# Green Approach in Water-Based Drilling Mud Design to Increase Wellbore Stability
## 1 Introduction  
The successful construction of a wellbore is an extremely challenging technical and technological process, as each well presents its own specific challenges. Due to a number of potential problems, drilling can be significantly slowed down, and, in extreme cases, the well may be abandoned, resulting in significant financial losses. With the increasing depth of drilled wells and the increasing number of horizontal extended reach wells, the oil and gas industry faces many issues that were not as pronounced in vertical wells. According to the available data, solving problems related to wellbore instability annually costs the oil industry between USD 0.5 and 1 billion [1, 2, 3]. The most complex problems in the technology of deep well drilling arise from the disturbed stability of the wellbore walls due to the effect of various factors. Wellbore instability is defined as any undesirable change in the diameter (narrowing or widening) of the wellbore relative to the diameter of the drill bit used to drill a particular section of the wellbore, and is one of the major problems in drilling operations. The consequences of wellbore instability can vary, such as the difficult cleaning of the well, the demanding execution of cementing operations and logging measurements, problems with drill string tripping or casing run-off, which ultimately leads to an increase in the time planned for drilling, and the need to allocate additional resources to eliminate the negative consequences [4, 5, 6, 7, 3]. The causes of wellbore instability are numerous and vary depending on how they affect the stability of the wellbore. In most cases, wellbore instability is the result of severalcauses acting simultaneously. They are usually divided into two groups [5;8]: mechanical and physicochemical causes. Figure 1 illustrates different situations that can cause wellbore instability. Mechanical causes of wellbore instability result from the mechanical properties of drilled subsurface formations and from changes in stresses on the wellbore walls, while physicochemical causes are due to the interaction between rock and drilling mud. This classification can only be considered to be a framework, while wellbore instability is usually in reality the result of both groups of causes [9; 10; 11]. To understand the mechanical causes of wellbore instability, it is necessary to compare the state of the rock immediately before and after the wellbore is drilled through. From a rock mechanics point of view, the rock is in a natural stress equilibrium before drilling. This equilibrium state of the rock at a given depth is primarily the result of overburden (lithostatic) pressure, but also of various additional stresses resulting from tectonic activity. The moment at which the drill bit penetrates a particular rock, the existing equilibrium state is disturbed, and a new stress system is created on the wellbore walls and in the zone close to the borehole. The new stress distribution is due to a certain drilled rock volume being replaced by a mud column with a certain density. Axial (\(\sigma_{\text{a}}\)), radial (\(\sigma_{\text{r}}\)), and tangential (\(\sigma_{\text{t}}\)) stresses occur at the wellbore walls, while the original local (in situ) stresses continue to act at a certain distance from the well [12]. The values and stress distributions on wellbore walls depend on the combined action of mechanical, chemical, thermal, and hydraulic effects [13]. Another form of instability that can occur during drilling is rock fracture or  
Figure 1: Examples of situations that can cause wellbore instability (according to [3]). tensile failure, which occurs when the effective value of any of the three principal stresses exceeds the tensile strength of the rock. In addition, the angle connecting the axis of the wellbore to a particular subsurface formation layer plays an important role in the stability of the wellbore. According to [PERSON] et al.