2.2.1 Measurement of zeta potential of the procured TEOS

TEOS is a chemical compound with the formula Si(OC₂H₅)₄ and is the most common alkoxide of silicon. It was used as a precursor for SiO₂ and a SiO₂ source in synthesizing PEG–SiO₂ NC. Before it is used in the synthesis process, the magnitude of its zeta potential (ZP) was checked by dispersing 0.001 ml TEOS in 100 ml deionized (DI) water, as reported by Wang et al. (2006) to find out the inherent charge on its surface. The ZP level of the procured TEOS was examined using Malvern Instruments Inc., USA (ZSP Malvern Nano Zetasizer), and Fig. 2 shows the result from such examination. The result in Fig. 2 indicates that the TEOS has a high negative ZP of –0.7 mV magnitude, which epitomizes incipient instability that can cause rapid agglomeration of composite fluid.

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Fig. 2

Surface charge of procured TEOS.

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2.2.2 Synthesis and formation of PEG-SiO₂ nanocomposites

PEG-SiO₂ NC was prepared by a simple one-step protocol which includes the hydrolysis and condensation of TEOS in EtOH and water mixture under alkaline conditions at room temperature. This technique is chosen due to the simplicity in using hydrophilic ethanol solvent. An overview of the synthesis protocol is illustrated in Fig. 3. To synthesize, 10.5 ml TEOS was added into 100 ml EtOH and stirred for 10 mins at 149 °F (65 °C) and 300 rpm using a mechanical stirrer. Then, 28 ml NH₄OH and 34 ml DI water was measured into a beaker and poured into the reactor. Next, 5 ml of PEG was introduced slowly into the reaction mixture dropwise. The mixture was stirred vigorously for 3 h at 149 °F (65 °C). After that, the mixture was centrifuged at 4000 rpm for 40 min, and the obtained composite was washed with DI water. Finally, the resulting composite was dried in an oven at 149 °F (60 °C) for 24 h to obtain the PEG-SiO₂ NC. Fig. 4 presents the formation mechanism of the particles of the PEG-SiO₂ NC.

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Fig. 3

Synthesis process of PEG-SiO₂ NC.

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Fig. 4

The formation mechanism of particles of the PEG-SiO₂ NC.

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2.2.3 Evaluating the zeta potential of synthesized PEG-SiO₂ nanocomposites

After synthesizing the PEG-SiO₂ NC, its ZP level was checked with a ZSP Malvern Nano Zetasizer at ambient temperature by dispersing 0.001 g NC in 100 ml DI water having 0.1 M NaOH and 0.1 M HCl and the pH was fixed at 4.0, as suggested by Jesionowski and Krysztafkiewicz (2001). The result of this investigation is shown in Fig. 5 and it shows that the ZP level of −24.6 mV is present on the surface of the synthesized PEG-SiO₂ NC (unmodified), indicating slight instability approaching the stability region of −30 mV. With this ZP, the propensity of fast particles gelling and agglomeration in a dispersion can occur. This charge confirms that enough –OH groups are present on the PEG-SiO₂ NC surface. The negative surface of the unmodified PEG–SiO₂ NC is because of silanol groups deprotonation (Aissaoui et al., 2012). The implication of this charge in a WBM containing bentonite is the occurrence of possible electrostatic repulsive force between SiO₂ NP and bentonite (Aissaoui et al., 2012); that is, mud deflocculation that can reduce the yield point and gel strength will occur. This is disadvantageous to the mud’s drilled cuttings lifting ability and suspension. For this purpose, anionic SDS was used to modify the SiO₂-bearing surface of the synthesized PEG-SiO₂ NC to prevent particle’s agglomerates and to cause effective mud’s dispersion.

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Fig. 5

Average ZP of synthesized PEG-SiO₂ NC (unmodified).

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2.2.4 Functionalization of PEG-SiO₂ nanocomposites

The often used organic modifier to modify the surface of SiO₂ NP is the silane coupling agents (Daniel and Francis, 1998; Posthumus et al., 2004) and anionic SDS molecules (Qiao et al., 2016). Through condensation reaction, the –OH groups on the SiO₂ surface reacts with silane or SDS molecules, at times with the support of a catalyst (Choi and Chen, 2003). This makes organic hydrophobic compounds to adhere to the hydrophobic phase to cause an adherence and modification at the surface (Xie et al., 2010). In this study, amphipathic anionic SDS surfactant that utilized aqueous mixing was used to modify the SiO₂-bearing surface of the produced PEG-SiO₂ NC. SDS is a surfactant with a polar head and a non-polar tail that enables it to self-associate or micellize in solution (Xie et al., 2010). It is a salt and organosulfate synthetic organic compound. Since SDS is soluble in water, the aqueous mixing technique is without an organic solvent and it provided a complete SDS and SiO₂ dispersion. That is, there was perfect interaction (contact) between SiO₂ and SDS in the functionalization reaction (Xie et al.,2010). The modification can participate to endow stabilized dispersed surface to the PEG-SiO₂ NC.

The technique utilized for SDS functionalized PEG-SiO₂ NC is based on previous study by Qiao et al. (2016), that uses the aqueous mixing technique. The aqueous mixing technique was conducted as follows: 3.5 vol% commercial SDS powder was added to 150 ml DI water in an Erlenmeyer flask. After that, the solution was subjected to magnetic stirring for 10 min until it dissolved. Next, the solution was vigorously stirred for 1 h to provide a good dispersion followed by centrifugation at 4000 rpm and spray drying at 150 °C after washing thoroughly with anhydrous EtOH to obtain the modified PEG-SiO₂ NC-SDS. As SDS is rather soluble in anhydrous EtOH, any unreacted SDS can be removed completely by the thorough EtOH washing. Fig. 6 shows the SDS modification mechanism of the PEG-SiO₂ NC.

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Fig. 6

Surface modification of PEG-SiO₂ NC by SDS.

