Wpływ ciśnienia na wyniki badań chemizmu wód porowych

Abstract

Szerokie wykorzystanie wyników badań chemizmu wód porowych dla celów rekonstrukcji paleohydrochemii, ochrony wód i zagadnień geologii inżynierskiej wymaga krytycznego spojrzenia na stosowane dotychczas metody wydzielania tych wód. W artykule przedstawiono przegląd stosowanych powszechnie metod otrzymywania wód porowych (wyciągi wodne, metody ciśnieniowe) oraz poglądy na zmianę ich składu chemicznego pod wpływem ciśnienia.

ON THE INFLUENCE OF PRESSURES ON RESULTS OF STUDIES ON PORE WATER CHEMISTRY

For obtaining pore water, two groups of methods are most widely used:

– extraction (washing rock samples with various media such as water and spirit),

– pressure methods (squeezing out pore water with the use of press or centrifugal machine).

Regardless of the used method, the degree of inhomogeneity of mineralization and chemical composition should be taken into account in studies on pore water. That is why either fractional washing of rock or analysis of pore water fractions successively separated under increasing pressure are carried out.

The analysis of several papers showed that the degree of pore water inhomogeneity depends on colloidal-chemical properties of a rock. According to empirical data, pore water obtained from coarse-dispersional (sandy-silty) rocks are characterized by stable mineralization and chemical composition, regardless of the amount of washing medium and applied pressures. In the case of finely-dispersional (especially clay) rocks, the methods of pore water extraction were found to bear a decisive influence on its mineralization and chemical composition.

According to several authors, concentration of ions in water extracts may be 3 to 5 times greater than mineralization of pore water obtained from the same rocks with the use of pressure methods.

However, the latter methods are also somewhat misleading as pore water chemistry was found to change depending on the applied pressure. In relation to that question, three groups of viewpoints may be recognized:

– mineralization of pore water increases along with increase in the applied pressure (Fig. 1);

– mineralization of pore water remains constant (Fig. 3); and

– mineralization of pore water decreases along with increase in the applied pressure (Fig. 4).

It should be noted that the authors holding the above mentioned view points were studying rocks differing in mineral character, depth of occurrence, porosity and moisture content as well as at different pressure ranges. The available data suggest that pore water chemistry is primarily shaped by magnitude of the applied pressure as well as mineral character and degree of diagenesis (natural moisture content and porosity) of rocks and mineralization of pore water.

At low pressure range, the extracted pore water is characterized by stable chemistry and mineral composition and the changes are not recorded until certain pressure value is overpassed.

In the literature, various estimates of that value may be found. For example, changes in concentration of ions are recorded when pressure exceeds the value of about 14,000 N/cm² in the case of bentonite, and not lower than about 38,000 N/cm2 in the case of kaolinite (Fig. 5). Generally, the more hydrophilous is the rock (i.e. the higher is the moisture content and the lower is the pressure), the earlier the inhomogeneity in chemical composition may be noted. It follows that the magnitude of the usable pressures should be empirically established for studied rocks.