The stability of a shale containing smectite with different exchangeable cations (Na+, Ca2+) was improved using optimum solutions containing polyether amine (PA), poly-alcohol (PO) or KCl. Two types of shale samples with Na+ and Ca2+ as the main exchangeable cations, respectively, were used and the optimized solutions were determined using X-ray diffraction (XRD), an adsorption test, an oedometer swelling test, and an immersion test. The use of KCl prevented intercalation of PA or PO and maintained the stability of the Na-smectite-bearing shale. PA or PO adsorption reduced water adsorption sites on the clay layer, and K+ reduced hydration of exchangeable Na+, resulting in good shale stability in mixed solutions of KCl+PA, and KCl+PO. More stable shale was achieved in KCl+ PA mixed solution, whereas in the KCl+ PO solution the transport of water or solute molecules in the shale was reduced. In the shale containing mainly Ca-smectite, PA, PO and KCl maintained shale stability when applied separately or in common, as PA or PO cannot exchange Ca2+ in the smectite interlayer. As a result, PA or PO should be used together with KCl during drilling in shale formations containing Na-smectite, whereas in shales with Ca-smectite, PA, PO or KCl may be used separately.

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