Petrophysics (from the Greek πέτρα, petra, "rock" and φύσις, physis, "nature") is the study of physical and chemical rock properties and their interactions with fluids. A major application of petrophysics is in studying reservoirs for the hydrocarbon industry. Petrophysicists work together with reservoir engineers and geoscientists to understand the porous media properties of the reservoir. Particularly how the pores are interconnected in the subsurface, controlling the accumulation and migration of hydrocarbons. 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-log tools 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 sidewall core or whole core 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.
Fundamental petrophysical properties The following are the fundamental petrophysical properties used to characterize a reservoir:
Lithology: A description of the rock's physical characteristics, such as grain size, composition and texture. By studying the lithology of local geological outcrops and core samples, geoscientists can use a combination of log measurements, such as natural gamma, neutron, density and resistivity, to determine the lithology down the well. Porosity: The pore space volume portion related to the bulk rock volume, symbolized as ϕ {\displaystyle \phi } . It is typically calculated using data from an instrument that measures the reaction of the rock to bombardment by neutrons or gamma rays but can also be derived from sonic and NMR logging. A helium porosimeter is the main technique to measure grain volume and porosity in the laboratory. Water saturation: The fraction of the pore space occupied by water. This is typically calculated using data from an instrument that measures the resistivity of the rock and applying empirical or theoretical water saturation models; the most worldwide used is Archie's (1942) model. It is known by the symbol S w {\displaystyle S_{w}} . Permeability: The quantity of fluid (water or hydrocarbon) that can flow through a rock as a function of time and pressure, related to how interconnected the pores are, and it is known by the symbol k {\displaystyle k} . Formation testing is the only tool that can directly measure a rock formation's permeability down a well. In case of its absence, which is common in most cases, an estimate for permeability can be derived from empirical relationships with other measurements such as porosity, NMR and sonic logging. Darcy's law is applied in the laboratory to measure the core plug permeability with an inert gas or liquid (i.e. that does not react with the rock). Formation thickness (h) of rock with enough permeability to deliver fluids to a well bore, this property is often called “net reservoir rock.” In the oil and gas industry, another quantity “net pay” is computed which is the thickness of rock that can deliver hydrocarbons to the well bore at a profitable rate.
Rock mechanical properties The rock's mechanical or geomechanical properties are also used within petrophysics to determine the reservoir strength, elastic properties, hardness, ultrasonic behaviour, index characteristics and in situ stresses. Petrophysicists use acoustic and density measurements of rocks to compute their mechanical properties and strength. They measure the compressional (P) wave velocity of sound through the rock and the shear (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. Converted-wave analysis is also determines the subsurface lithology and porosity. 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. They can also help interpret seismic signals from the Earth, either manufactured seismic signals or those from earthquakes.
Methods of petrophysical analysis
Core analysis Core samples are pieces of rock collected from a subsurface formation during drilling operations, to study the physical and mechanical properties of the formation in detail. They provide the only direct evidence of the reservoir's formation rock structure. Core analysis is the "ground truth" data measured in a laboratory to determine the key petrophysical features of the in-situ rock. In the petroleum industry, rock samples are retrieved from the subsurface and measured by oil or service companies' core laboratories. This process is time-consuming and expensive; thus, it can only be applied to some of the wells drilled in a field. Also, proper design, planning and supervision decrease data redundancy and uncertainty. Client and laboratory teams must work aligned to optimise the core analysis process.
Well-logging
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