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2.2.5 Evaluating the effect of SDS on PEG-SiO₂ nanocomposites

After the modification process, the influence of SDS on the studied PEG-SiO₂ NC was checked through ZP to ascertain the nature of the charge present on the SiO₂-bearing surface of the PEG-SiO₂ NC. This check was to understand how SDS affects the stability of the PEG-SiO₂ NC particles. From all the anionic surfactants, SDS is the most often applied and preferred due to its low-cost and ability to prevent particles oxidation and isolation. It has a C₁₂ alkyl chain, which infiltrates the oil droplet and increase in the concentration of SDS in micro-emulsion electrolyte lessen the electro-osmotic flow (EOF) because of the increase in the ionic strength of the electrolyte. As an anionic surfactant, SDS can increase the hydrophilicity of PEG and can further partake in stabilizing the charge of SiO₂ suspension (Qiao et al., 2016). As SDS absorbs water, it unsettles non-covalent bonds and break their interactions. As the intermolecular forces between aqueous molecule and SDS are much lower than those between two aqueous molecules, there will be reduction in surface and interfacial tension; hence, a stabilized interface will be obtained.

A ZSP Malvern Nano Zetasizer at ambient temperature was used to check the ZP of the modified material. 0.001 g PP–SiO₂ NC-SDS in 100 ml DI water having 0.1 M NaOH and 0.1 M HCl at a constant pH of 4.0, as suggested by Jesionowski and Krysztafkiewicz (2001) was applied. The result obtained from this analysis is shown in Fig. 7, and it showed a ZP level of −51.5 mV, which indicates propensity of high stability of the particles in a dispersion (refer: Table 2) (Oseh et al., 2019a). By inserting –OH molecules, an anionic reaction took place between the SiO₂-bearing negative surface of the PEG-SiO₂ NC and the –OH molecules on the surface of the SDS, causing a shift in the negativity from −24.6 mV of PEG-SiO₂ NC (Fig. 5) to −51.5 mV (Fig. 7) of the PEG-SiO₂ NC-SDS. Consequently, a steric repulsive force between –OH molecules and silanol group (Si-OH) increases, resulting in a stabilized-dispersed system (Xie et al., 2010). For smaller size particles with high specific surface area, high magnitude of ZP (positive or negative) will confer stability. This is attributed to the ability of attractive forces to exceed forces of repulsion (Hanaor et al. 2012). Colloids or colloidal particles with low ZP (positive or negative) are liable to coagulation, whereas colloidal particles with high ZP (positive or negative) are stabilized electrically, and the description of ZP indicator is shown in Table 2 (Oseh et al., 2019a).

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Fig. 7

Average ZP of PEG-SiO₂ NC-SDS (modified).

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Thus, it can be stated that with the ZP data, the SDS modified PEG-SiO₂ NC have a high stability that can confer good dispersion and effective interaction with WBM molecules as compared to the unmodified product, confirming the fulfilment of one of the research goals, which was to design a stable and dispersible material. This stability was further examined by different characterization techniques to understand the overall performance and attributes of the PEG-SiO₂ NC-SDS.

2.2.6 Material characterization tests

2.3.1 Spud mud formulation

The first drilling mud formulated is an unweighted water-bentonite-NaOH spud mud. This mud is used to start the drilling of a well after the rig has been examined and all of the systems tested. Its application can continue up to the drilling of first-hundred feet of hole (Nyland et al., 1988). After its formulation, different concentrations of the synthesized PEG–SiO₂ NC with or without SDS modification were added into the water-bentonite-NaOH spud mud to test for their mud properties, such as pH, density, rheological and API fluid loss volume without aging at 78 °F. The formulation of this muds is according to the API recommended standard bulletin of fluid test conditions for water-based drilling fluids (API RP 13B-1, 2017). The following concentration in grams (g) and sequence were followed to formulate the water-bentonite-NaOH-based spud mud:

Spud mud (base mud) = 320 ml tap water + 25 g sodium bentonite + 0.15 g NaOH.

After the spud mud formulation, other drilling muds which are the unmodified and SDS modified PEG–SiO₂ NC were formulated and represented as shown in Table 3.

2.3.2 Rheological properties measurements of spud muds with Fann 35 viscometer

To characterize the rheological properties, such as plastic viscosity (PV), apparent viscosity (AV), Yield point (YP), and gel strength (GS), Fann 35-viscometer model was applied. The viscometer dial data were then changed to equivalent shear stresses by applying befitting conversion factors. Afterwards, the shear stresses and effective viscosities were estimated with a non-Newtonian fluid model. The Fann 35-viscometer model consist of a plane concentric cylindrical surface and is broadly utilized in the industry for inspecting drilling fluid viscosity (Kelessidis et al., 2006). Fluids tests conducted with a 6-speed Fann 35-viscometer can achieve efficient approximations of fluid flow behaviour and yield stress. Fann viscometers are typically used and often selected to understand the lifting and suspension capabilities and to indicate solids accumulations, solids flocculations or deflocculations; it is also needed for drilling fluid hydraulic estimation. Furthermore, it is predominantly applied for the measurement of drilling fluid shear-stress/shear strain-rate relationship-from which viscosities and yield stress can be evaluated directly with a two-parameter Bingham plastic model. The instrument is also used to measure fluid thixotropic character and gelation can also be determined using this equipment. The two controlling viscometer readings of Fann 35-viscometer are the spinning rotor and deflection bobs, which are required for wall shear-rate prediction. Dial readings obtained by bob deflection in a known spinning rotor are equivalently changed into shear-stress with practicable precision (Kelessidis et al., 2006).

The data applying to shear-stress at several shear-rates between 5.11 s⁻¹ and 1022 s⁻¹ were registered along with the equivalent dial readings tested between 3 rpm and 600 rpm. Thereafter, the rheological properties were fitted using Eqs. (1) – (3). The measurements were conducted thrice to validate accuracy and the average data obtained were recorded.

