The impact of overbalance pressure on the alteration of sandstone geomechanical properties

The overbalance pressure drilling technique is well common and practiced during the drilling operations in the petroleum field. This parameter is very critical and affects the drilled formation characteristics, and therefore, the main objective of the current study is to evaluate the impact of overbalance pressure that is applied during the drilling operation on the rock elastic and strength characteristics by employing integrated lab experimental work. Berea Buff sandstone rock type was utilized to interact with water-based drilling fluid (barite-weighted) under an extended range of overbalance pressure [300 to 1000 psi]. The current research presents extensive lab analyses that cover filtration tests for rock-mud interaction, scratch testing for determining the rock strength profile, sonic data acquisition, elastic properties determination, and spectrometry of scanning electron microscope and nuclear magnetic resonance spectrometry to study the internal pore structure changes. The analysis was performed before the mud interaction for the saturated samples (base case) and after applying different overbalance pressure (pre-mud interaction) to evaluate the rock properties alterations. The obtained results showed the high degree of impact of the overbalance pressure on the sonic wave propagation as the compressional and shear wave velocities increased by 17 and 6%, respectively with increasing the overbalance pressure. Poisson's ratio showed a stable trend, while Young's modulus showed an increasing trend by 23% increase with the extended range of the overbalance pressure. The rock strength showed deterioration behavior with increasing the overbalance pressure. The unconfined compression strength decreased by 10% and the tensile strength showed a 9% reduction. The internal pore spectrometry showed the rock structure alterations due to mud solids precipitations and filtrate fluid impact on the rock integrity during the rock-mud interaction. In addition, statistical analysis illustrated the high degree of relationship between the recorded rock characteristics with the overbalance pressure as a controlling factor on the rock and mud interaction.