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.