Pressure solution: Theory and experiments

Abstract

Parts of a solid under stress show a higher solubility than stress-free parts. In geological formations, this effect causes a reaction which is called ‘pressure solution’. During this process grain contacts dissolve and the grain boundaries in contact with the pore solution grow. This leads to a loss of porosity and an increase in formation strength. Various attempts have been made to describe this process thermodynamically. Neither Weyl's (1959) nor Riecke's (1894) formula — both predicting only minute solubility differences —is appropriate to describe the process. The following equation —equivalent to an equation proposed by Bridgman (1916) describes the solubility increase caused by stresses a p = a o exp { ΔvP n − Δw RT } where: a p = the solubility of part of a grain under normal stress P n a o = the solubility under hydrostatic pressure p. This equation can be used to describe the pressure solution phenomenon and to predict preferred lattice orientation of anisotropic crystals. There is a tendency for crystals to dissolve in places where the stress component normal to the crystal face is high and tc precipitate simultaneously in places where this stress component is low. It has been found that this tendency is many orders of magnitude greater than any possible preference for a particular lattice orientation to grow or disappear. Preferred lattice orientation will therefore probably be determined by the dissolution and precipitation rates of the different crystal faces, and not by the highest thermodynamic stability. Experimental data on pressure solution are very scarce, since experiments are very time-consuming, and are usually carried out under circumstances that are widely different from those occurring in nature. In the Shell laboratories at Rijswijk (The Netherlands), experiments have been carried out on quartz sands compacted in a 1M NaCl-solution at pressure up to 700 atm. The temperatures in the experiments are between 200 and 330°C. At the highest temperature, very distinct indications of pressure solution have been observed in the sand pack after one year of compaction. At lower temperatures, the effect of pressure solution could only be deduced from the compaction behaviour of the sands.