3.2 Preparation of Water Based Bentonite Fluid Mixture
Bentonite is naturally occurring clay. It is inorganic, non-toxic, and non-irritating. It is not considered hazardous on skin contact and is employed in cosmetics and skin products as a suspender. It is therefore very safe to use the bentonite clay in the experiment.
In this work, a water-based drilling mud fluid was prepared by using pure water as the base fluid and bentonite clay as a viscosifier. The bentonite clay has a density of 2500 kg/m3 and the water density is 1000 kg/m3.
For mud velocity calculations, the following equations are used.
\(V_{s}+V_{m1}=V_{m2}\ \)…… (8)\(\rho_{s}V_{s}+\rho_{m1}V_{m1}=\rho_{m2}V_{m2}\) …… (9)
From equation (8):
\(V_{m1}=V_{m2}-\ V_{s}\) …… (10)
Substituting Eq (10) into Eq (9) yields:
\(V_{s}\ =V_{m2}\left[\frac{\rho_{m2}-\rho_{m1}}{\rho_{s}-\rho_{m1}}\right]\)…… (11)
Multiplying Eq (11) by the density of solid yields the corresponding weight (of solids) required.
\({\rho_{s}V}_{s}\ =\frac{\rho_{s}V_{m2}\left(\rho_{m2}-\rho_{m1}\right)}{\rho_{s}-\rho_{m1}}\)…… (12)
By re-arranging Eq (11), the density of final mixture is given by:
\(\rho_{m2}=\rho_{m1}+\left(\rho_{s}-\rho_{m1}\right)\left(\frac{V_{s}}{V_{m2}}\right)\ \)…… (13)
Re-arranging Eq (9) and dividing by \(V_{m2}\) , the volume percentage of final mixture is given by:
\(\frac{V_{s}}{V_{m2}}=\left[\frac{\rho_{m2}V_{m2}-\rho_{m1}V_{m1}}{\rho_{s}V_{m2}}\right]\)…… (14)
Bentonite swells when it is wet. This expansion is due to the adsorption of water. Hence the clay volume increases and the total volume (clay plus water) is, for practically purposes, unchanged (Othman 2007).
The particle concentration in weight percentage of final mixture is given by:
\(\frac{w_{s}}{w_{m2}}=\left(\frac{\rho_{s}}{\rho_{m2}}\right)\left(\frac{V_{s}}{V_{m2}}\right)\)…… (15)
According to Birgersson et al. (2009), Ca-bentonite forms a stable sediment with water at a solid mass ratio well below 10, which has sand-like properties. Due to low swelling capacity, Ca-bentonite is unable to form colloids.