Introduction

In oil and gas rotary drilling operations, drilling fluid has to perform various functions including but not limited to cooling and lubricating the drill bit, transporting rock cuttings to the surface, and suspending cuttings while circulation is stopped. By exerting appropriate hydrostatic pressure, the drilling mud also aids in preventing both the potential collapse of unstable strata into the borehole and the intrusion of undesirable fluid from potentially present permeable water-bearing strata. In order to function effectively under anticipated wellbore conditions, drilling fluid systems are designed and formulated (Bourgoyne et al. 1986). Drilling fluid is classified into three types: water-based, oil-based, and gas-based. The decision on which category to use is heavily influenced by the circumstances surrounding the drilling operations. Although all drilling fluids have their applications, water-based drilling fluid is the most effective because of its low cost of preparation, maintenance, and environmental friendliness. So, in light of these facts and environmental compatibility, user interests have sparked in water-based drilling fluid as compared to others (Amanullah 2007).

The essential rheological properties considered necessary of drilling fluid for decent functioning performance are that it can withstand sufficiently high stress to keep cuttings suspended, especially when fluid flow is ceased, while also procuring a lower viscosity profile to allow effective pumping. In steady shear flow, the fluid must have high-yield stress and shear-thinning characteristics to perform the above-mentioned functions properly (Alderman et al. 1988). The design of the drilling fluid need to be desirable to accomplish the required functions of both rheological and filtration properties in an economical manner. Fluid loss and mud cake formation are essential for the drilling fluid to perform as intended. Unnecessary extra fluid loss during drilling operation is a growing problem in the oil and gas industry. This problem can be solved by designing a proper drilling fluid which will form a low-permeable and thin mud cake on the borehole wall. There will be a significant fluid loss to the formation if the borehole wall mud cake has a high permeability. As a result, it is essential in drilling to design and prepare a drilling fluid that minimizes fluid loss volume. Some chemical additives, such as bentonite, carboxymethyl cellulose (CMC), low viscosity polyanionic cellulose (PAC-LV) and starch, are used to control filtration loss in order to avoid high levels of fluid loss (Moore 1986; Shakib et al. 2016). Moreover, nanoparticles (Bayat et al. 2018) and cellulose nanomaterials (Zhou et al. 2023) have already showed their potentiality in controlling fluid loss at the laboratory scale.

The petroleum industries of the twenty-first century are mostly concerned with sustainable technologies. Natural additives have been introduced into the drilling industry to reduce the cost of drilling fluid as well as the environmental impact of toxic chemicals. Numerous researchers have also recommended some local agro-waste materials as useful additives for substitution in the formulation of drilling fluids, which offer superior or equivalent results to their chemical and toxic counterparts. Some recent studies on environment-friendly drilling fluid additives are as follows:

Davoodi et al. (2018) presented a innovative field-applicable drilling fluid formula using pistachio shell powder (PSP) and successfully applied it to a oil field of Iran. The major source of concern was the substitution of polyanionic cellulose polymer with pistachio shell in two distinct fine particle sizes. Comparatively tiny particle size (less than 75 microns) demonstrated its ultra-practical quality as filtration loss control additive by reducing fluid loss upto 44% and developed thin and fine mud cakes. Moreover, the PSP mud exhibited excellent rheological properties and the cost study revealed that making drilling fluid with PSP saves 13% of the cost.

Al-Hameedi et al. (2019a) evaluated the applicability of grass powder (GP) to improve the fluid properties and compared the results with conventional starch. The experimental findings of the mud samples prepared with 0.5%, 1%, and 1.5% GP were analyzed with the same concentrations of starch. Authors concluded that GP was more successful as filtration loss control additive than the starch while starch increased rheological qualities more effectively at all concentrations than GP. Moreover, GP and starch had no influence on mud weight.