2.3.3 Complex drilling mud formulation

The formulation of complex drilling mud without NC (base mud system) was planned to reach a density of approximately 9.5 ppg, as reported by Fattah and Lashin (2016) to be within the domain of optimal density (9.0 to 10 ppg) for WBMs. To formulate the base mud, 450 cm³ of mud, which is equivalent to one laboratory barrel was prepared to meet the objectives of this study. Tap water (base liquid), sodium bentonite (for viscosity and control filtrate loss), NaOH (to modify pH), and Na₂CO₃ (to disperse sodium bentonite) were used to formulate the base-aqueous mud. Other additives used are XG (to serve as viscosifier and suspend drilled solid particles), PAC-R (to control filtrate loss), and BaSO₄ (for weight control). After the base mud formulation, only SDS modified NC were added to check the impact of this concentrations on the base mud rheological and filtration properties behaviour. The SDS bearing NC was used because the spud mud data of the SDS modified NC (MS-4, MS-5, and MS-6) showed better stability and enhancement of the water-bentonite-NaOH spud mud (S0) properties than the unmodified NC (US-1, US-2, and US-3). Prior to the formulation, the four concentrations of the SDS modified NC were dispersed into 100 ml of distilled water and sonicated for 2 h before they are introduced into the base mud system immediately after barite blending. The added 100 ml of distilled water was subtracted from the total volume of the water used to formulate the SDS modified NC drilling muds.

The following concentration in grams (g) and sequence and were followed to formulate the complex drilling mud (base mud) system:

Complex drilling mud (base mud) system = 450 ml tap water + 15 g sodium bentonite + 0.25 g NaOH + 0.25 g Na₂CO₃ + 0.20 g XG + 2.0 g PAC-R + 34 g BaSO₄.

After the base mud formulation, different concentrations (0.5 g, 1.0 g, 1.5 g, and 2.0 g) of only the NC with SDS modification were formulated and represented in Table 4.

2.4.1 Rheological properties test with rotational rheometer

Rheometry technique is applied to examine the rheological performance of a material and rheology involves the study of a material when it flows or distorts. Thus, rheology defines forces and distortions of a material over a period of time (Caenn et al., 2017). The NC’s rheological description is essential to provide adequate understandings into the flow characteristics against distortion and physical stability of a material. Therefore, rheological tests of SDS bearing NC mud samples were examined using a Rotational Rheometer (Cylindrical Coaxial Brookfield RST Rheometer) armed with water bath and temperature regulator for high temperature environments. The rheometer is branded with a robust and fast couple system attached for trouble-free spindle, and a testing compartment to hold the cylindrical coaxial spindle and mud sample for rheological examination. It has the capacity to generate results of shear-rate/viscosity and shear-rate/shear-stress at a specified temperature conditions. All the rheological testings of SDS bearing NC mud samples were performed under the shear-rate range of 1.0 s⁻¹ to 250 s⁻¹ and temperatures varied up to 78 °F and 250 °F in triplicate to establish data precision. Before making any test, the rheometer arrangement was standardized by measuring with tap water at variable temperatures. Afterwards, the NC samples with SDS were transferred into the cylindrical coaxial apparatus and tested. The uncertainty of this test is that in the rotational rheometer, the shear-rates were fixed and the mud viscosity was tested. However, in the borehole drilling environment, the shear-rate quite varies and it depends on both the inherent geometry and the fluid saturation hindering the aqueous flow.

3.1.1 PSD and laser SSA results

The PSD and laser SSA data of SDS-free PEG–SiO₂ NC and PEG–SiO₂ NC with SDS are presented in Fig. 8a, b, and c, respectively. The nature of the curves is identical, but PEG–SiO₂ NC with SDS sample (Fig. 8b) indicated a slight broader distribution of particles as compared to the SDS-free PEG–SiO₂ NC sample (Fig. 8a) because of increase in micro molecules (SDS molecules) in the modified particles (Fig. 8b) (Mao et al., 2015). Furthermore, the particles of both NCs are largely found not beyond 410 nm, wherein the SDS-free PEG–SiO₂ NC particles ranged between 65 nm and 380 nm with a D50 (median diameter) of 182 nm and 118 nm mean diameter (Fig. 8a). For the SDS bearing PEG–SiO₂ NC sample, the particles are distributed within the range of 82 nm to 410 nm with a D50 (median diameter) of 195 nm and 144 nm mean diameter (Fig. 8b). This data point out that both PEG–SiO₂ NC particles with/without SDS contain narrow range of size distribution within the nanometer domain.

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Fig. 8

PSD data of PEG-SiO₂ NC (a) without SDS (b) with SDS, and (c) Laser diffraction D50, SMD, and SSA of PEG-SiO₂ NC with and without SDS.

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A particle SSA is the means solid particles adopt to interact with the drilling fluid system. The particle SSA per unit mass or volume rises with a reduction in the size of the particles. A larger particle SSA implies better grip of particles, higher cohesion, and the larger contact area of the particles with other particles or molecules (Mao et al., 2017). The data of laser SSA of SDS-free PEG–SiO₂ NC and PEG–SiO₂ NC with SDS are summarized in Fig. 8c. The evaluated PEG–SiO₂ NC both with/without SDS showed a significantly high average SSA of 49.3 m²/g for unmodified and 41.4 m²/g for modified. These SSA values are practically sufficient to cause strong interaction with the base mud additives.

