Understanding Porosity: Core Analysis, Logs, Classification, Measurement, and Influencing Factors

What is porosity?

Porosity is the ratio of the pore volume to the bulk volume of a substance.

Factors Influencing Porosity:

• Degree of Sorting: In well-sorted and moderately rounded sand grains, the spaces between grains can result in 30-40% porosity when they settle in water. However, in poorly sorted sediments, smaller particles fill the gaps between larger ones, significantly reducing overall porosity.
• Compaction: A geological process, compaction decreases porosity due to the pressure exerted by the overlying sediments. Sandstones exhibit very low compressibility (around 3x10–6), while shales can be compacted to a small fraction of their original volume, greatly affecting porosity.
• Cementation: This process has the greatest impact on reducing the original porosity by altering the size, shape, and continuity of pore spaces within the rock.
• Clay Content: Clay often acts as a natural cementing agent. Although it deposits alongside sand grains and tends to stick to them, a small amount of clay may not greatly affect the porosity of sandstone, allowing significant porosity to remain even after deposition.

How do we get porosity?

1. Porosity from wireline:

• Sonic log
• Density log
• Neutron log
• NMR log

2. Porosity form core analysis: RCAL

• Glass pycnometer
• Russel volumeter
• Ruska porosimeter
• Gas expansion

Classification of porosity?

1. Primary porosity: the main or original porosity system in a rock or unconfined alluvial deposit.
2. Secondary porosity: A subsequent or separate porosity system in a rock, often enhancing the overall porosity of a rock. This can be a result of the chemical leaching of minerals or the generation of a fracture system. This can replace the primary porosity or coexist with it.
3. Effective porosity (also called open porosity): Refers to the fraction of the total volume in which fluid flow is effectively taking place and includes catenary and dead-end (as these pores cannot be flushed, but they can cause fluid movement by release of pressure like gas expansion[3]) pores and excludes closed pores (or non-connected cavities). This is very important for groundwater and petroleum flow, as well as for solute transport.
4. Ineffective porosity (also called closed porosity): Refers to the fraction of the total volume in which fluids or gases are present but in which fluid flow can not effectively take place and includes the closed pores. Understanding the morphology of the porosity is thus very important for groundwater and petroleum flow

Hint:

Dual porosity: Refers to the conceptual idea that there are two overlapping reservoirs that interact. In fractured rock aquifers, the rock mass and fractures are often simulated as being two overlapping but distinct bodies. Delayed yield and leaky aquifer flow solutions are both mathematically similar solutions to that obtained for dual porosity; in all three cases, water comes from two mathematically different reservoirs (whether or not they are physically different)..

Porosity Logs:

Porosity From Core Analysis:

Φ = Vp / Vb = ( Vb - Vm )/ Vb

Where:

Vp: pore space volume

Vm: matrix (solid rock) volume

Vb: bulk volume (Vp + Vm)

Bulk volume (Vb): can be determined directly from core dimensions if we have a fluid-saturated regularly shaped core (normally cylindrical), or by fluid displacement methods by weight where the density of the solid matrix and the displacing fluid is known, or directly by volume displacement.

Matrix volume (Vm): can be calculated from the mass of a dry sample divided by the matrix density. It is also possible to crush the dry solid and measure its volume by displacement, but this will give total porosity rather than effective (interconnected) porosity

Pore space volume (Vp): can also be determined using gas expansion methods.