Al-Hameedi et al. (2019b) examined the possibility of employing mandarin peel powder (MPP) as a new eco-friendly drilling fluid additive. The experimental investigations revealed that MPP lowered the alkalinity of the drilling fluid by 20–32% and improved its rheological characteristics such as plastic viscosity, yield point and gel strength. When compared to the reference mud, the fluid loss was reduced by 44–68% at MPP concentrations as low as 1–4%, and the filter cake was improved. Another experimental study on MPP was led by Medved et al. (2022) where they attempted to establish if the particle size of mandarin peel powder impacts the qualities of the drilling fluid. They concluded that smaller particle sizes exhibited better results in improving fluid properties. From the study of Al-Hameedi et al. (2019b), it was found that the maximal concentration of MPP was 4% by volume of water, but in their research, 2% MPP was the maximum tested concentration. They found that, the concentration above 2% caused significant drilling fluid gelation, making mixing impossible, and also suggested that, the ideal concentration of mandarin peel powder should be up to 1.5% by volume of water.

Al-Hameedi et al. (2020a) carried out a laboratory study on banana peel powder (BPP) to assess its ability to optimize drilling fluid properties. The study’s findings revealed that the BPP-added material significantly enhanced the filtration characteristics and rheological properties while also boosting the chloride content. However, as compared to the reference fluid, BPP additions lowered pH levels, calcium content, and resistivity. Another laboratory study of food waste; potato peel powder (PPP), was led by Al-Hameedi et al. (2020b) to enhance the rheological and filtration properties of drilling fluid. The findings of the investigation were not satisfactory for the higher concentration of PPP but lower concentrations (1–2%) of PPP performed better to improve both the rheological and filtration properties. Moreover, the authors examined the applicability of palm tree leaves (Al-Hameedi et al. 2019c) and black sunflower seeds’ shell powder (Al-Hameedi et al. 2020c) as biodegradable drilling fluid additives in other studies.

Amory and Almahdawi (2020) investigated the effects of pomegranate peel and grape seed powders as natural additives when introduced with the WBM. Rheological properties, filtration properties as well as alkalinity and pH of the mud samples were examined for different concentrations of pomegranate peel powders and grape seed powders. The authors observed a decrease in pH values with an increase in the concentrations of the natural additives. Moreover, they suggested that both the natural additives can be used to reduce PV and filtrate loss of the WBM.

Prakash et al. (2021) carried out an experimental study to evaluate the performance of litchi leaves powder (LLP) as a filtration loss control additive in the water-based drilling fluid. The experimental results revealed that a 5% concentration of LLP greatly decreased the filtration loss of drilling fluid by 70.6% in contrast to conventional filtration loss control agent carboxymethyl cellulose (CMC) under the effect of 100 psi pressure and considerably improved the rheological parameters such as plastic viscosity (PV), yield point (YP) and gelation of drilling fluid in comparison with reference mud.

An experimental and modeling study was conducted on Okra powder as a fluid loss control additive by Murtaza et al. (2021). Their experimental investigation found that adding a 1% okra solution might minimize fluid loss by 20%. In comparison with starch, okra has a lower influence on rheological characteristics and has no effect on the pH of the solution.

Zhou et al. (2021) studied the possibility of adding a natural ingredient called wild jujube pit powder (WJPP) to enhance the drilling fluid’s performance. The findings of the study concluded that WJPP has positive impact on viscosity, decrease filtration loss, improve shear thinning and thixotropy, and lower the drilling fluid’s lubrication coefficient. In addition, the mud cake microstructure test revealed that WJPP could create a grid structure, which, when combined with particle blocking, the structure might block water molecules from flowing through, reducing filtering loss.

