Rice husk ash as a sustainable and economical alternative to chemical additives for enhanced rheology in drilling fluids

Performance evaluation of drilling fluids is essential for a successful drilling project, as they not only remove drill cuttings but also prevent undesired penetration or outflow of formation fluids by sealing off wellbore walls. However, concerns have been raised about the use of chemical additives in drilling fluids due to their toxicity and non-biodegradability. To this end, agricultural waste materials are recognized as a promising alternative as they are cost-effective, environmentally sustainable, and can be used as a substitute for lost circulation materials. Rice husk ash (RHA) has become popular as an additive due to its renewable characteristics, including its large surface area, silica content, and microporous structure. This research article explores the rheological properties of drilling fluid with RHA as a filter control medium. The results showed that increasing concentrations of RHA in the drilling mud significantly improved its rheology, particularly at higher concentrations (15 and 20 wt.%). The addition of RHA modified the filtration and rheological properties of the drilling mud, resulting in improved plastic viscosity, yield point, density, gel strength, and thixotropy. However, filter loss and mud cake thickness increased at elevated RHA concentrations. Furthermore, the pH test revealed that the mud's properties shifted toward the acidic region as the RHA concentration increased. The results indicate that RHA could be used as a sustainable and cost-effective alternative to conventional chemical additives with a positive environmental impact. This study may also provide valuable insights into the use of RHA in water-based bentonite mud and could serve as a guide for future research in the drilling industry.


