Porosity Measurement | What is porosity and how is it measured?

Precision of Core Porosity Measurements

The summation-of-fluids and resaturation techniques for porosity measurement usually generate porosity values accurate to within ± 1 unit of porosity when the limits imposed on each method are properly observed. Boyle’s Law porosity is more accurate, with a standard deviation of 0.07% (as described above). In cases where the summation-of-fluids data have been found to generate high porosity values because of the presence of clays, and when information is required with rapid turnaround for operational decision purposes, both the summation-of-fluids and Boyle’s Law porosity data can be determined. The summation-of-fluids data provides rapid information for early operational decisions. The plugs to be used for permeability measurements are cleaned, dried and subsequently used for a Boyle’s Law porosity value comparison. This generates more accurate data for application in reservoir engineering calculations.

Overburden Pressure Effects

Rocks analyzed at surface conditions have been relieved of the subsurface confining pressure. Porosity decreases with increasing net overburden pressure (lithostatic pressure less the pore pressure). In particular, unconsolidated and poorly consolidated rocks expand considerably when released from their natural confining stresses. In clastic rocks, the stress sensitivity normally increases with increasing clay content and decreasing cement content. As a result of the porosity being dependent on the stress, laboratory porosity measurements should be made at the subsurface stress conditions.

Such measurements are often made with specially designed Boyle’s Law porosimeters which apply hydrostatic stress to the core sample. However, in the reservoir the resolved stress component is uniaxial, which is lower than the hydrostatic stress. Consequently, the hydrostatic strain measured in the core analysis laboratory must be converted to an equivalent reservoir strain (uniaxial).

A hydrostatic load cell used for simulating downhole stresses is shown in Figure 1.

The loading imposed on the core sample is referred to as hydrostatic pressure, or hydraulic loading, because pressures are equal in all directions. A schematic of reservoir loading on a core sample and the concept of net overburden pressure is shown in Figure 2.

The laboratory overburden pressure test simulates the net overburden pressure, being the difference between the overburden pressure caused by the weight of sediments and the reservoir pressure. In Figure 2, reservoir pressure is estimated to be the pressure related to a normal hydrostatic gradient and is approximately equal to 0.5 psi/ft. (11.3 kPa/m) multiplied by the depth in feet. When the actual reservoir pressure is known, it should be substituted in this equation.

Reservoir loading is thought to be essentially uniaxial, whereas the loading on the core sample in the laboratory is essentially equal in all directions. In well-indurated rocks, this is observed in the laboratory to cause a reduction in pore volume believed to exceed that actually seen in the reservoir. Numerous research studies indicate that in unconsolidated cores the application of hydrostatic loading adequately approximates reservoir loading. This conclusion was reached after long-term tests on carefully handled unconsolidated cores.

Figure 3 illustrates the reduction in porosity observed for samples of various levels of cementation.

These curves exhibit typical shapes, in that the greatest rate of porosity reduction is typically seen at lower net overburden pressures. Pore space reduction with overburden is low in the well-cemented core and becomes more important as one moves from friable to unconsolidated formations.

A significant reduction in the porosity with an increase in the overburden pressure has been observed in high porosity chalk formations. Such a reduction in pore space may also be associated with a reduction in permeability and, hence, reduced formation productivity. Excessive deformation in chalks should supply reservoir energy to displace oil, but could also lead to casing collapse, related reservoir problems or even the requirement to jack up platforms to counteract subsidence as oil is produced and removed, as shown in Figure 4.