Adding sawdust to examine the filtration properties in water-based drilling fluid was carried out by Agwu et al. (2019). They investigated the potential of using rice husk and saw dust as a filter loss control agent. The main focus of their research was to evaluate the performance of filtration properties after adding rice husk and sawdust as filtration loss control additives to the reference fluid. The filter loss tests revealed that rice husk reduced filter loss by an average of 77%, compared to 63% for sawdust. Furthermore, at greater concentrations, sawdust and rice husk were shown to reduce fluid loss. Rice husk produced 14% thicker mud cakes (2.8 to 3.8 mm) than sawdust (2.6 to 3.3 mm). Mud cakes formed for both the sawdust and rice husk failed to meet the 2 mm API standard (Agwu et al. 2019). Moreover, the rice husk mud produced smooth and slick cakes, but the sawdust mud produced rough texture, sticky and hard cakes. According to the author’s explanation, due to rough and hard mud cake, there will be more frictional drag on the drill pipe when it contacts the borehole walls and will not prevent the differential pipe sticking. The authors presented a detailed comparative experimental analysis of filtration properties such as filtrate volume; the texture of the mud cake; mud cake thickness; and permeability of the mud cake; of drilling fluid after adding different concentrations of rice husk and sawdust. But it is also important for an additive to exhibit significant rheological performances after adding with drilling fluid to be considered for field application.

In their research, the rheological properties of the formulated drilling fluid were not evaluated and the effects of concentrations of sawdust on mud weight, plastic viscosity, yield point, and gel strength were not measured. The sawdust was sieved to 125 µm in their experiment though recent studies revealed that relatively fine particle size (less than 100 µm) additives perform well to improve filtration properties (Agwu and Akpabio 2018; Davoodi et al. 2018; Zhou et al. 2021). Ultimately, their study could not provide a detailed analysis of rheological behavior, the effects of different concentrations of additives on rheological properties and an acceptable solution to mitigate the roughness of the mud cake. A detailed experimental study is required to check the applicability of sawdust to be used as a green additive as well as to answer the research gaps found in the previous study.

In this study, we addressed those research gaps and examined the applicability of sawdust to be used as a green additive to improve both the rheological and filtration properties of WBM. The main objective of this study is to assess the applicability of sawdust to be used as a green additive in WBM. To assess the effectiveness of sawdust, this study investigated the rheological and filtration characteristics of mud samples incorporating sawdust. The findings were then compared with those of the base mud, aiming to determine sawdust’s capacity for enhancing the properties of water-based mud. Additionally, this study tried to overcome the limitation of designing a thin and low-permeable mud cake by lowering the concentrations of sawdust in the mud sample. In order to conclude the research objectives, the following experimental analyses were performed in this research:

  • API-based full sets of drilling fluid experiments including but not limited to mud balance test, rheology tests using rotational viscometer and low temperature- low pressure (LTLP) filtration test for filtration properties were carried out.

  • A fine particle-sized additive was used; sawdust was carefully sieved to 63 µm to improve filtration properties.

  • The sawdust was utilized at lower concentrations in the mud samples to improve the mud cake’s characteristics.

  • Effects of the sawdust concentrations on mud weight and mud rheological properties were investigated.

  • Fluid flow index was calculated for all mud samples using the power law model.

  • Filtration properties such as mud filtrate volume, mud cake thickness, mud cake permeability and mud cake texture were examined.

  • Though drilling fluid is formulated by using tap water/fresh water, the effect of water mineralogy on the formulation of drilling fluid was analyzed in this study.

  • The samples of drilling fluid were kept in ambient condition for 48 h to check the bacterial concerns and the fitness of the mud.

The performances of the sawdust-containing mud samples were compared with the base mud sample. The experimental outcomes are concluded in the conclusion section.

Materials and methods

This section provides a comprehensive description of the procedures used to collect and prepare biodegradable additive sawdust and the various experimental steps used in the laboratory evaluation. Freshwater, bentonite, sodium hydroxide, xanthan gum, barite, and sawdust were used as reagents in the experiment. Chemical reagents are collected from a reputed local chemical provider called “Taj Scientific”. In addition, base mud and mud samples formulated by adding different concentrations of sawdust have been introduced in this section.

