Insight on the inhibitive property of potassium ion on the stability of shale: a diffuse double-layer thickness (\u03ba\u22121) perspective

Talal Al-Bazali

doi:10.1007/s13202-021-01221-2

Abstract

It is believed that potassium ions reduce the hydration energy and swelling of clays and thus promoting stability to shales. This belief was made based on volumetric and linear expansion data obtained from shale and KCl solutions interactions. However, swelling data alone is not adequate to mitigate wellbore instability in shale. Such data must be incorporated with mechanical and physicochemical data for complete and accurate wellbore instability analysis.This paper presents clear experimental evidence showing that concentrated potassium chloride solutions tend to suppress shale swelling as higher concentration of potassium ions collapses the diffuse double layer of clay particles causing shale shrinkage which confirms the notion that the Debye length (κ−1) decreases as the ionic concentration increases.Results show that there exists a KCl concentration threshold above which shale’s compressive strength deteriorates significantly. This concentration threshold was found to hover around 5% by weight. The amount of water and ions uptake into shale was quantified using gravimetric measurements. Significant potassium ions invasion into shale was experimentally measured as KCl solution concentration increased which proved the leaky nature of shale’s membrane. The reduction of shale’s compressive strength seems to be well correlated with the amount of ions uptake into shale. Moreover, data suggests that shale’s compressive strength was not significantly impacted by swelling. It was possible to gravimetrically separate osmotic water from associated water as shale interacted with KCl solutions. Results suggest that osmotic water is responsible for shale swelling since it is unattached to ions which makes it free to move around inside shale. On the other hand, data suggest that associated water does not contribute to shale swelling as it is bound to potassium ions which makes it unfree to move around. It is fair to state, based on our experimental data, that osmotic water is responsible for shale swelling while associated water contributes to shale’s compressive strength alteration.

Highlights

Background and prior workUltra-low permeability, negatively charged surface and high clay content distinguish shale from other sedimentary rocks
Inter-crystalline swelling results from hydration of exchangeable cations of dry clay while osmotic swelling is caused by ionic concentrations imbalances between clay surface and aqueous solutions
A standard stop watch was used to record the time of shale swelling as shale sample interacts with the KCl solutions

Summary

Introduction

Ultra-low permeability, negatively charged surface and high clay content distinguish shale from other sedimentary rocks. Their tiny pore throat diameters which range from 3 to 100 nm coupled with their negatively charged clay surfaces create a favorable environment for water and ions exchange when exposed to aqueous solutions (Bybee 2009). The low permeability of shale together with their affinity to adsorb water promoted by the existence of negatively charged clay surfaces made shale very susceptible to the development of swelling and hydrational stresses and loss of mechanical cohesive strength. According to Madsen and Müller-Vonmoos (1989), the swelling behavior of clay rocks depends on the type and quantity of clay minerals encountered, their surface charge and the valence of the cations in the diffuse double layer.

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