The Effect Of Fluid pH On Clays And Resulting Formation Permeability

Clays are present in a majority of hydrocarbon-bearing formations and their presence can cause many problems in the production of oil and gas. The difficulties that are encountered are complicated by the fact that clay composition and location in the rock can vary extensively. In reality, the presence of clays in any part of the drilled hole can present a variety of problems in all phases of the life of the well. There are shales that show a high degree of deterioration when contacted by the common types of water-base drilling fluids. Fluid loss from cement slurries and certain water-base stimulation fluids are another source of potential damage to permeability. Clays may be found in all types of rock. Formations made up of such rocks have been called water sensitive and may be defined as formations where a reduction in effective permeability is noted when a foreign water invades the formation. Normally, sandstone formations exhibit the greatest degree of water sensitivity. Clays may also be present in carbonate rocks but are frequently encapsulated in the rock matrix. Thus, they are not in a position to be contacted by the invading fluids. Permeability damage is primarily caused by migration of clays and other fines with fluid flow. High pH fluids, particularly those with low salt content, can cause dispersion of clays which increases their possibility of migrating. Subsequent bridging of particles at flow restrictions in the rock flow channels causes a plugging action which results in a permeability reduction. This overall effect is primarily found during the flow of water.