The basis for choosing sawdust

Cellulose content: Cellulose is a long chain of glucose molecules that are held together primarily by glycosidic bonds. Cellulose fibers have the unique property of quickly forming a seal at the face, preventing the entry of undesirable fluids or solids into the formation. In oil and gas drilling operation, carboxymethyl cellulose (CMC), low viscosity polyanionic cellulose (PAC-LV) and starch, are usually used as fluid loss control additives to minimize the filtration loss (Moore 1986; Shakib et al. 2016). Moreover, the potentiality of cellulose nanomaterials has been investigated recently (Zhou et al. 2023) to be used as filtration control additive. Table 1 shows that sawdust has a high cellulose content, indicating that it has the potential to be used as a filtration loss control agent.

Table 1 Cellulose content of woods for sawdust (Agwu et al. 2019)
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Availability According to Paulrud et al. (2002), about 10–13% of the total volume of a wood log is reduced to sawdust during milling operations. The estimated annual sawdust production in Bangladesh is approximately 0.99 million tons (Islam et al. 2015). A small amount of sawdust is used as cooking fuel in the country, while the remainder has no practical use and is discarded. As a result, sawdust generated in sawmills during the processing of timber should be used more efficiently and economically.

Cost The prices of various cellulosic materials that can act as filter loss agents in drilling fluids are shown in Table 2. Because sawdust had lower costs than the other cellulosic materials, it ranked first in the list of choices for this study. It should be noted that these prices are subject to change over time; however, the availability of sawdust compensates for this.

Table 2 Cost of filter loss control materials (Agwu et al. 2019)
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Collection and physical pre-treatment of sawdust

The sawdust utilized for this experiment was sourced from a local timber sawmill market located in the Cumilla district of Bangladesh. The primary motivations for utilizing this type of waste were to examine its potential to upgrade both rheological properties and filtration properties and discover a new alternative eco-friendly additive. Furthermore, sawdust is generally available and extremely simple to collect and prepare. At first, foreign particles were manually removed from the raw sawdust. After that, they were dried in an oven at \(85^\circ{\rm C} -90^\circ{\rm C}\) for about 30 min. The sawdust was sieved to a particle size of 63 µm by using a sieving mesh to confirm sample homogeneity and lower the degree of particle size uncertainty (Fig. 1a, b). Finally, the finely sieved sawdust was kept at room temperature.

Fig. 1
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a Raw sawdust, b Sieved sawdust

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Drilling fluid preparation

Base mud was prepared by mixing barite, bentonite, xanthan gum, and sodium hydroxide in water. Reagents were mixed using a standard Hamilton Beach Commercial high-speed mixer and a bottom mixer cup. An electronic balance machine was used to weigh each solid additive before gradually introducing them into 370 ml of fresh water while the impeller was blending. To avoid overflow of the fluid, each sample was blended at a low speed for 10 min. The mixing order of the reagents is given in Table 3 with their functions. Three fluid samples were formulated by adding 0.25%, 0.5%, and 0.75% concentrations of sawdust separately to the base mud and leveled as sample 1, sample 2, and sample 3, respectively (Fig. 2). The concentrations of sawdust were measured as a weight percentage of the total volume of the base water. The concentration of sawdust above 0.75% caused drilling fluid gelation, making mixing difficult. Moreover, the study by Agwu et al. (2019) revealed that sawdust concentrations of more than 1% produce a thick and sticky mud cake, which causes additional frictional drag on the drill pipe and decreases hole cleaning efficiency. To reduce the excessive gelation of the mud and to produce a thin mud cake to avoid frictional drag, this study has chosen sawdust concentrations of less than 1%. As sawdust is a biodegradable material, mud samples containing sawdust were left in the laboratory at room temperature for 48 h to examine the effects of bacteria. It was discovered that there are no issues of bacteria with sawdust. The following samples were formulated for laboratory experiments:

  • Base mud

  • Sample 1: Base mud + 0.25 wt.% of sawdust by volume of water

  • Sample 2: Base mud + 0.50 wt.% of sawdust by volume of water

  • Sample 3: Base mud + 0.75 wt.% of sawdust by volume of water

Table 3 Raw materials used for mud formulation
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Fig. 2
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Mud samples used in the study

