Petrophysics

In petrophysics , the physical properties of rocks are examined. One or more petrophysical parameters are defined for the various properties, by means of which they can be recorded quantitatively. In addition, research is being carried out into which processes and mechanisms can determine the rocks and how they change with external conditions.

Subject and structure of petrophysics

There are close interrelationships between the individual sub-areas of petrophysics and those branches of geophysics and geotechnics that investigate the relevant property on the earth's body ( in situ ). For the development of petrophysics as an independent science, however, it was sometimes disadvantageous in a certain sense if the individual branches were only operated as appendages to geophysical institutes, because this often meant that a profound study was not carried out.

However, the elastic properties have been investigated particularly intensively because the knowledge of the elastic parameters is a necessary prerequisite for the evaluation of seismological measured values. For example, the elastic properties and the absorption coefficient can be used to calculate the shear and compression wave speed. Some sub-areas are also directly important for technical applications, such as the investigation of fracture and deformation processes for tunnel construction and engineering geology .

From the standpoint of strict systematics, petrophysics can be viewed as a special branch of solid state physics . In fact, many definitions of terms, experimental and theoretical methods and findings from solid state physics have been adopted. On the other hand, however, the object of investigation, the rock, differs significantly from the objects that are processed in solid-state physics due to its statistical composition of individual grains of different sizes of different minerals, which can still be connected by amorphous material or separated by a partially saturated pore space. In this respect, petrophysics has its own very special problem, which justifies its independent position.

Petrophysical properties can be divided into the following ways:

• electrical properties
• magnetic properties
• elastic properties
• thermal properties
• Properties of pore space and matrix ( porosity , density , permeability , capillary pressure )
• Breaking and deformation processes
• Radioactivity and heat production

Each of the properties represents an external, macroscopic symptom that should be explained by internal processes and mechanisms on the atomic to microscopic scale.

Problems of determining petrophysical parameters

The rock samples belonging to a certain rock type can vary greatly in their mineralogical composition, structure and texture. It is therefore necessary to determine the petrophysical parameters on a large number of samples in order to obtain the possible range of variation of the values ​​for each individual rock type. With sufficiently good characterization of the individual samples through previous mineralogical and petrographic investigations, information about the dependence of the petrophysical parameters on the mineral stock , the grain sizes and the porosity can be obtained in this way .

It is obvious to try to theoretically calculate the parameter values ​​of a rock from the values ​​of the minerals it contains. Of course, the proportions of the minerals must also be taken into account. A large number of so-called mixing rules have been derived partly theoretically and partly empirically that pursue this objective. For some petrophysical parameters, such as density, specific heat and radioactive heat production, this method gives very good results. However, this methodology logically fails to the extent that the properties depend on structural conditions and grain boundary effects, which are not taken into account in these calculations. The mixing rules also fail if the parameter values ​​of the various minerals of a rock differ very greatly and a mineral occurring as a minority has such high parameter values ​​that it decisively determines the overall behavior despite a small proportion. This situation is common with electrical and magnetic properties.

literature

• Jürgen Schön: Petrophysics. Physical properties of rocks and minerals . Enke, Stuttgart 1983, ISBN 3-432-92971-4 ( table of contents [PDF; 217 kB ; accessed on April 26, 2013] also Akademie-Verlag, Berlin 1983).