Introduction
After drilling operations, the cementing process is a crucial step in hydrocarbon exploration. A mud slurry is placed between the wellbore casing and rocks to isolate formation layers and tubing. The primary objective of this operation is to prevent the entry of formation fluids into the tubing and achieve zonal isolation between different oil and gas reservoir layers (Feng et al. 2018;Luke et al. 1981;Vidal et al. 2018). To accomplish this, a complete drilling mud system is designed to construct a well completion path with key functions such as removing drill cuttings, maintaining back pressure, and sealing wellbore walls to prevent filtration (Annudeep 2012;Feng et al. 2009). One of the most important properties of drilling mud is to minimize fluid loss, which is generally achieved by reducing the permeability of mud filtrate into the wellbore side walls (Agwu et al. 2018;McCosh and Getliff 2004;Okon et al. 2020).
The nature of chemical additives plays a vital role in drilling fluids, as chemical additives control properties such as rheology, filtration loss, mud cake thickness, and mud density (Kelvin and Dune n.d.). These additives, which may be solid or liquid in water-based drilling mud, promote rheology and mud density by reducing filtration losses and mud cake thickness (Ali et al. 2022; Barreno-Avila et al. 2022;Ricky et al. 2022). The choice of these additives is determined by the type of drilling operation. Some additives Responsible Editor: Philippe Garrigues increase drilling fluid viscosity, while others change it differently. These chemical additives can raise drilling costs and damage health, safety, and the work environment due to their high cost and mostly hazardous nature (Alhazzaa and Nutskova n.d.). Consequently, degradable materials are recognized as viable options as drilling mud additives as they are associated with several benefits in terms of no environmental risks, being inexpensive, and being readily available relative to non-degradable chemical additives (Patidar et al. 2021).
Several materials have been explored for their utilization as water-based drilling fluids, including potato peel powder (Al-Hameedi et al. 2019c), rice husk (Okon et al. 2014;Silva et al. 2022), pomegranate peel powder and sugar cane (Al-Saba et al. 2018a, b), palm tree leaf powder (Al-Hameedi et al. 2019a, b, c, d), fibrous food waste (Al-Hameedi et al. 2019c), pomelo peel powder (Zhang et al. 2020), banana peel powder (Al-Hameedi et al. 2020), grass powder (Al-Hameedi et al. 2019c), and Moringa oleifera leaves (Biwott et al. 2019). Agwu et al. (2018) conducted an analysis of various agro-waste materials as lost circulation control agents and found that combining two or more materials is more effective than using a single material (Agwu et al. 2018). Rice husk ashes, a prevalent agricultural byproduct, are obtained from rice, which is cultivated globally. Approximately 20% of rice production accounts for rice husks, with 20% of this total being ash (Malhotra 1993). In some countries, rice husks are used as fuel in rice factories or burned in open fields, leading to air pollution (Biwott et al. 2019). Rice husk ash (RHA) has various applications, such as a concrete additive (Akeke et al. 2013;Hadipramana et al. 2016), for brick-making, as silica powders, insulators, biochar, activated carbon, epoxy coatings, heat-absorbing glass (Prasara-A et al. 2017), and drilling mud (Ali et al. 2022;Biltayib et al. 2018;Yalman et al. 2022).
The rice husk contains approximately 50% cellulose, 25-30% lignin, 15-20% silica, and 10-15% water, making up about 20% of the total weight of rice. Its bulk density ranges from 90 to 150 kg/m 3 . The rice husk ash is generated through controlled burning and is added to drilling mud to determine its rheological properties. According to the findings of Okon et al. (2014), smaller-sized particles are more effective in preventing filter loss than larger-sized particles. Similarly, a range of particle sizes as filter loss agents in drilling mud can significantly reduce the filtrate rate, as observed by various scholars in experimental studies (Adebayo et al. 2020;Azizi et al. 2013;Henry et al. 2017;Okon et al. 2014). Vidal et al. (2018) studied the efficiency of RHA in high-temperature bottom-hole conditions of 300 °C and reported that RHA exhibited a 30.55% higher compressive strength of drilling fluid compared to silica flour. In one such study, Igwilo et al. (2019) incorporated rice husk as an additive in water-based drilling mud with a particle size of 1.25 × 104 µm and varying concentrations (0.005, 0.01, and 0.015 kg) and found that the filter loss significantly decreased with increasing concentration (Igwilo et al. 2019). The excellent mechanical and resistive properties of RHA provide long-lasting strength and durability to the cemented formation, and its low volatilization makes it less harmful to the respiratory systems of workers (Torres-Carrasco et al. 2019).
The current study presents experimental investigations to evaluate the effectiveness of RHA as an additive in waterbased drilling mud. The objective is to assess the impact of RHA on various experimental parameters such as rheology, fluid loss, and mud cake thickness. The article emphasizes the importance of using RHA as a sustainable and cost-efficient substitute for chemical additives that are commonly used in drilling operations. The study demonstrates that the incorporation of RHA as a filter control medium can minimize the environmental impact of drilling, reduce drilling expenses, and enhance drilling efficiency. The primary aim of this study is to evaluate the applicability of rice husk ash materials in formulating a drilling fluid with reduced toxicity levels, thereby mitigating the adverse environmental effects associated with drilling operations. Furthermore, this study serves as a useful reference for reducing drilling fluid expenses and minimizing the quantity of non-biodegradable waste that accumulates in landfills, thereby promoting environmental sustainability.
Therefore, this research provides new insights into the utilization of rice husk ash in drilling operations that can have favorable implications for the environment and the economy.

Collection of rice husk
The source of the rice husk utilized in the experiments was a Lar rice mill situated near Multan, Pakistan. The experiments were performed in compliance with the American Petroleum Institute's (API) recommended practice, API-RP-13B-1.

Physical pre-treatment of the rice husk
The collected samples undergwent a physical cleaning process to eliminate any impurities and were subsequently oven-dried for 3-4 h at a temperature of 45 °C. Further manual cleaning was carried out on the sample. Subsequently, the rice husk was subjected to a sieving process using a 1 3 vibratory sieve shaker (Fritisch Miling Analysette 3 Pro) with a 1.4-mm size for 5 min on the sieve. Finally, the samples were burned in a muffle furnace at a temperature of 650-700 °C for 1 h at room temperature.

Thermogravimetric analysis
The proximate analysis was performed with a thermos-balance TGA/DTG in accordance with ASTM D5142-02a. A sample ranging from 250 to 710 µm was placed in the TGA microbalance pan. The thermogravimetric analysis was performed at a heating rate of 20 °C/min from room temperature to 850 °C. Nitrogen gas was used as both sample and balance gas, with a flow rate of 5 L/min. A 10-mg sample was placed in an alumina crucible under non-isothermal conditions, and zones were categorized accordingly.