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Laboratory measurements

Laboratory experiments were conducted, and experimental results were recorded to analyze each drilling fluid sample’s rheological and filtration properties. Each of the laboratory measurements was carried out by following the procedures listed in “API Recommended Practice 13B-1,” a manual for the recommended practice of field testing water-based drilling fluids. Mud weight and specific gravity were measured by using OFITE Metal Mud Balance. By determining the density of fresh water, the instrument’s calibration can be quickly verified. Assuming no wallslip condition, rheological properties were calculated from the dial reading data obtained from OFITE Model 800 viscometer. Dial readings were recorded for different rotation speeds by following API testing procedures from the user manual. The OFITE API low-temperature low-pressure (LTLP) filter press was used to measure the different types of filtration properties of the mud samples. The filtration characteristics of drilling fluid samples were measured at ambient temperature and 100 psi pressure. Filtrate volumes at 7.5 min and 30 min were recorded from the graduated cylinder. After 30 min, the filter paper and deposited mud cake were carefully retrieved. The mud cake was then gently washed with the base fluid (water) to remove any excess fluids that are not part of the mud cake. A physical examination of the mud cakes textures was conducted immediately before drying up the cakes and mud cake thickness was measured by dipping a millimeter scale into the mud cake. Mud cakes examined in this study are shown in Fig. 3. The photographs of the mud cakes depicted in Fig. 3. were taken right after the removal of any excess fluids that were not integral to the mud cakes, and this was done prior to the measurement of the cakes’ thickness. The mud cakes hold significant importance as they safeguard wellbore stability, control filtration, offer insights for formation evaluation, enhance drilling efficiency, mitigate potential formation damage, and play a critical role in coring operations by maintaining core integrity during retrieval. Further elaboration on the thickness, textures, and permeabilities of the mud cakes is described in the Results and Discussion section.

Fig. 3
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Mud cakes examined in this study

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Results and discussion

The experimentally obtained readings for the base mud and mud samples with various sawdust concentrations are presented and thoroughly discussed in this section.

Effect of water mineralogy on drilling fluid formulation

During the formulation of base mud, two different water samples (sample A and sample B) with different mineralogy were used to observe the effect of water mineralogy on the formulation of base mud. The mineral compositions of water samples are given in Table 4. Two base mud samples were created by blending solid chemical additives with water samples of varying mineralogical compositions, all at the same concentrations. The mixing sequence remained consistent, and each mixture was allowed to stand for the same specified duration, as outlined in Table 3.

Table 4 Mineral composition of water samples
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Since no direct test was conducted to observe the mineralogy’s impact on the drilling fluid, subjective judgment was employed. A physical examination of the formulated drilling fluid indicated that solid additives mixed well with water sample A compared to water sample B. Solid chemical additives were loosened up gradually from the water and formed discrete lumps of clotted additives in the base mud which was formulated with water sample B as shown in Fig. 4b. This clotted mud will fail to prevent both the potential collapse of unstable strata into the borehole and the intrusion of water from potentially present permeable water-bearing strata by exerting appropriate hydrostatic pressure. Additionally, it will unable to transport rock cuttings to the surface due to the poor rheological characteristics. As a result, the base mud prepared with water sample A (Fig. 4a) was selected over water sample B and used for further experiments in this study.

Fig. 4
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Base mud formulated with a water sample A, b water sample B

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Effect of sawdust concentrations on mud weight and specific gravity

Sawdust concentrations had a insignificant effect on mud weight and specific gravity at 0.25% concentration while a minor impact at 0.5% and 0.75% concentrations were observed. From Fig. 5 it can be observed that mud weight decreased with respect to base mud as sawdust concentrations increased. The mud weight results are given in Table 5. The basis of the decrease in mud weight was the formation of foams in the mud which was observed during the laboratory work. It is obvious that the majority of conventional drilling fluid additives such as CMC, starch, PAC-LV, and salt clay cause some foam during the preparation of fluid. The presence of foam has the disadvantage of slightly lowering the mud weight (density) because of the entrapped bubbles.