3.1.2 FTIR result

The FTIR peak assignments were represented in Figs. 9 and 10 plotted in transmittance (%) and wavenumber (cm⁻¹). Fig. 9 presents the difference in FTIR spectra of SiO₂ and SDS-free PEG-SiO₂ NC. These spectra clarify whether the synthesis of the newly-developed PEG-SiO₂ NC was effectively conducted in aqueous medium and if PEG and SiO₂ are combined successfully through bonding network_. In Fig. 9, the transmittance peak at 3441 cm⁻¹ was allocated to –OH groups on the particle surface of both SiO₂ (Fig. 9a) and PEG-SiO₂ NC (Fig. 9b) (Oseh et al., 2020a). The small peak that appeared at 1701 cm⁻¹ in both particles was ascribed to H-O-H bond, in which the hydrogen atom from a C–H group (alkane) was replaced with an –OH groups thereby permitting the association of the molecules via H–bonding (Qiao et al., 2016). The transmission peak at 1101 cm⁻¹ that appeared in both the spectra of SiO₂ (Fig. 9a) and PEG-SiO₂ NC (Fig. 9b) was due to the Si–O–Si groups stretching vibration. Two other transmittance peaks found at 490 cm⁻¹ and 800 cm⁻¹ belonged to the Si–O stretching vibration (Xu et al., 2018). All these transmission peaks belonged to SiO₂ (Fig. 9a) and were found in the peak assignment of PEG-SiO₂ NC (without SDS) shown in Fig. 5b. Moreover, the characteristic peaks that appeared at 1400 cm⁻¹ and 3001 cm⁻¹ (Fig. 9b) belonged to the alkyl groups due to bending and stretching vibrations, respectively. A classic characteristic peak that occurred at 960 cm⁻¹ of C–H (methylene) group corresponds to –CH₂ of PEG. These peaks occurred in the spectrum of PEG-SiO₂ NC in pink spotted lines (Fig. 9b) but were absent in SiO₂ spectrum depicted in (Fig. 9a), validating the successful formation of the PEG-SiO₂ NC particles.

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Fig. 9

FTIR spectra of (a) SiO₂ and (b) SDS-free PEG-SiO₂ NC.

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Fig. 10

FTIR spectra of (a) SDS powder and (b) PEG-SiO₂ NC with SDS.

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The FTIR spectra of SDS and PEG-SiO₂ NC with SDS investigated to verify the success of inserting SDS molecules on the PEG-SiO₂ NC surface are shown in Fig. 10. In Fig. 10a, the spectrum ascribed to SDS has the following characteristic peaks: the transmission peak at 3460 cm⁻¹ belonged to H–OH stretching vibration, while the two peaks at 2914 cm⁻¹ and 2846 cm⁻¹ belonged to CH₂ group of SDS molecules stretching and bending vibrations. The peaks at 1240 cm⁻¹ and 1218 cm⁻¹ encompass the S–O stretch bridge and they correspond to the skeletal mode. The transmittance peak found at 1070 cm⁻¹ belonged to C–C stretch, while the two peaks that occurred at 580 cm⁻¹ and 830 cm⁻¹ were ascribed to C–H asymmetric bending mode of the CH₂ group (Qiao et al., 2016). These peaks all belonged to the SDS spectrum (Fig. 10a) and they are all found in the spectrum assignment of PEG-SiO₂ NC with SDS indicated in red spotted lines, as shown in Fig. 10b. Through the FTIR analysis, it was found that SDS molecules are present on the synthesized PEG-SiO₂ NC surface. Thus, newly-developed PEG-SiO₂ NC was dispersed by using SDS surfactant; protecting the PEG-SiO₂ NC from oxidation and isolation.

3.1.3 FESEM and EDX results

The FESEM technique was introduced to investigate the morphology of the newly-developed PEG–SiO₂ NC with/without SDS. The motive for this investigation is to comprehend the morphological features of these materials and to ascertain the performance of SDS in the modified PEG–SiO₂ NC. The morphological configuration of these materials are depicted in Fig. 11. The FESEM micrograph of PEG–SiO₂ NC without modification depicted in Fig. 11a showed that the particles of PEG-SiO₂ NC is spherical with partial aggregation. The particles tend to stick and cluster to each other to form an aggregates. The particle’s aggregation may possibly be the results of non-uniform TEOS self–assembly (Qiao et al., 2016). According to the FESEM morphology, the particle size of PEG-SiO₂ NC was around 200 nm.

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Fig. 11

FESEM morphology of PEG–SiO₂ NC (a) without SDS, and (b) with SDS.

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In Fig. 11b of the PEG-SiO₂ NC with SDS, the particle’s morphology was a general particulate, which is a SiO₂–PEG core shell particles supported by SDS (Xu et al., 2018). The particles are well–dispersed and the morphology mainly contain individual separated particles because of the support from SDS. The few clustered white spot that appeared in the morphology could be unreacted SDS or SiO₂ and this cannot adversely impact the physical properties of the newly-developed PEG–SiO₂ NC with SDS (Qiao et al., 2016). Moreover, the morphology indicates a particle size of about 200 nm, which is consistent with the PSD results.

EDX method is an accessory to electron microscopy like FESEM or TEM gadgets. The EDX examination was introduced to detect the elements that constitute the newly-developed PEG–SiO₂ NC with/without SDS. Fig. 12a and b presents the elemental composition of (a) PP–SiO₂ NC without SDS and (b) PEG–SiO₂ NC with SDS. The elemental composition showed that the synthesis and modification protocols were effective. The elements that occured in both tables (insets) in Fig. 12a and b are O (Oxygen), C (Carbon), and Si (Silica). From this examination, H (hydrogen) which is the lightest of all elements was not found, nonetheless a notable amount of the element possibly exists. From the table (inset) displayed in Fig. 12b, the newly-developed PEG–SiO₂ NC with SDS exhibits the presence of Na (Sodium) and S (Sulphur), which were not seen in the EDX spectroscopy of synthesized PEG–SiO₂ NC without modification (Fig. 12a). The EDX spectra further corroborate that PEG and SiO₂ were combined successfully to form the PEG–SiO₂ NC and effectively modified to produce PEG–SiO₂ NC–SDS.

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Fig. 12

EDX spectra of PEG–SiO₂ NC (a) without SDS, and (b) with SDS.

