Rheological Behaviour of Eco-friendly Drilling Fluids from Biopolymers

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Structure of xanthan gum polysaccharide

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In the present paper, the effects of adding various quantities of different biopolymers (Polyanionic cellulose, carboxymethyl cellulose and xanthan gum polysaccharide) to a drilling mud sample containing bentonite will be reported. The rheological properties of the drilling fluids will be determined and comparison made. Xanthan gum is an anionic polysaccharide polymer with high molecular weight generated by starch after fermentation of xanthomonas campestris. Carboxymethyl cellulose (CMC) is a semi-flexible anionic cellulose ether polymer ^[27] that is produced by reacting alkali cellulose with sodium monochloroacetate under rigidly controlled conditions. It is a chemical derivative of cellulose where some of the hydroxyl groups (-OH) are substituted with carboxymethyl groups (-CH₂COOH) while polyanionic cellulose (PAC) is a kind of anionic cellulose ether of high purity and high degree of substitution, prepared with natural cellulose through chemical modification. Its sodium salt is often utilized. The primary difference between the CMC and PAC production processes is in the radicalization step and the high degree of substitution in PAC. The structures of the biopolymers used are illustrated below. As is well known for CMC, PAC and other cellulose ethers, the characteristics of these materials can be varied by having different average numbers of substituents per glucose unit and different molecular weights ^[28].

2. Theory

The concepts of shear stress and shear rate and their measurement enable the mathematical description of the flow of drilling muds. The amount of force applied to a fluid determines the shear rate, which in oil field terms is determined by the flow rate of the fluid through a particular geometric configuration. Flow models are plots either of flow pressure versus flow rate or of shear stress versus shear rate.The Power law and Herschel –Bulkley models are described here and used forthe characterization of the drilling fluids studied.

2.1. Power Law Model

Power law is a two – parameter model that relates shear stress to shear rate in a nonlinear manner ^{[17, 23]}. The model does not consider an excess yield stress and states the relation as:

(1)

where k, and n are consistency and flow indices respectively, is the shear stress, and is the shearrate. Taking the logarithm of the equation, the following term is obtained:

(2)

Thus, n = slope and k = intercept.

2.2. Herschel-Bulkley Model

Herschel-Bulkley is a three-parameter model that describes the behaviour of yield-pseudoplatics fluids ^[21].

(3)

where is the shear stress, is the yield stress, k is the consistency index, is the shear rate, and n is the flowindex.

3. Materials

The xanthan gum polysaccharide used for this study was obtained from Baker Hughes Houston USA under the trade name of XAN-PLEX™. Polyanionic cellulose regular under the trade name DRISPAC from Yu Long Chemicals China was among the natural polymers used in this study. Bentoniteand carboxylmethyl cellulose were obtained from Global Oil Company Nigeria Limited. The equipment/apparatus used in this study include: Fan V-G meter 8 speed model viscometer; hamilton beach mixer; stop watch; electronic weighing balance; spatula, viscometer cup and hamilton beach mixer cup; standardized weighing plate and thermometer.

3.1. Experimental Procedure

The suspension of bentonite samples were prepared by immersing 15g of bentoniteclay into 350cm³ of deionized water. The mixture was stirred vigorously for 15 minutes to achieve homogeneity while intermittently dislodging any bentonite clinging to the wall of the mixing container. The bentonite suspension was then stored at room temperature in a sealed container for 24 hours to give room for aging. Polyanionic cellulose Regular (PAC-R)was added to the Bentonite suspension at varying concentrations (0.50 g, 0.75g, 1.0g) and the viscometric studies carried out to determine its rheological behaviour on the drilling fluid. Suitable amountsof sodium hydroxide (NaOH) and Potassium chloride (KCl) were added to the samples and mixed to adjust the pH and for inhibition respectively. The procedure was then repeated for Carboxylmethyl cellulose (CMC) and xanthan gum polysaccharide (Xanplex D) biopolymers respectively. The shear-stress was determined using the power law model which assumes that all fluids are pseudoplastic.

