Bulk Density - Top Dog Engineer

In its simplest form, the subsurface consists of two things: 1) Rock grains and 2) fluid. Because we as petroleum engineers are so interested in pressure, we need a way to calculate the density of fluid-grain mixtures which can eventually get us pressure. This is where bulk density comes into play. In petroleum engineering, bulk density is a weighted average between the density of the fluid in the pore space AND the grain density of the rock. They use the word “bulk”, because it takes into account fluid and grains. The figure below shows a visual of what components contribute to bulk density:

From the figure above, it is clear that the fluid occupies the entire void space between the rock grains. Thus the fractional density contribution of the fluid is a function of the porosity ( $\phi$ ). The rest of the volume is occupied by the grains therefore it’s fractional contribution to the bulk density is $(1-\phi)$ . The general expression used to calculate bulk density is given by the equation below:

(1) $\begin{equation*}p_b = \phi p_f + (1-\phi)p_g\end{equation*}$

where:

$p_b$ = bulk density, $\frac{gm}{cc}$

$\phi$ = porosity, $unitless$

$p_b$ = fluid density, $\frac{gm}{cc}$

$p_g$ = grain denstiy, $\frac{gm}{cc}$

The density of the fluid ( $p_f$ ) is going to be close to the density of water which happens to be 1 $gm/cc$ . Density of the grains ( $p_g$ ) depends on the minerals that make up the formation. The table below shows the density’s for common minerals:

Table 1: Mineral grain densities

Mineral	density (gm/cc)
quartz	2.65
calcite	2.71
dolomite	2.87
clay	2.57
anhydrite	2.98
gypsum	2.35
salt	2.17

For a grain that consists of more than one mineral, which is often the case, the grain density is calculated as a weighted average (I’ll let you figure this out).

So why do we need to understand bulk density?? Well there are multiple applications however the one I will focus on is fracture pressure. Bulk density can be used to determine overburden pressure which can help us estimate fracture pressure of the formation. If you don’t know what overburden pressure is, don’t worry about it because we will cover it shortly.