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3.1.4 TGA results

The TGA data of PEG, SDS, and PEG-SiO₂ NC with/without SDS are shown in Fig. 13. When examining the Fig. 13, it seems that two thermogravimetric stages occurred overall. There is no weight loss for both PEG and SDS from 30 °C to 100 °C. The first (initial) weight loss occurred between 101 °C and 250 °C for PEG, while that of SDS occurred between 151 °C and 200 °C, and this phase recorded a small margin of weight loss, which may perhaps, is due to complete exclusion of moisture content in the PEG and anionic SDS, since the boiling point of water is 100 °C (Xu et al., 2018; Qiao et al., 2016). A very sharp decrease in the thermogravimetric curve was observed for the PEG and SDS, and this stage represents the final weight loss; the weight loss is from 251 °C to 400 °C for PEG, and between 201 °C and 326 °C for SDS. For the SDS, the weight loss is due to its phase change as it has a melting point range between 205 °C and 210 °C (Qiao et al., 2016), while the degradation of PEG molecules is the cause of PEG weight loss (Xu et al., 2018).

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Fig. 13

Thermal stability of PEG, SDS, PEG–SiO₂ NC, and PEG–SiO₂ NC–SDS.

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For the PEG–SiO₂ NC, there is no weight loss from 30 °C to 240 °C; its initial weight loss occurred from 270 °C to 350 °C, which is due to the disintegration of –OH ions and its final thermogravimetric curve occurred from 351 °C to 400 °C, and remains constant from 400 °C up to 600 °C, and this phase suffered a quite severe weight loss, which is as a result of deterioration of PEG molecules (Xu et al., 2018). For the SDS modified PEG-SiO₂ NC, there is no weight loss from 30 °C to 340 °C and the only weight loss observed occurred between 341 °C and 450 °C, and in this phase, the weight loss is quite sharp. It remains unchanged from 450 °C to 600 °C; it is caused by the degradation of the particles of SDS and PEG molecular chains (Qiao et al., 2016). Thus, the increase in the SDS modified PEG-SiO₂ NC residual weight validates the inclusion of SDS on the surface of the modified PEG-SiO₂ NC. In general, the newly-developed PEG-SiO₂ NC with/without SDS possess satisfactory thermal stability.

3.2.1 pH and density data of spud mud

pH level indicates the liquid hydrogen ion concentration (H⁺) and it is a measure of how acidic or alkaline is the drilling fluid. The study of pH is integral in the control of drilling fluids properties, especially the rheology. Fluids pH impacts the effectiveness and solubility of chemicals and clay dispersion (Singh and Dutta, 2018). A modification in the level of pH during drilling often represents contamination by materials, such as gypsum, cement, or carbon dioxide. Studies revealed that drilling mud pH ought to be preserved at level between 8 and 10, because increasing mud pH will raise its rheological parameters (Singh and Dutta, 2018). Table 6 presents the pH data of water-bentonite-NaOH spud mud with different contents of SDS-free PEG-SiO₂ NC and PEG-SiO₂ NC with SDS at 78 °F. The pH level of the spud mud at 8.5 remained unchanged with both SDS-free PEG-SiO₂ NC and PEG-SiO₂ NC with SDS, which is due to PEG inability to alter the pH of aqueous solution as it is not a surfactant.

Also presented in Table 6 is the density of the spud mud with three concentrations (0.5 g, 1.0 g, and 1.5 g) of PEG-SiO₂ NC-SDS. For the mud density, precise data on the changing character of fluid variables, especially the density of drilling mud is indispensable for designing a fruitful drilling program under such environment. The mud density constantly changes along with reduction in temperature and rise in pressure for HPHT wells. In typical conventional drilling, the mud density ought to be in surplus of the equivalent fluid pressure of the drilled formation to avert any incursion of formation fluids into the drilled hole (referred to as kicks) (Fattah and Lashin, 2016). As shown in Table 6, the density of the spud mud at 8.6 ppg was constant with SDS-free PEG-SiO₂ NC, but slightly changed to 8.7 ppg (S4-M) and remain constant at 8.8 ppg for S5-M and S6-M. The little variation in density could be due to more solids content of the PEG-SiO₂ NC-SDS (Oseh et al., 2020a).

3.2.2 Rheological properties data of the spud mud

The rheological data of all the spud muds showed that the interaction of PEG-SiO₂ NC with or without SDS and base mud additives generally changes the rheological parameters (AV, PV, YP, YP/PV, and GS), as presented in Table 6.

3.2.3 Filtration control properties data

Today, drillers are concentrating more on ground-breaking technologies to apply in drilling operations to prevent or reduce fluid loss into the reservoir formation and fluid invasion towards the porous and permeable media. It was found that to obtain a homogenous and stabilized NPs or NC-based muds for fluid loss control, highly efficient polymers or surfactants containing large neutralizing potentials are to be used (Medhi et al., 2020). Therefore, investigation of the filtration properties performance of NC with/without SDS surfactant in a water-bentonite-NaOH mud system was evaluated in this study and the data obtained are summarized in Table 7.

3.3.1 Rheological properties characterization of complex drilling mud systems

The universal phrase ‘rheology’ denotes the science relating to the study of flow behaviour and distortion of materials. It is governed by its features, principally the viscosity. The rheological characterization of a material provides a holistic knowledge on the material viscoelastic flow character. Undoubtedly, rheology is very vital to every drilling fluid material because the responses of rheological properties are analogous to the ultimate configurations of the material (Luo et al., 2017).

Rheological properties tests for mud samples are indicator of the nature and rate of deformation or degradation that take place when the mud’s materials are stressed (Nelson and Guillot, 2006). Fluids exhibit different characters with stress after a while. Overtime, as stress increases, fluid decreases in viscosity. This is known as thixotropic fluids. Most drilling muds used in the circulation process are thixotropic, shear thinning, and non–Newtonian with a yield stress having an increasing shear-rate with a decreasing viscosity (Nelson and Guillot, 2006). Drilling mud show an internal arrangement because of its compositions which is liable to modification giving to shear and flow environments (Sayindla et al., 2017). The non-Newtonian flow character of drilling mud has been ascribed to the transmission of the shear-stress through the continuous medium; distortion or orientation of the suspended particles opposing the random influence of Brownian motion (Nelson and Guillot, 2006).