Fourier-transform infrared spectroscopy analysis
This technique was used to identify organic, polymeric, or, in some cases, inorganic materials. The test samples were scanned using a Fourier-transform infrared (FTIR) (Perkin Elmer Spectrum Version 10.5.2) analyzer, which observes the chemical properties of the samples using infrared light. The technique allowed for the determination of various bonds between the atoms in different regions of the curve.

X-ray diffraction analysis
The crystallographic structure of RHA was investigated using X-ray diffraction (XRD) analysis. The analysis was performed on the sample utilizing the Bruker D2 Phaser equipment with a Cu source and Lynxene detector, employing a particle size of 1.4 mm.

Scanning electron microscopy analysis
The FEI Nova NanoSEM 450 was used for scanning electron microscopy (SEM) analysis to explore the structure and particle arrangements of RHA samples. The particle size was measured using a high magnification power of × 200,000, WD 5.8 mm, SEM mode, and det TLD.

Energy dispersive X-ray analysis
The elemental composition of the RHA was determined through energy dispersive X-ray (EDX) Oxford INCA XACT EDX. The analysis enabled the determination of the elemental composition of the material.

Brunauer-Emmett-Teller
The specific surface area and gas adsorption properties of RHA were determined by Brunauer-Emmett-Teller (BET) analysis. The Quantachrome Nova 2200e instrument was used for BET analysis, with the sample being degassed at 120 °C and 32 points of mesoporous material being used for isotherm formation.

Preparation of drilling fluids systems
In order to examine the impact of RHA as an additive on various parameters, a controlled mud sample was prepared according to API standards by mixing 15 g of bentonite, 22.8 g of barite, and 1.05 g of xyenthan gum in 350 cm 3 of water. In addition, four samples with the same composition as the control mud but with varying percentages of RHA (from 5 to 20% by weight of water) were prepared and compared to the control mud sample to assess the effectiveness of RHA as an additive in water-based drilling mud. The RHA mud and control mud were evaluated by comparing different parameters, and the filter loss and mud cake thickness were recorded.

Mud apparent viscosity
The rotational viscometer ZNN-D6 was used to measure the Fann 6-SPEED rheology. Once the rotor was set, the mud was filled into the viscometer cup. The apparent and plastic viscosities were calculated at 3, 6, 100, 200, 300, and 600 r/min. The plastic viscosity (PV), apparent viscosity (AV), and yield point (YP) of each mud sample were calculated using the appropriate formula.

Plastic viscosity and yield point
After analyzing the initial parameters, the plastic viscosity and yield point were evaluated, which are considered crucial criteria for the drilling mud while removing drill cuttings from the borehole to the surface. A cylindrical cell was used to collect the controlled mud sample to the indicated level and to set it on the stand dip with the viscometer rod inside. The viscosity was determined by rotating it on a 6-speed rotational viscometer at various RPMs ranging from 3 to 600 rpm. Using the Bingham plastic model, the plastic viscosity of the control mud was determined based on the viscosity results. The yield point was determined using Eq. (2). The same method was performed with 5%, 10%, 15%, and 20% RHA on the viscometer, and a digital meter was used to measure the viscosity.
A slight increase in the viscosity of the sample was observed as the percentage of RHA in the drilling mud increased. However, this increase is considered acceptable as higher-viscosity fluids would require more power from the mud pump to deliver, which is not feasible or desirable. The addition of RHA to the base mud results in a proportional increase in the yield point.

Evaluation of gel strength
The drilling mud gel strength was evaluated using plastic viscosity and yield point values obtained from API-compliant mud samples. Viscosity and yield point were used as indicators to estimate the gel strength of the drilling mud. The samples underwent two rounds of testing, first at 600 rpm for 10 s and then at 300 rpm. The results were recorded after 10 min at 300 rpm for analysis. The gel strength was found to increase with the RHA foundation mud.

Determination of density
To evaluate the effect of RHA on mud density, the density of a reference mud sample was calculated using a mud balance (specifically, the RCMB-4 Model China 2-Scale Stainless Steel Mud Balance). The mud balance features a cup on one side and a lobe on the other, and the mud of interest is placed in the cup. The balancing rod is then adjusted with a knob until the bar is level and the density of the reference mud is measured. The calculated density was approximately 9 pounds per gallon (ppg). The density of the first sample, which had the same composition as the control mud but with the addition of 5% RHA, was also determined using a mud balance.

