EXPERIMENTAL INVESTIGATION OF SHALE SWELLING BEHAVIOR UNDER DIFFERENT SALINITY CONDITIONS

ABSTRACT Wellbore instability caused by shale swelling is a major technical and economic challenge in drilling operations, leading to significant nonproductive time and costs. While water-based drilling fluids (WBDFs) are environmentally preferred, they can react with clay minerals in shale, causing swelling. This study investigates the effect of various saline inhibitors; Potassium Chloride (KCl), Sodium Chloride (NaCl), Calcium Chloride (CaCl₂), and Ammonium Chloride (NH₄Cl) in mitigating shale swelling. A comprehensive laboratory based methodology was employed, involving the preparation of shale and bentonite samples mixed with different salt concentrations (1g, 2g, 3g). Key parameters analyzed over five days included pH, density, rheology, fluid loss, filter cake thickness, and a direct swelling index. The results indicate that all salts provided effective shale inhibition, with minimal volume change observed in the swelling index test. However, significant differences were noted in their impact on drilling fluid performance. NaCl consistently demonstrated superior fluid loss control, with volumes decreasing from 32 ml to 28 ml as concentration increased. CaCl₂ also showed effective inhibition and produced an exceptionally thin filter cake at higher concentrations. In contrast, NH₄Cl was the least effective. Rheological analysis confirmed that the shale and salt mixtures lacked the necessary gel strength and viscosity, which were proficiently provided by the bentonite-based fluids. The study concludes that NaCl and CaCl₂ are highly effective saline inhibitors for controlling shale swelling and improving filtration properties in WBDFs. The findings provide critical insights for optimizing drilling fluid formulations to enhance wellbore stability in shale formations, balancing inhibition effectiveness with essential fluid performance characteristics