The role of overbalance pressure on mud induced alteration of sandstone rock pore system

Overbalance pressure is a very critical parameter in drilling operations. It has a great impact on formation damage, depending on other downhole parameters such as temperature, time, type and composition of mud, and rock mineralogical content. The objective of this study is to determine the degree of the impact of overbalance pressure on mud-rock interaction and the resultant effects on the rock pore system. This research presents an experimental study for the interaction of a Berea Buff sandstone and barite water-based under different overbalance pressure (300, 700, and 1000 psi) under the same temperature and interaction time. The experiments involved the use of the scanning electron microscope and nuclear magnetic resonance relaxation measurements to monitor changes in the pore system of the rock samples. A modified filtration cell was used to accommodate the rock samples and mud at different overbalance pressures. The obtained results showed that the filtration properties, rock flow characteristics (rock permeability, pore throat radius, and pore system scale type) are all affected by increasing the overbalance pressure. The filtration properties increased in terms of mud cake thickness and filtrate volume by 111% and 36% respectively when the overbalance pressure was increased from 300 to 1000 psi. The total rock porosity showed a decrease from 21.6% (pre-mud interaction) to 17.6, 15.2, and 14.2% under 300, 700, and 1000 psi, respectively. The rock permeability decreased by 75% under 1000 psi overbalance pressure while pore throat radius decreased by 45%. However, the rock pore type remains on the same scale (Macro) after interaction with the mud. Statistical analysis showed that the rock porosity and permeability decreased with the overbalance pressure increase through a polynomial relationship with a high determination coefficient of 0.99. Analysis of the internal pore system by the scanning electron microscope showed that the formation damage is mainly attributed to the precipitations of mud solids as overbalance pressure is increased.