Fluid loss and mud cake thickness determination
The API RP 13B-1 (American Petroleum Institute, 2003) test was conducted at room temperature for 1800s under low temperature and pressure conditions using a cylindrical cell as a mud scale. To perform the test, a filter paper (Whatman No. 50) is attached to the base of the cell, and the filter press connections are connected. An air compressor pump is linked to a back pressure regulator, and a nitrogen tank is used to apply 100 psi pressure to the top of the cell. After 1800s, the filtrate is collected in a graduated cylinder placed beneath the cell, and the API filter loss is calculated in millimeters of filtrate every half an hour. The filter paper is (1) Plasticviscosity(up) = θ 600 −θ 300 (2) Yieldpoint =θ 300 − up then removed from the cell by dismantling it, and the mud cake thickness that has covered the filter paper is measured to the closest millimeter using a ruler. As the RHA content in the drilling mud increases, the volume of the filtrate also increases.
During this test, a cylindrical cell is used, and the test is conducted at room temperature for 30 min. The cell is lined with filter paper (Whatman No. 50) and is used for weighing the mud. After attaching the filter press, a pressure of 100 psi was provided to the cell's top by an arrangement consisting of a pump, a pressure regulator, and a nitrogen storage tank. After 1800s, the filtrate is collected in a graduated cylinder and the API filter loss is measured in cubic meters per half hour. The filter paper is then removed, and the mud cake that has covered the filter paper is measured to the nearest millimeter. As the RHA content in the drilling mud increases, so does the volume of the filtrate.

Determination of pH and resistivity
The rheological properties of drilling mud can also be affected by pH and resistivity, making it essential for a mud engineer to evaluate these parameters accurately. To determine the pH and resistivity of drilling mud samples, a pH meter (Milwaukee Tool Ground Meter MW101) and a resistivity meter (Gardco Model SR-2 Soil Resistivity Meter) were used. After placing the samples in a beaker, readings for pH and resistivity were taken using a digital meter by immersing the pH meter rod for 10 s. As the percentage of RHA in the base mud increases, the conductivity increases while the resistivity and pH decrease. This is attributed to the polarity introduced by RHA in the mud, which facilitates the free movement of electrons through it.

Rice husk ash characterization
The thermal stability, structural, morphological, and textural characteristics of rice husk ash were evaluated using TGA, FTIR, SEM, EDX, XRD, and BET.

Thermogravimetric analysis
The evaluation of rice husk ash stability is a crucial aspect, and thermogravimetric analysis (TGA) provides valuable information for this purpose as RHA possesses a high volatile matter (VM) content and a low H/O ratio, as shown in Tables 1 and 2, respectively. These factors often result in synthesis gases having low H 2 concentrations and H 2 / CO ratios. Moreover, RHA's low nitrogen and sulfur contents suggest minimal emissions of NO x and SO 2 during the pyrolysis process. RHA's higher heating value indicates its potential as a feedstock for small-scale applications, as it generates sufficient heat required for pyrolysis or gasification (Loy et al. 2018). RHA's thermochemical process is influenced by its lignocellulosic composition, which comprises cellulose, hemicellulose, lignin, and silica as its primary components. RHA comprises approximately 32-39%, 19-22%, and 13-24% of cellulose, hemicellulose, and lignin, respectively. TGA analysis of RHA reveals three distinct thermal decomposition regions between 20 and 260, 260 and 347, and 347 and 850℃, as shown in Fig. 1, where weight loss occurs due to water evaporation, hemicellulose decomposition, and cellulose decomposition. Notably, hemicellulose, cellulose, and lignin decompose at 150-350℃, 275-350℃, and 250-500℃, respectively, with a significant weight loss observed between 250 and 348℃. Researchers have conducted TGA and kinetic studies on the catalytic pyrolysis of rice husk pellets utilizing its ash as a low-cost in situ catalyst (Wibowo et al. 2022).

