Numerical and experimental study of the interaction between surface swelling soil and gas well casing, based on unsaturated soil mechanics

• Innovative Testing and Modeling: Used triaxial unsaturated tests and regional climate data to model soil expansion and well impact. • Enhanced Shear Strength Insights: Shear strength increases with confining stress and matric suction, providing new insights into soil stability. • Volumetric Strain Analysis: Found that higher confining stress increases volumetric strain, while higher matric suction reduces deformation. • Impact of Precipitation: Significant soil swelling occurs approximately 3.4 years after heavy rain, leading to increased soil volume. • Well Stability Prediction: Developed a model predicting up to 163 kN tensile force on the well casing, with structural stability maintained under swelling forces. This research presents an in-depth analysis of the volumetric and mechanical behavior of expansive soils surrounding the Khangiran gas well in Sarakhs, emphasizing the effects of matric suction and confining pressures on soil mechanical properties. The study employs both laboratory and numerical approaches, utilizing an unsaturated triaxial apparatus and GeoStudio software, to assess (1) the influence of matric suction and confining pressure on volumetric deformation and shear strength, (2) the impact of annual precipitation on soil swelling, and (3) the tensile stresses exerted on the well casing due to soil expansion. Laboratory results reveal that shear strength increases from 195 kPa to 235 kPa as confining stress rises from 100 to 200 kPa, while cohesion climbs from 68 kPa in saturation to 95 kPa under 100 kPa of matric suction, signifying enhanced resistance in drier soil conditions. Numerical modeling indicates that annual precipitation induces a maximum tensile force of 163 kN at a depth of 13 m, with the expansive zone extending approximately 15 m from the well. The thickness of the steel used for tensile strength resistance against soil swelling is sufficient, and if extensive corrosion of the steel casing is not a concern, tensile strength failure will not occur. These findings offer critical insights into soil-structure interaction in expansive soils and provide practical guidance for the design of resilient gas well casings in similar geotechnical settings.