand, horizontal wells have become a common method that enable economical access to oil or gas reservoirs from existing vertical wellbores, enable access to reserves that were previously out of reach, and reduce the negative impact on environment [11].The main purpose of drilling horizontal wells, i.e., wells whose production section occupies a horizontal position in space, is increasing the contact area of the wellbore with the oil or gas reservoirs [12], thus increasing the productivity/injectivity of these wells. Therefore, despite all the risks arising from this type of wellbore trajectory, in certain cases this is either a better or the only option for the economical production of hydrocarbons from certain underground formations [13; 14]. Also, the capacity of fluid injection is better compared to vertical wells under equivalent differential pressures [15], therefore, horizontal wells are used for EOR projects, and are widely used in offshore projects [15; 16]. The productivity of horizontal wells largely depends on the length of the horizontal section of the wellbore through the production formation [17]. If it is determined that it is cost effective, extended-reach drilling (ERD)is the best variant for the maximum exploitation of the underground formation potential compared to a development of the same reservoir with a few vertical wells. Horizontal wells with an extended reach can have a very long wellbore and a relatively small true vertical depth, so the kick off point can be located shallowly, as shown in Figure 1(5). Also, an extended-reach well can be used as the best option for the development of the offshore field, ensuring distant parts of the reservoirs are reached from one fixed location above the sea. Extended-reach drilling uses directional and horizontal drilling techniques and represents an advanced form of directional and horizontal drilling technology. An extended-reach well can be defined as a well where the ratio between the horizontal reach and true vertical depth is larger than 2, or where the horizontal displacement is greater than 6096 m (20,000 ft) [18; 19; 20]. With the development of the industry, the technical limits that once existed in the construction of this type of well are becoming less of a problem nowadays, and the longest extended-reach well today has a measured depth of 15,240 m (50,000 ft) [18]. Although these records are broken primarily due to the development of the equipment needed to drill such demanding wells, special attention should be placed on the drilling fluid design, as this is probably the most important and most expensive component of the drilling process [21]. Drilling fluid selection for extended-reach drilling must fulfill all the functions needed for a successful drilling process, such as removing drill cuttings from the well to the surface (hole cleaning), this the primary barrier to formation pressure, providing lubrication for the drill string and drill bit, stabilizing the wellbore and ensuring filtration through its walls, and many others [22; 23; 24; 25].