Fig. 5
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Effect of sawdust concentrations on mud weight

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Table 5 Rheological properties of mud samples
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To prevent or minimize the development of foam in the drilling fluid, environment-friendly anti-foam additives can be used. Some environment-friendly silicon products are generally used for treating water in water treatment plants to expel foams. Additionally, an-appropriate mud weight can be achieved by using the weighting agent barite. Therefore, there is no reason to be concerned that sawdust will have an adverse impact on mud weight, and all of the above-mentioned remedies will uphold the study goal of evaluating the applicability of sawdust as biodegradable additive.

Effect of sawdust concentrations on rheological properties

Plastic viscosity and yield point

The effects on rheological properties due to sawdust concentrations are given in Table 5. Sawdust increased both plastic viscosity (PV) and yield point (YP) at each concentration. Plastic viscosity increased by 28.5, 42.8, and 71.4% compared to the base mud at 0.25, 0.5, and 0.75% sawdust concentrations, respectively. These results recommend the sawdust applicability as utilized as a viscosity modifier.

The capability of drilling fluid to lift or extract cuttings from the annulus is indicated by the yield point. Cuttings will be transported more effectively by drilling fluids with higher yield points compared to those with lower yield points. In the case of the yield point, sawdust-containing mud samples exhibited a gradual increase in YP with respect to base mud as shown in Fig. 6.

Fig. 6
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Effect of sawdust concentrations on PV and YP

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Gel strength

By definition gel strength refers to the measurement of inter-particle forces in drilling fluid and describes how much gelling will happen when circulation stops. Sawdust-containing mud samples exhibited excellent gel strength performance as the numerical deviation between the initial (10 s) and final gel strengths (10 min) was less than 10 (lb/100ft2) as shown in Fig. 7. During pump-off condition, this lower value of deviation will make a positive impact on drilling operations. As a result, additional pressure will not be required to break the gel strength when switching from pump-off to pump-on conditions (Al-Hameedi et al. 2020a, b, c). Moreover, it will add no effect on pump efficiency and may prevent fractures in weak formations.

Fig. 7
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Effect of sawdust concentrations on gel strength

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Fluid flow index

Several developed rheological models have described the rheology of drilling fluid. These models depict and characterize the flow characteristics of non-Newtonian fluid under various stress conditions. The power law model is one of these models. It is a typical rheological model for quantifying the shear-thinning nature of a fluid sample. The power law model can be expressed mathematically as (Hossain and Al-Majed 2015):

$$\tau =k{\gamma }^{n}$$
(1)

where, \(\tau\) is the shear stress \((Pa)\), \(k\) represents the flow consistency coefficient \((Pa.{S}^{n})\), \(\gamma\) is the shear rate \(({s}^{-1})\) and \(n\) is a dimensionless fluid flow behavior index that indicates a fluid’s tendency to shear-thinning. When \(n<1\), the fluid is shear thinning or pseudoplastic fluids, and for \(n>1\), the fluid is shear thickening (Hossain and Al-Majed 2015). Equation (1) can be simplified by taking the logarithmic function on both sides as follows:

$${\text{log}}\;\tau = {\text{log}}\;k + n\;{\text{log}}\;\gamma$$
(2)

The fluid flow index, \(n\) can be determined from the slope of the straight line of the Eq. (2).

In this study, Eq. (2) was used to produce straight lines for the sawdust-containing mud samples as well as base mud (Fig. 8). Fluid flow indexes were calculated from the slopes of produced straight lines to determine whether the mud samples prepared for the experiment are share thinning or not.