Nowadays, in drilling programs, the shear-rate hardly ever go beyond 250 s⁻¹, essentially for a hole section of 8.5 in. and equivalent. If a hole section of 17.5 in. and equivalent is considered, the shear-rate hardly go above 50 s⁻¹. Hence, for higher precision and to checkmate the rheological variables against over-approximation during the course of drilling, fluid rheological behaviour and description can give a suitable approximation of the necessary possible pump rates required (Oltedal et al., 2015). Therefore, Bingham plastic and Ostwald-de-Waele models were employed in this study to describe the rheological fluid behaviour of all the complex mud samples obtained from a rotational rheometer tests ranging from 1.0 s⁻¹ to 250 s⁻¹ shear-rates.

3.3.2 Bingham rheological Characterization-Shear stress/shear rate relationship

In conventional drilling, Bingham plastic and Power law (Ostwald-de-Waele) models are two mathematical models predominantly applied in the field for description of rheological behaviour. The Bingham plastic model is a theoretical model that specifies the elements that impacts rheological variables (Nelson and Guillot, 2006). This model is widely introduced to describe the features of different forms of drilling fluids. Typically, drilling muds obeying this model demonstrate a linear shear-rate/shear-stress relationship after the threshold, initial shear-stress (YP) is surpassed. Thus, the shear-rate/shear-stress relationship and shear-rate/viscosity relationship of the complex drilling muds were fitted using Bingham plastic model according to Eq. (4):

τ symbolizes shear stress (cP), ${\tau }_0$ denotes the threshold stress (intercept) known as the YP (lb/100ft²), $\gamma$ signifies the shear rate (s⁻¹), and ${\mu }_p$ which indicates the PV (cP) symbolizes the slope of the line.

Fig. 14a and b shows the rheometer data of shear-rate and shear-stress of formulated complex mud samples tested at two temperatures (T = 78 °F and 250 °F), respectively. With an increase in shear-rate from 1.0 s⁻¹ –250 s⁻¹, the shear-stress of all the mud samples was increased at the temperature of 78 °F (Fig. 14a). This increase in shear-rate/shear-stress is substantially higher when SDS modified NC samples were used. SDS bearing NC increases the shear-stress of the base mud (CD0) with shear-rate with an increase in concentration from CD1 – CD3 and drop a little from CD3 – CD4. At mud composition CD3 (29.5 cP) relating to 250 s⁻¹ shear-rate, the margin of increase over mud composition CD0 (17.8 cP) is in the margin of 65.7%, while that of mud composition CD4 (25.8 cP) registered 44.9% over CD0. According to the trend lines of the SDS modified NC mud samples, there is a drop in shear-stress beyond 1.5 g concentration of the modified NC which is due to the reduction in the internal friction between the molecules (Boyou et al., 2019). Mud composition CD2 (23.7 cP) displayed 33.1%, while the lowest margin of increase over CD0 was found in CD1 (21.9 cP) by 23.6%. The positive hydrogen ions (H⁺) present in the SDS modified NC created hydrogen bonds with aqueous mud molecules to increase the strength of the intermolecular interaction between the aqueous molecules and the SDS modified NC; resulting in the improvement of shear-stress of the base mud (CD0) (Boyou et al., 2019; Qiao et al., 2016). Furthermore, the negative hydroxyl ions (OH⁻) in the SDS modified NC surface are adsorbed on the surface of bentonite clay platelets to form strong hydrogen bridges that increased the rheological properties of the SDS bearing NC mud samples (Mao et al., 2015).

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Fig. 14

Shear-stress of drilling muds against shear-rate ((a): T = 78 °F; (b): T = 250 °F).

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After oven treatment, the inclusion of SDS bearing NC in the complex base mud also cause the shear-stress to increase over CD0 (Fig. 14b). The observed trend lines of shear-stress in Fig. 14b at 250 °F are similar to those represented in Fig. 14a at 78 °F. For example, the mud compositions CD1 – CD4 containing SDS modified NC raised the shear-stress of the CD0 (14.2 cP) by 23.7% for CD1 at 17.6 cP, 40.1% for CD2 at 19.9 cP, and recorded the highest increasing margin with CD3 (24.8 cP) by 74.6% and a little drop by 59.2% with CD4 (22.6 cP) at a shear-rate of 250 s⁻¹. For the rest shear-stress, similar trend lines to that of 250 s⁻¹ shear-rates were observed. In the Fig. 14, it was observed that a very little change occurred despite the change in temperatures. This is because of the high dispersion, good physical stability and thermal stability of the SDS bearing NC particles (Qiao et al., 2016). This is a confirmation of the ZP and TGA data shown in Figs. 5 and 13, respectively.

3.3.3 Bingham rheological characterization-shear rate/viscosity relationship

Fig. 15a and b presents the rheometer viscosity character of SDS bearing NC in complex base mud at 78 °F and 250 °F, respectively. The rheogram shown in Fig. 15a illustrates that the base mud (CD0) and all the SDS bearing NC concentrations (CD1 – CD4) display a non-Newtonian fluid character. The viscosity of all the drilling mud samples decreases with increasing shear-rate, therefore, establishing a pseudoplastic character at low, moderate, and high shear rates considered (Oseh et al., 2020a; Boyou et al., 2019). According to the trend lines shown in Fig. 15a, the SDS bearing NC samples exhibited a momentous margin of increase in the viscosity of the base mud (CD0) at 3.35 cP at the lowest shear-rate of 1.0 s⁻¹. For CD1 (4.43 cP) and CD2 (5.57 cP), the margin of increase was lower by 32.2% and 66.3%, respectively, compared to the one recorded with CD4 (6.72 cP) by 100.6%, that drop behind CD3 of 7.85 cP. This mud composition (CD3) recorded a significantly largest increase by 134.3% over CD0 at the lowest shear-rate of 1.0 s⁻¹. The viscosity trend lines of all the mud samples at other shear-rates greater than 1.0 s⁻¹ up to 250 s⁻¹ exhibited similar flow pattern from lowest to highest shear-rates. The resistance due to internal friction is highest at the lowest shear-rate of 1.0 s⁻¹; thus, highest viscosity. Also, as shear-rate increases, more particles, molecules, and polymer chains orient in the flow direction reducing the internal friction resulting in decrease in viscosity with increasing shear-rate (Gbadamosi et al., 2019; Mao et al., 2015). Thus, the pseudoplastic character shown on the plot of shear-rate/viscosity in Fig. 15. This interprets that the SDS bearing NC particles possess a beneficial viscosifying component for drilling operations. The viscosifying component of the SDS bearing NC particles is the ability of the dispersed particles (as was confirmed by FESEM results in Fig. 11) to embed in the particle pore structure on the clay surface and conferred links with sodium bentonite clay platelets, that in effect supports clay particles gelation; hence, the viscosity (Aftab et al., 2017).