Fourier-transform infrared spectroscopy
FTIR spectroscopy was used to analyze RHA to uncover important information about its functional groups and surface alterations. In Fig. 2, a comparison of the FTIR measurements of rice husk ash and silica-enabled identification

Scanning electron microscopy
SEM was used to analyze RHA to explore important insights into its structural features. Figure 3 illustrates SEM images of RHA, which highlight a macrohole responsible for burning the organic components. Furthermore, the pore size of the ash material was found to range from 5.0 µm to 500 nm, as shown in Fig. 3a, b (Xie et al. 2012). The SEM analysis also revealed that the RHA exhibits pores of varying sizes, contributing to an increase in its specific surface area (SSA) values. The SEM image of RHA taken at magnifications ranging from × 100,000 to × 25,000 showed that its structure was not entirely spherical but composed of pseudo-spherical particles. Additionally, the image indicated that RHA particles formed agglomerates, with fine particles clustering around the surface of larger particles. The smallest particle size of RHA was found to be 80-100 nm.

Energy dispersive X-ray analysis
The elemental composition of RHA is presented in Fig. 4 and Table 3. The high carbon ratio observed in the EDX analysis implies that the rice husk ash has undergone complete combustion and that the volatile and moisture content have been removed from the RHA sample. The findings from the EDX analysis are crucial as they provide valuable insights into the chemical composition of RHA, which can be used in various applications, such as construction materials, soil stabilization, and wastewater treatment.

X-ray diffraction
The XRD pattern of RHA is shown in Fig. 5, which indicates the existence of the cristobalite, tridymite, and anorthite phases in RHA. The identified phases, comprising quartz, mullite, hematite, tridymite, and aragonite, confirm the sample's high crystalline nature, which conforms to the high mineral content within the RHA sample. Moreover, the XRD pattern shows amorphous silica at 22 ϴ, and its existence in RHA enhances the reactivity and thermal stability of drilling mud, which leads to reduced penetration and boosts cutting recovery.

Brunauer-Emmett-Teller
The BET analysis was utilized to determine the porosity and surface area of RHA. According to Table 4, the BET analysis shows that the surface area of RHA is 3.863 m 2 /g with a pore volume of 0.003 cm 3 /g. These results indicate

Evaluation of % RHA in water-based bentonite drilling mud rheological and filtration properties
The present study investigates the impact of different concentrations (5, 10, 15, and 20 wt.% of RHA in water) of RHA on the rheological and filtration properties of waterbased bentonite drilling mud.

Mud density measurement
Figure 6 depicts mud density measurements in a waterbased bentonite drilling mud with varying amounts of RHA. The results showed that as the percentage of RHA increased, the mud density improved progressively. This is Fig. 4 Scanning electron micrographs with EDX   useful as the addition of RHA enhanced mud density while avoiding the addition of solid particles that could potentially cause problems during drilling operations. However, excessive RHA addition may result in a density that exceeds the specified range, resulting in an overall reduction in the rate of penetration (ROP) during drilling due to the rise in the mixture's solid content. Hence, the addition of RHA should be carefully controlled to ensure that the mud density remains within the specified range for optimal drilling operations.