Fig. 8
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Effect of sawdust concentrations on fluid flow indexes of mud samples

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The majority of drilling fluids are non-Newtonian thixotropic shear-thinning fluids with yield stress that decreases viscosity as the shear rate increases and increases viscosity as the shear rate decreases (Coussot et al. 2004). All the mud samples examined in this study exhibited shear-thinning characteristics as the value of \(n\) was less than 1 for each sample (Table 5). Compared to the base mud sample, sample 1 (0.25% sawdust), sample 2 (0.50% sawdust), and sample 3 (0.75% sawdust) presented a lower fluid flow index, \(n\). Lower values of the fluid flow index indicate the improvement of hole cleaning performance by developing the efficient annular viscosity and flattening the annular velocity profile. It will lessen the turning effect; as a result, the solids will be transported more directly up the hole and particle breakage will be reduced (Akinade et al. 2018).

Effect of sawdust concentrations on filtration properties

Filtrate volume

Filtrate volume was measured at 100 psi using a standard API filter press to determine the applicability of sawdust as a filtration control additive. The fluid loss was measured after 7.5 min and 30 min. The recorded filtrate volumes of mud samples are given in Table 6 with other filtration properties. It can be observed that the inclusion of sawdust leads to a significant reduction in fluid loss. Mud samples containing sawdust showcase markedly lesser filtrate volume compared to the base mud, as illustrated in Fig. 9. Though all mud samples with sawdust reduced filtration loss, the lowest filtration loss was recorded for 0.5% sawdust (sample 2) concentration for both 7.5 min and 30 min. Reducing fluid loss is a representation of the good filtration properties of sawdust mud as it will prevent formation damage as well as borehole instability.

Table 6 Filtration Properties
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Fig. 9
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Filtrate volume for mud samples at 7.5 min and 30 min

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Mud cake thickness

The mud cake quality and build-up characteristics are very significant in preventing drilling and completion problems. In the case of mud cake thickness, the addition of sawdust decreased mud cake thickness by more than 30% for all concentrations of sawdust. Sawdust-containing mud samples formed a thin mud cake compared to base mud as displayed in Fig. 10.

Fig. 10
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Effect of sawdust concentrations on mud cake thickness

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The mud cake thickness is presumed to be proportional to filter loss and is affected by the concentration of solids in the mud as well as the amount of water retained in the cake. The filter loss decreases as the solids concentration increases, but the cake volume grows (Caenn et al. 2011). The relationship among filtrate volume, mud cake thickness, and solids concentrations is displayed in Fig. 11.

Fig. 11
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Relationship among filtrate volume, mud cake thickness and solid concentrations

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Thick mud cakes and too much filtration loss raise the possibility of pipe sticking, poor well-log data, formation damage and lost circulation through fines and filtrate invasion into the permeable zone (Aftab et al. 2017). So, it can be concluded that sawdust containing mud will not raise the above-mentioned difficulties as they formed thin mud cakes.

Mud cake permeability

The development of a compact, thin, and low-permeability mud cake is a precondition for superior filtration performance. Darcy’s law in the form of Eq. (3) was used to calculate the permeability \(({k}_{\mathrm{mc}})\) of the mud cake:

$${k}_{\mathrm{mc}}=\left({Q}_{f}\times {l}_{t}\times \mu \right)\times \frac{1}{2P\times F\times t}$$
(3)

where,

\({Q}_{f}\) = filtrate volume (cm3) recorded after time \(t\)

\({l}_{t}\) = thickness of the mud cake(cm).

\(\mu\) = viscosity of filtrate (cP).

\(P\) = filtration pressure (atm).

\(F\) = effective filter surface area (cm2).

\(t\) = time (s).

Table 4 shows the mud cake permeability at various sawdust concentrations which are calculated using Eq. (3). The results showed that when sawdust was added, the permeability of the mud cake decreased by nearly 50% for all concentrations of sawdust as compared to the base mud as displayed in Fig. 12. This provides an explanation for why the sawdust mud samples’ filter loss volumes were significantly lower than those of the base mud. A good mud cake should have a low permeability because one with a higher mud permeability will permit fines and filtrates to move into the porous formation, potentially damaging it.

