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Petrophysics (from the Ancient Greek πέτρα, petra, "rock" and φύσις, physis, "nature") is the study of physical and chemical rock properties and their interactions with .
A major application of petrophysics is in studying reservoirs for the hydrocarbon industry. Petrophysicists work together with reservoir engineers and Earth science to understand the porous media properties of the reservoir. Particularly how the pores are interconnected in the subsurface, controlling the accumulation and migration of . Some fundamental petrophysical properties determined are lithology, porosity, water saturation, permeability, and capillary pressure.
The petrophysicists workflow measures and evaluates these petrophysical properties through well-log interpretation (i.e. in-situ reservoir conditions) and core analysis in the laboratory. During well perforation, different Well logging are used to measure the petrophysical and mineralogical properties through radioactivity and seismic technologies in the borehole. In addition, core plugs are taken from the well as Core sample samples. These studies are combined with geological, geophysical, and reservoir engineering studies to model the reservoir and determine its economic feasibility.
While most petrophysicists work in the hydrocarbon industry, some also work in the mining, water resources, geothermal energy, and carbon capture and storage industries. Petrophysics is part of the geosciences, and its studies are used by petroleum engineering, geology, geochemistry, exploration geophysics and others.
Petrophysicists use Acoustics and density measurements of rocks to compute their Geomechanics. They measure the P wave velocity of sound through the rock and the S wave velocity and use these with the density of the rock to compute the rock's compressive strength, which is the compressive stress that causes a rock to fail, and the rocks' flexibility, which is the relationship between stress and deformation for a rock. (2025). 9781444308914, Blackwell Pub. ISBN 9781444308914 Converted-wave analysis is also determines the subsurface lithology and porosity. (2025). 9781560803195, Society of Exploration Geophysicists. ISBN 9781560803195
Geomechanics measurements are useful for drillability assessment, wellbore and open-hole stability design, log strength and stress correlations, and formation and strength characterization. These measurements are also used to design dams, roads, foundations for buildings, and many other large construction projects. (2025). 9781420041217, CRC Press. ISBN 9781420041217 They can also help interpret seismic signals from the Earth, either manufactured seismic signals or those from earthquakes.
An example of wireline logs is shown in Figure 1. The first “track” shows the natural gamma radiation level of the rock. The gamma radiation level “log” shows increasing radiation to the right and decreasing radiation to the left. The rocks emitting less radiation have more yellow shading. The detector is very sensitive, and the amount of radiation is very low. In clastic rock formations, rocks with smaller amounts of radiation are more likely to be coarser-grained and have more pore space, while rocks with higher amounts of radiation are more likely to have finer grains and less pore space.
The second track in the plot records the depth below the reference point, usually the Kelly bush or rotary table in feet, so these rock formations are 11,900 feet below the Earth's surface.
In the third track, the electrical resistivity of the rock is presented. The water in this rock is salty. The electrolytes flowing inside the pore space within the water conduct electricity resulting in lower resistivity of the rock. This also indicates an increased water saturation and decreased hydrocarbon saturation.
The fourth track shows the computed water saturation, both as “total” water (including the water bound to the rock) in magenta and the “effective water” or water that is free to flow in black. Both quantities are given as a fraction of the total pore space.
The fifth track shows the fraction of the total rock that is pore space filled with fluids (i.e. porosity). The display of the pore space is divided into green for oil and blue for movable water. The black line shows the fraction of the pore space, which contains either water or oil that can move or be "produced" (i.e. effective porosity). While the magenta line indicates the toral porosity, meaning that it includes the water that is permanently bound to the rock.
The last track represents the rock lithology divided into sandstone and shale portions. The yellow pattern represents the fraction of the rock (excluding fluids) composed of coarser-grained sandstone. The gray pattern represents the fraction of rock composed of finer-grained, i.e. "shale." The sandstone is the part of the rock that contains the producible hydrocarbons and water.
The shale/clay volume is an essential petrophysical parameter to estimate since it contributes to the rock bulk volume, and for correct porosity and water saturation, evaluation needs to be correctly defined. As shown in Figure 2, for modelling clastic rock formation, there are four components whose definitions are typical for shaly or clayey sands that assume: the rock matrix (grains), clay portion that surrounds the grains, water, and hydrocarbons. These two fluids are stored only in pore space in the rock matrix. Due to the complex microstructure, for a water-wet rock, the following terms comprised a clastic reservoir formation:
Vma = volume of matrix grains.
Vdcl = volme of dry clay.
Vcbw = volume of clay bound water.
Vcl = volume of wet clay ( Vdcl + Vcbw).
Vcap = volume of capillary bound water.
Vfw = volume of free water.
Vhyd = volume of hydrocarbon.
ΦT = Total porosity (PHIT), which includes the connected and not connected pore throats.
Φe = Effective porosity which includes only the inter-connected pore throats.
Vb = bulk volume of the rock.
Key equations:
Vma + Vcl + Vfw + Vhyd = 1
Rock matrix volume + wet clay volume + water free volume + hydrocarbon volume = bulk rock volume
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