4. Results and Discussion

The rheological parameters such as plastic viscosity, yield stress, and apparent viscosity of all of the drilling fluids were calculated and presented in Table 1. Figures 1 – 3 illustrates the relationship between shear stress, and shear rate of the non-Newtonian fluids at different concentration of the biopolymers.

The pattern of these curves showed an initial high stress after which there was a less stress with increasing shear. This indicates that the fluids are pseudoplastics and obey the Herschel-Bulkley model. According to Hemphill et al. ^[24], Herschel-Bulkley fluid model requires a certain stress to initiate flow, but less with increasing shear.

Table 1. Rheological properties of the drilling fluids

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Tables index

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The three parameters of Herschel-Bulkley model, i.e., yield stress, fluid consistency, and fluid index were calculated by fitting the experimental data of shear stress as a function of rate of shear to the model in Equation 3. The results indicate that the increment of the amount of biopolymers, increase the yield stress of the final drilling fluid as the flow resistance is increased. This finding is in agreement with the results of Khalil and Jan ^[25]. The increase in yield stress with concentration resulted from an increase in shear stress required to break the gel structure of the mud before flow started. Furthermore, it could be considered that, at rest, the mud’s chains were entangled and the gel formation resulted from the polymer network formed by the physical aggregation with region of local order acting as network junctions.

Figure 1. Plot of shear stress versus shear rate for bentonite gel modified with 0.5g of different biopolymers

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Figure 2. Plot of shear stress versus shear rate for bentonite gel modified with 0.75g of different biopolymers

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Figure 3. Plot of shear stress versus shear rate for bentonite gel modified with 1.0g of different biopolymers

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Figure 4. Plot of log of shear stress versus log of shear rate for bentonite gel modified with 0.50g of different biopolymers

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Figure 5. Plot of log of shear stress versus log of shear rate for bentonite gel modified with 0.75g of different biopolymers

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Figure 6. Plot of log of shear stress versus log of shear rate for bentonite gel modified with 1.0g of different biopolymers

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The flow index, n, and consistency index, k, of the drilling fluids were analysed using Equation 2 andthis relationship is illustrated graphically in Figure 4 – Figure 6. The n values presented in Table 1 shows that the flow index, n, is less than 1 for each drilling fluid at the various concentrations of the biopolymers studied indicating a shear thinning behaviour. According to Mewis et al. ^[21], a fluid for which n valueis less than 1 (n < 1) is said to have pseudo plastic flow behaviour. Further examination of the table reveals thatthe flow index n, of the drilling fluid decreased with increasing concentration of polyanionic cellulose. This is contrary to the findings of Chike–Onyegbulaet al ^[26] who studied biodegradable polymer drilling mud prepared from Guinea corn starch. The consistency index, k, for the polyanionic cellulose modified drilling fluid was found to increase with increase in the concentration of the polymer in the fluid. The increase in consistency index, k, with polyanionic cellulose concentration resulted from the increase in the overall thickness of the fluid. In the case of drilling fluids modified with xanthan gum polysaccharide and caboxymethyl cellulose, the consistency index, K and flow index, n appears not to be affected by the quantity of the biopolymer. Contrarily, Khalil and Jan ^[25], found that the calculated fluid consistency of their drilling fluids appears to be strongly dependent on the presence of glass bubbles, xanthan gum, and clay. However, their fluid consistency appears not to be affected by the presence of starch.

Figure 7. Plot of viscosity as a function of shear rate for bentonite gel modified with 0.50g of different biopolymers

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Figure 8. Plot of viscosity as a function of shear rate for bentonite gel modified with 0.75g of different biopolymers

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Figure 9. Plot of viscosity as a function of shear rate for bentonite gel modified with 1.0g of different biopolymers

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