Abstract

Based on a containment transport model developed for hydrogeological purposes, a numerical method for the analysis of intrusion of potassium ions into a shale has been developed. The scheme has been applied in the back-analysis of KCl-brine exposed specimens of smectite-rich Tertiary Paleocene shale from the North Sea. The specimens, exposed under effective confining stresses in a triaxial cell at 80 C, shrank during the KCl-exposure. Two tests with different KCl concentrations (5wt% and 20wt%) have been back-analysed.

The ion transport is modelled by diffusion Using a finite difference scheme. In the back-analysis it was assumed that the observed shrinkage is due to the ion exchange when the bound Na+-ions are exchanged with K+-ions from the exposure fluid. The agreement between simulated and measured shrinkage rate was good, indicating that the assumed relation between ion transport, ion exchange and shale shrinkage is a valid mechanism.

The simulations showed tensile stresses near the specimen boundaries, where the shale first shrinks. A downhole situation was therefore analysed with a linear elastic model, and the development of tensile stresses with time was investigated. This linear elastic analysis showed that in the vicinity of the borehole wall, large tensile stresses develop as the front of the K+ ions progresses into the shale. Such tensile stresses may lead to the development of cracks and fissures, which in turn increases the surface area of the brine-exposed shale. An accelerating mechanism of cracking may thus develop, increasing the potential of destabilising the shale.

For each practical case there is thus an upper limit to the KCl-concentration which should be used in smectite-rich shale. Up to this limit, stability is improved due to a reduced stress concentration. Above this limit, stability problems will increase with increasing KCl-concentration.

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