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Fig. 15

Viscosity of drilling muds against shear-rate ((a): T = 78 °F; (b): T = 250 °F).

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As shown in Fig. 15b, the trend lines of the viscosities of CD1 – CD4 at 250 °F temperature are analogous to that of CD0 and the CD1 – CD4 displaying more improved viscosyfing character. These compositions represented in Fig. 15b showed little lower viscosities with increasing shear-rates as compared to the ones presented in Fig. 15a. With higher temperature, the intermolecular attraction existing between the mud’s molecules tends to deteriorate resulting in reduced viscosities of all the mud samples (Aftab et al., 2017). As observed earlier at 1.0 s⁻¹ shear-rate, the behaviour of the mud samples with/without aging is alike. CD1 (3.34 cP) and CD2 (4.43 cP) improved the viscosity of CD0 of 2.25 cP by 48.4% and 96.9%. For CD3 (6.62 cP), the highest margin of improvement by 194.2% was achieved, while a drop in the viscosity to 145.3% by CD4 of 5.52 cP over CD0 was recorded at 1.0 s⁻¹ shear-rate. With this rheological trend lines demonstrated by the SDS bearing NC particles, the mud may not be thick enough to induce high ECD in a deep water low–temperature formations. Due to a narrow safe–density hole-in-the-wall of such formations, a high ECD will cause formation fracture and lost circulation (Alsaba et al., 2014). This is a problem in drilling operation that often requires huge capital to address or can even lead to loss of entire well. Moreover, the viscosities of the SDS modified NC mud samples are stable and did not undergo notable fluctuations, largely due to its enough small size, sufficient high SSA (refer: Fig. 8), and high dispersion stability (refer: Fig. 5).

3.3.4 Ostwald-de-Waele rheological characterization of flow index and consistency factor

The Power law (Ostwald-de-Waele) model was also used to fit the rheological parameters using Eq. (5) and the data achieved are presented in Figs. 16 and 17. This model is an empirical model derived from analysis of laboratory data (Casson, 1959). Today, this rheological model gets greater attention in the field because it gives a high precision of fluids behaviour (Casson, 1959).

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Fig. 16

Flow behaviour index of drilling mud samples (T = 78 °F and 250 °F).

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Fig. 17

Fluid consistency factor of drilling mud samples (T = 78 °F and 250 °F).

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μ = viscosity (cP), γ̇ = shear rate (s⁻¹), n = dimensionless flow behaviour index, and K = fluid consistency factor (lb/100ft²ⁿ).

Fig. 16 presents the results of the Ostwald-de-Waele rheological characterization of flow index (n) of all the mud samples at temperatures of 78 °F and 250 °F. According to Fig. 16, SDS bearing NC concentration reduces from CD0 to CD1 and increases to CD2, then drops from CD2 to CD3 before increasing again to CD4 at 78 °F. Similar observation was seen at 250 °F but in this case the reduction is from CD0 to CD3 before a little increase from CD3 to CD4. The range of the values of n from CD0 to CD4 is between 0.172 and 0.126 at 78 °F and between 0.193 and 0.132 at 250 °F, indicating a flat viscosity-profile. For the observed highest shear-thinning character (CD3), it improved the shear-thinning property of the base mud (CD0) by 26.7% and 31.6% at 78 °F and 250 °F, respectively. For the two evaluated temperature conditions, all the drilling mud systems are much below one (n $\le 1)$ ; specifying all the mud compositions to possess high shear-thinning character, suitable for drilling operations.

For the fluid consistency factor (K) depicted in Fig. 17 for both temperature conditions (T = 78 °F and 250 °F), the SDS modified NC mud samples increases with an increasing concentration from CD1 to CD3 over CD0, but decreased a little from CD3 to CD4. The highest margin of increase occurred with mud composition CD3 by 87.2% and 145.2% at 78 °F and 250 °F, respectively. All the mud samples exhibited beneficial rheological properties for drilling operations; they lie between 73.42 lb/100ft²ⁿ and 113.3 lb/100ft²ⁿ for 78 °F and between 52.14 lb/100ft²ⁿ and 109.6 lb/100ft²ⁿ for 250 °F, indicating an improvement in the fluid’s viscosity for better lifting and suspension of drilled solids (Ismail et al., 2016).

3.4.1 Filtrate loss control data of complex drilling mud systems

Having a low loss of filtrates into the drilled formation is important to achieving optimal hole stability while drilling. Drilling muds are tested to investigate the filtrates that seeps via a certain medium (filter paper), leaving formed or deposited cake on the medium (filter paper). A low permeable and thin filter cake is desirable in drilling jobs; it prevents differential stuck pipe problems. Thick and high permeable filter cake is undesirable while drilling; the effective hole diameter is reduced and subsequently, the contact area between the drill string and filter cake is increased resulting in a greater problems of differential stuck pipe (Caenn et al., 2017). The filtrate loss control of different drilling mud systems under API and HPHT conditions are represented in Fig. 18. The data displayed in Fig. 18 show that the base mud sample (CD0) recorded the highest filtrate loss into the reservoir formation under both API and HPHT conditions. The API FL of the CD0 was 10.2 ml, and with the SDS bearing NC, it decreased by 15.7% for CD1 at 8.6 ml and 24.5% for CD2 at 7.7 ml. It was best controlled by mud composition of CD3 by 47.1% at only 5.4 ml, while mud composition CD4 with 2.0 g (highest concentration of the NC) showed 33.3% control at 6.8 ml and drop behind CD3 of 1.5 g. The newly-developed SDS bearing NC was efficiently dispersed in aqueous phase and it was able to permit less filtrate to seep via its hydrophilic film that offered a passage for filtrates to escape into the permeable formation (Oseh et al., 2020c).