Mud apparent viscosity measurement
The measurement of mud's apparent viscosity in drilling fluid is a crucial parameter that requires careful monitoring to ensure efficient drilling operations. Sufficient viscosity is needed to suspend and clean drilled cuttings, and a lack of apparent viscosity can result in well abandonment. The results of the apparent viscosity measurement of the drilling mud containing RHA showed an increase in viscosity with the increase in concentration of RHA, as depicted in Fig. 7. Generally, for efficient drilling operations, high viscosity is desirable; however, it may require greater power from the mud pump, which may not be feasible due to highenergy inputs, leading to an overall uneconomical operation. Thus, it is essential to carefully consider the concentration of RHA in the drilling mud to achieve the appropriate apparent viscosity without requiring excessive power from the mud pump. Furthermore, monitoring the appropriate level of apparent viscosity also ensures the effective removal of drilled cuttings and the overall success of the drilling operation. Yalman et al. (2021) reported that the inclusion of rice husk ash in the samples led to a notable elevation in apparent viscosity in comparison to the base drilling fluid devoid of rice husk ash. The introduction of rice husk ash at a concentration of 15 wt.% resulted in a significant 60% increase in the apparent viscosity of the fluid. It is noteworthy to underscore that prior studies conducted by Al-Hameedi et al. (2019a, b, c, d) and Al-Saba et al. (2018a, b) did not investigate variations in apparent viscosity associated with the utilization of environmentally sourced materials. Figure 8 depicts how the addition of RHA to bentonite mud causes a gradual increase in its plastic viscosity because of particle friction caused by the solid components. As the viscosity of the plastic increases, so does the solid content, and vice versa. Drilling with low plastic viscosity lowers frictional pressure loss during fluid circulation (such as in the surface system, drill string, drill bit, and annulus). As a result, pump pressure and circulation density (ECD) rise. A high ECD may result in formation cracks in cases where the difference between pore pressure and fracture pressure is insignificant, which can lead to greater drilling costs. Furthermore, the yield point of water-based bentonite mud increases with the increase in concentrations of RHA, and the maximum yield point value for RHA was achieved at a 20% concentration, as shown in Fig. 9. In order to investigate rheological properties (plastic viscosity and yield point measurements), different concentrations of RHA were added to water-based bentonite drilling mud, as shown in Fig. 8, which depicts that the plastic viscosity of the bentonite mud increases with the increase in RHA. The particle friction in the drilling fluid

Gel strength (10 min) measurement
Gel strength measurement is a crucial indicator of drilling fluid thixotropic behavior, which gauges its capacity to maintain cutting suspension in a static state while being able to flow when sufficient force is applied (Culver 1998;Rabia & Technology 1985). This property is vital in preventing cuttings from settling, which can cause bit and drill pipe adhesion while tripping. The thixotropy of the fluid is determined by calculating the 10-min and 10-s gel strength values. Figure 10 demonstrates that the increase in RHA concentration in the base mud results in an increase in gel strength. When incorporated into a gel, RHA serves as a filler or reinforcing agent, enhancing the gel's structural integrity. Initially, at a concentration of 5%, RHA particles are uniformly dispersed in the gel, imparting strength to the gel through reinforcement. The RHA particles form a network-like structure within the gel, rendering it more resistant to deformation when subjected to stress. However, as the RHA concentration surpasses this optimal level (5%), several factors contribute to a decline in gel strength. At higher concentrations, the RHA particles tend to agglomerate, disrupting the uniformity of the gel network and creating weaker points in the structure, thus reducing overall gel strength. Additionally, the presence of RHA particles can impede the movement of gel molecules, making it more difficult for the gel to reform its structure after deformation, leading to reduced mobility and weakened gel strength. Furthermore, increasing RHA concentration reduces the available space within the gel for the formation of a strong network. As a result, the gel may be unable to create an optimized and robust structure, resulting in diminished gel strength. Beyond a certain concentration, the RHA particles is responsible for the impact of the solid content on its plastic viscosity, and an increase in plastic viscosity directly corresponds to an increase in solid content and vice versa. Furthermore, an increase in RHA concentration led to an increase in the yield point of water-based bentonite mud, and the highest yield point value was observed at a concentration of 20% RHA, as demonstrated in Fig. 9. The existence of RHA encourages the saturation of micropores and the development of increased amorphous gel components, culminating in a compact arrangement characterized by a minimal water absorption rate (Hossain et al. 2021).
can dominate the gel matrix, essentially becoming the primary component rather than a reinforcement. This can overwhelm the properties of the gel matrix, leading to further weakening of the gel. Collectively, these factors contribute to the decrease in gel strength as RHA concentration surpasses the optimum level. Hence, achieving the desired properties in the gel requires finding the appropriate balance of RHA concentration, which may vary depending on the specific application and gel composition.

pH and resistivity
The pH of drilling fluid plays a critical role in determining the effectiveness of additives, clay dispersion, and solubility. The observed changes can be attributed to the alterations in polarity caused by the addition of RHA in the mud, which enhances the movement of electrons through the fluid. Hence, pH and resistivity measurements are important indicators of drilling fluid properties and can provide valuable insights into the fluid's behavior during drilling operations. The measurement of pH and resistivity was executed by varying the concentration of RHA from 5 to 20%, respectively. From the results displayed in Figs. 11 and 12, an increase in RHA percentage in the base mud resulted in increased conductivity with a simultaneous decrease in resistivity and pH.

