Formation permeability damage, treatment

Formation permeability damage caused by precipitation of dissolved minerals such as colloidal silica, aluminum hydroxide, and aluminum fluoride can reduce the benefits of acidizing (132-134). Careful treatment design, particularly in the concentration and amount of HF used is needed to minimize this problem. Hydrofluoric acid initially reacts with clays and feldspars to form silicon and aluminum fluorides. These species can react with additional clays and feldspars depositing hydrated silica in rock flow channels (106). This usually occurs before the spent acid can be recovered from the formation. However, some workers have concluded that permeability damage due to silica precipitation is much less than previously thought (135). 

However, production engineers have been reluctant to use particle bridging because of the possibility of particle transport into the formation, resulting in formation damage and/or costly and often ineffective stimulation treatments. A particle bridging fluid has been developed that quickly and effectively controls fluid loss in a wide range of permeabilities and pore diameters [916]. 

Acidizing treatments are used to remove wellbore damage, enhance matrix permeability or both. The acid reacts with the rock matrix and, as a result, the permeability of the formation will increase. Reagents commonly used to stimulate carbonate reservoirs are hydrochloric (15 or 28 wt%), formic (9 wt%), and acetic (10 wt%) acids [J]. Recently, Fredd and Fogler [2, 3] introduced ethylenediamine tetraacetic acid (EDTA) as an alternative fluid to acidize carbonate formations. 

Lower-permeability sands should be treated with lower acid volumes, because of the increased sensitivity to damage caused by the treatment. Also, if a formation contains high clay content, especially the swelling variety (e.g., smectite or illite-smectite), volumes may need to be reduced further. 

Naturally, during injection, add takes the path of least resistance, as does any injected fluid. Anyone who has worked on acid treatment design or treatment execution has no doubt heard or said that before. The paths of least resistance will be those formation sections or layers with the highest permeability and the least damage. Higher-permeability streaks or natural fractures will accept fluid more readily than zones of lower permeability or injection capacity. 

Relatively low-cost formation damage removal treatments, such as acidizing, are unrivaled in their potential financial significance. This is especially true in medium- to high-permeability reservoirs, which are not often ideal candidates for hydraulic fracturing. ... 

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