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Fig. 18

Filtrate loss control data of drilling mud samples at API and HPHT conditions.

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Increasing temperature can participate to reduce the aqueous phase viscosity, thus raising the amount of filtrate loss. According to Fig. 18, the filtrate loss of all the mud samples tested under HPHT conditions were increased with the base mud (CD0) exhibiting the highest loss. It can be said that, at 250 °F, molecules of the base mud tend to degrade faster and needs the support of effective filtrate loss control material to support the PAC-R used, if not high loss of filtrate may happen. The margin of reduction in the filtrate loss volume of the CD0 at 20.5 ml is highest with mud composition CD3 by 53.2% at only 9.6 ml followed by 40.5% for mud composition CD4 at 12.2 ml. Mud compositions CD1 and CD2 showed a decrease of 15.1% and 30.2% at 17.4 ml and 14.3 ml, respectively. The reduction in the filtrate loss control performance of the SDS modified NC samples at 250 °F is likely to have come from the PEG that has the poorest thermal resistance compared to SiO₂ and SDS; thus, the particles could no more offer enough to seal the pore spaces of the formation (Medhi et al., 2020). From this data, similar behaviour of filtrate loss control was experienced in the rheological characterization of all the mud samples using the Bingham plastic and Ostwald-de-Waele models.

3.4.2 Filter cake thickness data of complex drilling mud systems

Filter cakes indicate the residues that are deposited on filter papers (permeable media) when drilling muds under a certain pressure are compelled against the media. The liquid that seeps via the media, leaving the cake on the media is the filtrate. Filter cake performs essential task to making permeable porous formation to become stable. The ideal filter cake should be tough, thin, flexible, and low permeable (Mao et al., 2015). Filter cake properties, such as thickness and permeability were studied in this research; they are vital because the deposited cake on permeable regions in the wellbore can induce incident of differential pipe sticking and wellbore damage. Wellbore damage is associated with less petroleum production when a poor filter cake permits high filtrate intrusion. A certain level of filter cake thickness ( $\le$ 2.0 mm) and permeability range (10^-2 to 10^-4 mD) is desired for drilling under API conditions (Bageri et al., 2015).

Fig. 19 contains the API FCT and HPHT FCT of the drilling mud samples. The cake thickness of all the mud systems is very low and is found to be $\le$ 2.0 mm for API and within the recommended field range for HPHT conditions. Mud composition CD3 has the lowest cake thickness at both API and HPHT conditions with about 61.5% and 36.7% less thick than CD0 (base mud), respectively. The rest SDS bearing NC mud systems (CD1, CD2, and CD4) displayed less thickness of 14.4%, 23.7%, and 30.7% (API conditions) and 25.6%, 31.1%, and 33.3% (HPHT conditions) over that of CD0 (base mud), respectively. Drillers often prefer this property to be as low as possible to permit less filtrate loss to protect the wellbore and reservoir formation.

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Fig. 19

Filtrate loss control data of drilling mud samples at API and HPHT conditions.

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3.4.3 Filter cake permeability data of complex drilling mud systems

The estimated cake permeabilities of the filter paper used for all the mud samples are shown in Fig. 20 under API and HPHT conditions. These permeabilities are calculated from Darcy’s law using the following relations (Eqs. (6) – (8)) proposed by Hubbert (1957):

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Fig. 20

Filter cake permeability data of drilling mud samples at API and HPHT conditions.

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From Eq. (6), $\mathit{dv}/dt$ represents the rate of filtration, $K$ indicates the proportional constant representing the cake permeability, $A$ defines the cross-section area, $\mathrm{\Delta }P$ defines the differential pressure, $\mu$ represents the mud (dynamic) viscosity, and $h$ represents the thickness of the filter cake accomplished using Eq. (7).

From Eq. (7), $f_{\mathit{sc}}$ represents the deposited fraction amount of solids in the filter cake, $f_{\mathit{sm}}$ represents the fraction amount of solids in the drilling mud, and $t$ is the time used in the filtration test. The filter cake area of cross-section was set constant at 31.2 cm² and $V_f$ represents the viscosity of the filtrate that was calculated at a 500 psi differential pressure $\left(P_o-P_i\right)$ using Eq. (8). P_o is the outlet fluid pressure and P_i is the inlet fluid pressure.

From Fig. 20, the SDS modified NC samples (CD1 to CD4) displayed more permeable filter cakes as compared to the base mud (CDO). Mud compositions CD1 to CD4 will be more preferred in drilling operations for the protection of wellbore integrity. The base mud (CD0) has the highest cake permeability at both conditions, while mud composition CD3 has the lowest; being less permeable than CD0 with about 67% (API conditions) and 50% (HPHT conditions). Furthermore, the permeabilities of all the mud samples at both conditions are found to be within the suggested range of 0.01 – 0.0001 mD by Bageri et al. (2015). With these filtration data, the newly-developed SDS modified NC reduced the filtrate loss volume of the base mud; has less cake thickness and is more permeable than the base mud. Along with the rheological characterization data, the optimum concentration of the newly-developed SDS bearing NC in the complex drilling mud system is found to occur at mud composition CD3 (CD3 = CD0 + 1.5 g SDS modified NC); it displays much improved drilling mud properties. This translates that beyond the 1.5 g concentration under the studied prevailing conditions, the drilling job program may possibly no longer be optimized.