Filtration loss
Fluid loss can be characterized as the quantity of external fluid intrusion into a porous and permeable geological stratum stemming from elevated hydraulic pressure exerted by drilling mud. It is of paramount importance to attain minimal fluid loss volume, as this serves to avert a spectrum of formidable and economically burdensome drilling complications that arise from excessive fluid loss. These complications encompass, but are not limited to, formation impairment, adherence of the drill string, inadvertent fluid escape into subsurface formations, unproductive temporal intervals, and analogous challenges. The measurement of filtration loss is taken as a crucial parameter in the drilling industry as it has a significant impact on drilling efficiency and economics. As demonstrated in Fig. 13, the results indicate a positive correlation between the RHA percentage in the drilling mud and the filter loss. An increase in the RHA percentage leads to an increase in filter loss and mud cake thickness. This phenomenon is due to the higher mud hydrostatic Fig. 11 Effect of rice husk ash concentration on water-based bentonite mud's resistivity pressure created by the addition of RHA, which causes fluid loss in porous and permeable formations. The control of fluid loss is vital in preventing various detrimental effects such as formation damage, clogged pipes, lost circulation, and non-productive time. Therefore, the study emphasizes the significance of optimizing drilling mud formulation to minimize fluid loss and improve drilling efficiency, which ultimately leads to reduced operational costs. The effect of RHA concentration in drilling mud on mud cake thickness is presented in Fig. 14, which shows that as the concentration of RHA increases in drilling mud, there is a corresponding increase in filtration loss, which in turn affects the thickness of the mud cake. The thicker mud cakes are formed when there is greater filtration loss. A thin, impermeable drilling fluid is desirable to prevent circulation loss, pipe clogging, and formation damage. This study also found that the mud cake thickness increased with an increase in RHA concentration up to 20 wt.% in water-based bentonite drilling fluid, resulting in a denser mud cake than the basic fluid, and the results are consistent with previous studies (Okon et al. 2014). Thus, the addition of RHA to drilling mud leads to the formation of a denser mud cake and an impermeable drilling fluid to mitigate circulation loss, pipe clogging, and formation damage during drilling operations.

Conclusion
This study investigates the viability of using RHA as an eco-friendly and cost-effective filter control medium in water-based bentonite drilling fluid. The results revealed that higher concentrations of RHA in the drilling mud improved its rheological properties, especially at 15 and 20 wt.%. Incorporating RHA into the drilling mud altered its filtration and rheological characteristics, leading to increased plastic viscosity, yield point, density, gel strength, and thixotropy. However, higher RHA concentrations also resulted in increased filter loss and mud cake thickness. The pH test indicated that as RHA concentrations rose, the pH level shifted toward acidity. These findings offer valuable insights into utilizing RHA in water-based bentonite mud and can guide future research in the drilling industry, contributing to sustainable development goals related to education quality and industrial innovation.
RHA is a highly promising sustainable and eco-friendly material derived from agricultural solid waste, specifically from rice milling. Its ready availability makes it an attractive alternative to synthetic additives in various applications, including drilling fluids. RHA's inherent low content of heavy metals sets it apart from certain drilling fluid additives, reducing the risk of environmental contamination and emphasizing its eco-friendly nature. Moreover, RHA exhibits a higher degree of biodegradability compared to synthetic additives, further supporting its environmentally benign profile. Being able to naturally break down in the environment ensures minimal ecological impact. Additionally, RHA demonstrates lower biotoxic effects compared to certain chemical additives found in drilling fluids, making it a safer and more sustainable choice. In conclusion, replacing traditional drilling fluid additives with RHA has the potential to yield positive environmental outcomes. Its origin as an agricultural byproduct, combined with its low heavy metal content, enhanced biodegradability, and reduced biotoxicity, underscores RHA as a viable and environmentally friendly option in the pursuit of more sustainable drilling practices. Nonetheless, careful assessment and monitoring remain crucial to ensure responsible usage and to determine the specific environmental benefits it can provide in each application.