Fig. 12
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Effect of sawdust on mud cake permeability

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Mud cake texture

Mud cake texture is a subjective description of the mud cake generally recorded by the mud engineer in the mud report. The mud cake texture was examined in this study by the physical observation of the freshly formed mud cake. Mud cakes are displayed in Fig. 3. After examining the mud cakes, it was observed that sample 1 and sample 2 had a smooth, soft and slippery texture, while sample 3 exhibited a sticky texture. It is essential to remember that a good mud cake might possess smooth and slippery qualities. This is because, in contrast to mud cake which is nearly solid and dry, mud cake with these characteristics will impede differential pipe sticking because of its slippery nature. Furthermore, the soft and slippery qualities of the mud cakes are desirable for avoiding too much torque and frictional drag when the drill pipe comes into contact with the wellbore walls (Agwu et al. 2019).

Finally, after analyzing the filtration performance, it is evident that a lower concentration of sawdust has the potential to enhance filtration performance along with its rheological properties. Table 6 shows that sample 2 (0.5% sawdust) performed well in terms of filtration properties when compared to the other mud samples. The mud cake from sample 2 had the lowest permeability, indicating a small filtrate loss into the formation. The fact of less filtrate loss is also supported by the experimental record of filtrate volume. Furthermore, a thin mud cake validates the filtration performance of sample 2, as thick mud cakes and too much filtration loss raise the possibility of pipe sticking, poor well-log data, formation damage, and lost circulation through fines and filtrate invasion into the permeable zone.

Conclusions

This study explored the prospective utilization of sawdust as a green additive in drilling operations. To assess the effectiveness of sawdust, this study investigated the rheological and filtration characteristics of mud samples incorporating sawdust. The findings were then compared with those of the base mud, aiming to determine sawdust’s capacity for enhancing the properties of water-based mud. The general guidelines provided by API for drilling fluid field testing were followed during the laboratory measurement. The followings are the key conclusions drawn from this experimental study by examining the laboratory outcomes:

  • Sawdust concentrations had no significant effect on mud weight and specific gravity.

  • Sawdust increased both plastic viscosity and yield point of the mud samples compared to the base mud with the increase of sawdust concentration.

  • No additional pressure will be required to break the gel strength when switching from pump-off to pump-on conditions as the numerical deviation between the primary (10 s) and final (10 min) gel strengths is less than 10 \((lb/{100ft}^{2})\).

  • The value of the fluid flow index,\(n\), from the power law model was found to be less than 1 for each sample, which indicates the shear-thinning nature of the sawdust-containing mud samples.

  • The addition of sawdust decreased mud cake thickness by more than 30% for all concentrations of sawdust compared to the base mud.

  • The incorporation of sawdust resulted in a nearly 50% decrease in mud cake permeability across all sawdust concentrations, relative to the base mud.

Based on these findings, this study concludes that lower concentrations of sawdust can be an effective and cost-efficient green additive for water-based drilling fluid systems, enhancing both rheological and filtration properties. Sample 2 (0.5% sawdust) showed particularly promising performance in improving filtration properties compared to other sawdust concentrations. It is recommended to implement the results of this study in field applications to evaluate the efficacy of the sawdust-containing mud in different geological formations and conditions.

Limitations

  • The study was conducted in controlled laboratory settings, which may not fully replicate actual drilling operations.

  • The findings are specific to the tested sawdust concentrations and drilling mud formulation, limiting their applicability to different contexts.

  • The study focused on immediate effects and did not extensively explore long-term performance or durability.

  • The impact of contaminants commonly found in geological formations was not explicitly examined.

Recommendations

Further research should be conducted to assess the stability of the proposed mud formulation in the presence of potential contaminants commonly found in geological formations, such as anhydrite, calcium, magnesium, salts, clays, and others. Additionally, future studies can explore the impact of wall slippage on measured viscosity and yield strength at lower shear rates by considering the effect of wall slip on the rheometer. These additional investigations will provide a more comprehensive understanding of the proposed sawdust-based drilling fluid system and its potential applications in the industry.