Thermodynamics of Salinity Changes Accompanying Compaction of Shaly Rocks
Authors: Smith J.E. 
Published: December 2007

When a clay or shale with fixed cation exchange capacity is saturated with salt water and the solid matrix is slowly squeezed, so that equilibrium is maintained between the remaining pore water and successive increments of squeezed-out water, these successive increments will contain salt of decreasing normality

L N = --- [1 - (n/2L)2], 2

provided the water is conducted away from and out provided the water is conducted away from and out of diffusional contact with the clay as it is produced. Here, n is the cation exchange capacity per unit pore volume and L is a constant determined so that pore volume and L is a constant determined so that this equation correctly relates the initial concentration of solution in equilibrium with the clay, No, and the initial exchange capacity per unit pore volume, no. The pore water remaining in the clay or shale at any point in the squeezing process will contain positive and negative ions of normality:

1 N = -- [L n + n2/4L). 2

In the pores, the positive-ion concentration increases steadily while the negative-ion concentration and N decrease to zero for n 2L. The squeezed-out pore water, if collected and mixed, will contain pore water, if collected and mixed, will contain salt of decreasing normality:

1 N = ---- [L - non/4L]. 2

In deriving these equations, it is assumed that the ratio of activity coefficients of salt inside and outside the porous material is unity, and that there is no association between cations and either anions or exchange sites. Association of cations and exchange sites is treated briefly. Approximations used are thatan increment of salt water passing out of the porous medium does not change volume andthe matrix material does not compress during compaction.

The process must be carried out so slowly that the changing porosity and salinity remain uniform throughout the porous material. The given thermodynamic results correspond to a common experiment in which a water-saturated clay cake is compressed and the squeezed-out water is conduced away through a short capillary of minimal volume and dropped into a receiver. The results also approximately describe some geologic situations in which compaction water from shales drains into comparatively thin sands that conduct the water away.

Introduction

A number of authors have reported progressive changes in the salinity of water squeezed out of clay cakes. The trend of the data is for the salinity of the squeezed-out water to decrease generally as the porosity is reduced. Chilingar et al. reported that the salinity decreases if the compaction is very slow, but can be made to increase if the compaction pressure increases abruptly. Experimental objectives have been to understand salinity changes in shales and sands in geologic settings. Knowledge of expected salinity changes can be applied to quantitative interpretation of electric logs, to interpretation of the direction of hydrodynamic flow over geologic time in compacting sand-shale sequences and to determining whether the more rapid water influx into producing petroleum reservoirs may have come from surrounding shales. Fowler has interpreted detailed data from the Chocolate Bayou field, Brazoria County, Tex., in terms of the last two phenomena. Patchett has shown that logged shale conductivities in wells from a number of sedimentary basins, when converted to a common temperature basis, fall within a narrow range for any given porosity. This is in accord with a finding here that ion concentrations in compacting shales should evolve to values depending on cation exchange capacity but not on the initial salt concentration in the depositional environment.

SPEJ

P. 377