Petrophysics
In the Petrophysics the physical properties of the rocks are examined. For the different properties of each one or more petrophysical parameters are defined, through which they can be quantified. In addition, it examines the processes and mechanisms by which rocks can be determined and how they change with the external conditions.
Subject and outline of Petrophysics
From the individual sub-areas of petrophysics interacts closely to those branches of geophysics and geotechnical engineering, examining the property in question at the Earth's body ( in situ). However, for the development of Petrophysics as an independent science, there was sometimes a disadvantage, in a sense, if the individual branches were used only as an adjunct in geophysical institutes, as this often remained an in-depth processing.
However, the elastic properties have been intensively studied because the knowledge of the elastic parameters is a necessary condition for the analysis of seismological measurements. One can for example use the elastic properties and the absorption coefficient for the calculation of the shear and compressional wave velocity. Some sub-regions are also important directly for technical applications such as the examination of the fracture and deformation processes for tunnel construction and engineering geology.
From the standpoint of a strict system, you can view the Petrophysics as a special branch of solid state physics. In fact, many definitions, experimental and theoretical methods and insights from solid-state physics have been adopted. On the other hand, however, differs the object of investigation, the rock through its statistical composition of individual differently sized grains of different minerals, which may be more connected by amorphous material, or separated by a partially saturated pore space, clear of the objects that are processed in the solid state physics. As such, the petrophysics has their very specific problem that justifies their separate identity.
Petrophysical properties can be divided in the following manner:
- Electrical properties
- Magnetic properties
- Elastic properties
- Thermal properties
- Properties of the pore space and matrix (porosity, density, permeability, capillary pressure )
- Fracture and deformation processes
- Radioactivity and heat production
Each of the characteristics constitutes an outer macroscopic symptom should be explained by internal processes and mechanisms of nuclear to microscopic sizes.
Problems of determination of petrophysical parameters
The belonging to a certain type of rock rock samples may vary greatly in their mineralogical composition, structure and texture. Therefore, it is necessary to determine the petro- physical parameters of a large number of samples, to obtain the variation range of the possible values for each type of rock. With enough good characterization of the individual samples by previous mineralogical and petrographic studies to win in this way, information on the dependence of the petrophysical parameters from the mineral composition, grain size and porosity.
It is natural to try to compute the parameter values of a rock from the values of the minerals contained in theory. Of course, also the proportions of the minerals must be considered. There is a large number of so-called mixing rules derived partly theoretical, partly been tested empirically that pursue this goal. In some petrophysical parameters, such as density, specific heat and radioactive heat production this way gives very good results. However, this approach fails logically to the extent that the properties of structural conditions and grain boundary effects are dependent, which are not considered in these calculations. The mixing rules fail even if parameters are different values of the various minerals of a rock very strong and as a minority occurring mineral so high parameter values is that it determines the overall behavior despite low share crucial. This situation is common in the electrical and magnetic properties.