# Green Approach in Water-Based Drilling Mud Design to Increase Wellbore Stability
## 1 Introduction
(2003), the probability of instability increases as the angle between the axis of the wellbore and the dip angle (undip or downdip) of the formation layer decreases, while the greatest stability is achieved when the formation layer is drilled through at an angle of 90\({}^{\circ}\). When designing a well, the relationships between the individual components of the local (in situ) stresses and their orientation must be taken into account [4]. Simultaneously with the process of stress redistribution on the wellbore walls that occurs during drilling, physicochemical interactions between drilling mud and rock may also occur. Various mechanisms that destabilize the wellbore can occur if the composition of the mud is not properly defined. Wellbore instability caused by physicochemical causes is most commonly associated with shale rocks due to their high clay mineral content and extremely low permeability [14; 15; 16]. Shale is usually composed of the following minerals: quartz, calcite, and clay from the smectic group (mostly montmorillonite), illite, chlorite, and kaolinite in varying proportions [3; 17; 18]. In shale, which comprises a significant amount of clay in its composition, hydration can occur during the drilling of this formation, which is a significant obstacle to successful well construction. In addition, shale is composed of mixed clay minerals in varying proportions, and their hydration behavior is difficult to predict. The behavior of shale as a rock directly depends on the behavior of a particular type of clay mineral. Each of these clay minerals has a specific crystalline structure that also determines its reaction with the mud, especially in water-based mud with respect to the reactivity of water with the clay minerals [3]. This problem is even more significant, because this type of mud is the most commonly used for drilling [19; 20; 21; 22], as it is more environmentally friendly and cheaper than oil-based muds. Due to the relatively low permeability of the shale, mud cake is not formed on the wellbore walls [23]. The direct consequence of the nonexistence of mud cake is the rapid movement of water molecules into the pore space of the shale and an increase in pore pressure in the near-wellbore zone over time. In addition to the increase in pore pressure due to unfavorable interactions between mud and shale rock, there may also be changes in the mechanical properties of the rock, such as strength and Young's modulus [24; 25]. According to literature, 90% of all well stability problems occur when drilling through shales, formations that account for 75% of all formations through which a well is drilled [8; 26; 27; 28; 29]. The contact of shale with water from mud can cause shale hydration, changing the volume of the rock and reducing its cohesive strength. The excessive hydration of shales during drilling can lead to several problems. The main problem is the swelling of clay minerals, which creates a condition for the occurrence of wellbore instability. In addition to wellbore instability, there are other negative consequences of excessive hydration during drilling, such as excessive mud viscosity, stuck pipes, lower mechanical drilling speed, and bit balling [30; 31; 32]. However, it is very difficult to single out hydration as the cause of shale swelling and wellbore instability. In addition, several different processes occur during the contact between mud and rock, such as the aforementioned hydration (adsorption or absorption of water), ion diffusion flow (change in the interlayer space of the clay particles), mud filtrate flow due to overpressure in the wellbore, and capillary action [11]. A common feature of these processes is the movement of water and ion molecules into or out of the shale, and that they occur simultaneously. In some situations, different mechanisms can simultaneously cause water and ions to move in opposite directions. These processes continue until equilibrium is reached between rock and mud.