Petrophysics is the link between geological objects and geophysical methods of studying well logs.
Petrophysics of sedimentary rocks includes studying various occurrences of rock porosity, fluid saturation properties and the ability of rocks to let fluids pass through the pore space, as well as determining the mineralogic and granulometric composition of rocks. These petrophysical properties are individually or collectively related to specific physical parameters. These include the electric conductivity or electrical resistivity, adsorptive capacity, density, hydrogen concentration, and natural radioactivity.
Reservoir rock properties and reservoir types are assessed via the quantitative interpretation of well logging data which requires certain petrophysical support. Dependencies for similar sediments in adjacent areas or corresponding literature-based or model dependencies are used in first prospecting wells, when there are no petrophysical correlations for the sediments in question. The petrophysical support required for well log data interpretation is only sufficiently developed at the stage of report preparation, which includes calculation of oil and gas reserves.
The petrophysics-based quantitative interpretation of well log data includes determining the types (classes) of reservoirs being studied, selecting physical models, and determining the petrophysical interrelations between the measured geophysical parameters and the expected reservoir properties for the selected models. This may require determining the lithologic composition of rocks, their porosity and permeability, fluid properties and basic physical properties measured with geophysical methods, such as electric resistivity, slowness (travel time), density, natural radioactivity, diffusion and adsorption activity, and nuclear magnetic properties for the same core samples. Special studies aimed to measure the permeability to phase and relative permeability, residual water and oil saturation, salinity of formation waters, etc. are performed to a lesser extent.
Petrophysical measurements must be performed for core samples from the first wells drilled in the field. The core must be sampled at small intervals but with complete recovery. This is required to compare reservoir properties measured at the core and found in well logging data, and to control the reliability of measurements.
Petrophysical modelling tasks include: