An experimental study was undertaken to investigate the mud solids and filtrate invasion in reservoir rocks while using drilling muds. The objective of these experiments is two fold: one is to evaluate the performance of various reservoir rocks and select the most compatible drilling fluid, and second is to delineate the pore blocking mechanism that causes formation damage. Core plugs of Berea sandstone, dolomite and limestone were examined by petrographic methods and tested for permeability reduction during mud circulation. Both of water-base and oil-base muds that are typically used in field studies were obtained and tested for their performance evaluation.
Data from these tests shows that particles from these drilling muds penetrated well into the interstices of the formation, causing a significant reduction in absolute permeability of the rocks. Higher the initial rock permeabilities of the rocks, higher and much deeper were the impairments. An average reductions of 50 to 80% of oil mobilities were found in case of Berea sandstones when exposed to various muds. In cases of light formations, like limestones tested here, reduction in oil mobilities were found to be 30 to 40%. The location and extent of damage, also varies with other parameters such as mud annular velocity, differential pressure, characteristics of the rock as well as the characteristics and composition of the mud filtrate and solids that flow with it under dynamic conditions. A mechanism of permeability reductions in reservoir rocks is proposed based on the filter cake properties and the test data. Effective guidelines for selecting a proper drilling mud can be obtained as an outcome of this work.