The Shrinkage Rate of KCL-Exposed Smectitic North Sea Shale Simulated By a Diffusion Model

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. P. 273

Clay mineralogy and shale instability: an alternative conceptual analysis

“…Transport rates of water depended on shale type and at the drilled surface advection is the dominant process with rates depending on overbalance pressure and shale permeability. Bostrøm et al (1998) and Horsrud et al (1998a) experimented on the effect of KCl on preserved smectitic North Sea shales and also concluded that ion transport took place by diffusion and not osmosis. The mechanism of stabilization was considered to be by cation exchange and shale shrinkage.…”

How do Anions in Water-Based Muds Affect Shale Stability?

“…Shale is carefully checked to avoid any laboratory artifact due to capillary effects 10,17,26,27 . Pressure transmission tests.…”

“…Shale hydration has been one of the first problems addressed in shale/fluid interaction studies, and potassium chloride has been suggested as a mean to prevent it 9 . In spite of some successes, recently a few questions have been raised on the potassium chloride role in some wellbore failures (shale matrix shrinking 10 , mineralogical transformations 11 , kaolinite hydration/ swelling 12 ) demanding for more work on the topic. Soluble glycols [13][14][15] has been also proposed to control shale hydration, even with modified chemical structure to be used without potassium chloride 16 .…”

Effects of Sorptive Tendency of Shale on Borehole Stability

“…However, there are various types of chemical reactions that affect the concentration of solute during transport in porous media, such as cation exchange and sorption phenomena [49]. The available mathematical models that describe wellbore stability in shale formations have several limitations; the interaction between solvent and pore space is neglected in the transport equations, the drilling fluid is assumed to be an ideal solution.…”