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Most sandstone formations contain clays that can have a significant effect on the flow of aqueous fluids. The clays most frequently detected are smectite, mixed layer, illite, kaolinite, and chlorite. All of these clays are capable of migrating and causing permeability damage when they are contacted by waters foreign to the formation. Normally, these waters alter ionic environments around the clays, which causes the clays to be dislodged from their original positions. Thus, any time clay is present in the rock, it can be assumed that permeability damage can occur. The degree of damage will depend upon the concentration and types of clays present, their relative position in the rock, the severity of the ionic environmental change, and fluid velocity.
Aqueous fluids that may be classed as foreign waters are effluents being injected into disposal wells, fresh water storage in underground aquifers, flooding waters used for secondary oil recovery, and waters that may contact hydrocarbon-producing formations during numerous phases in a well's life.
Permeability damage has been minimized in oil and gas wells through the use of the potassium and ammonium ions. These ions will control the amount of water adsorbed by clays but, since they are exchangeable, they do not affect permanent protection against clay migration. A hydroxy-aluminum ion has been used effectively for formation treatment, but its use is limited to formation temperatures below 93.3°C (200°F) and where pH values do not vary more than a few integrals from neutral. The most effective material that provides long-time control of clay migration under a large variety of conditions is a select group of organic polymers. These polymers are strongly adsorbed by clays, thus helping to prevent clay movement in the rock matrix.
flow, water, oil effluents, clays, permeability damage, migration, polymers, damage prevention, water injection
Research associate, Halliburton Services, Duncan, Okla.