# Green Approach in Water-Based Drilling Mud Design to Increase Wellbore Stability
## 1 Introduction
Whether and to what extent such processes develop primarily depend on the mineralogical composition of the clay rock, but also on the properties of the mud in contact with the rock. In order to avoid the physicochemical causes of wellbore instability, the composition of the mud used when drilling through a particular rock must be precisely defined. Although a certain composition of drilling mud gives satisfactory results in wellbore stabilization, recent research indicates that the problem has not yet been completely solved. The most commonly used additives to prevent clay hydration are various types of salts such as KCl, NaCl, NH\({}_{4}\)Cl, or CaCl\({}_{2}\), and permanent inhibitors such as quaternary amine polymers [3]. When drilling through rocks that are prone to wellbore instabilities, mud composition can be optimized to have the lowest possible filtration value to minimize the amount of filtrate that penetrates the rocks of the near-wellbore zone, reacting with the clay components. However, a small amount of water always penetrates the rock before the mud cake forms, if it is created at all, as in the case of drilling through the shale formation. Therefore, it is necessary to minimize filtration by using additives such as starch, gum, and cellulose. Since some commercially available water-based drilling mud additives fall into the category of environmentally hazardous substances (NaOH, KOH, K\({}_{2}\)SO\({}_{4}\), polyamine, chromium-containing thinners, many shale stabilizers and mud loss additives, etc. ), the new stricter environmental norms require the development of new ecofriendly additives for the adequate control of drilling mud properties with minimal environmental impact [33; 34; 35]. In the last decade, researchers have conducted laboratory tests to examine if waste materials can be added to water-based mud to optimize filtration properties. Table 1 lists food waste that has been used to optimize the filtration properties of water-based mud. Most of the tests were conducted with bentonite-based mud (bentonite suspension), which mainly consists of water, bentonite, and additives for pH adjustment, so Table 1 does not provide information on the composition of each mud type. Although the additives were added at different concentrations, most of the examined additives generally reduce API filtration by increasing the concentration, and Table 1 shows the highest measured value. Exceptions are green olive pits, which decrease API filtration up to a concentration of 0.75% of water volume and then increase it [36], and rice husk ash, which decreases API filtration up to a concentration of 9.6% and then increases it [37]. The range of filtration reduction for different waste materials is thus very wide, ranging from 9% to 68% for mandarin peel powder. To gain better insight into the potential of using these additives to optimize drilling mud, additional properties need to be tested. The effect of waste materials on rock swelling has not yet been studied in detail. [PERSON] et al. (2020) measured the swelling of a bentonite pellet in a pomelo powder solution. The pellets were compressed at 34.48 MPa (5000 psi) for 30 min. The concentration of the pomelo powder was 0.3, 0.5, 0.7, and 1% of the volume of water added to the base mud consisted of 600 mL of water and 18.6 g of bentonite. They concluded that, as the concentration of pomelo powder increased, the swelling of the pellet decreased to about 62, 60, 50, and 48% after 24 h compared to the swelling in distilled water (about 70%) [38].