Sandstone acidizing near-wellbore formation damage

The primary purpose of matrix acidizing in sandstones is to remove formation damage caused by clay and other siliceous fine particles plugging near-wellbore permeability. Particles may be naturally occurring or may have been introduced into... 

Acid reaction in sandstones is fast, and only formation damage that occurs very near the wellbore can be treated effectively. This is a function of the very high surface-area-to-volume ratios of siliceous minerals such as clays, feldspars, and zeolites—clays having the highest ratio. Large quartz grains have a very low surface-area-to-volume ratio. 

Shut-in of an HF acid treatment in a sandstone near the wellbore, increases the chance of damage in the formation from precipitation of HF reaction products. Acid, especially spent HF, should be produced back out immediately or as soon as possible. If immediate turnaround cannot be accomplished, then acid should be kept moving and a healthy overflush should be employed. This is so that reprecipitation of acid reaction products, which inevitably takes place, will be far enough beyond the near wellbore that its effect on radial permeabihty will be insignificant. 

As previously discussed, the most common purpose of matrix addizing is to restore near-wellbore permeability in or through a damaged formation zone. Because it is bypassed, rather than directly removed, formation damage in a candidate for carbonate acidizing need not be acid removable by contrast, it does need to be add removable in a candidate for sandstone acidizing. 

Matrix acidizing has application in both carbonate and sandstone formations. In sandstone formations, matrix acidizing treatments should be designed primarily to remove or dissolve add-removable damage or plugging in the perforations and in the formation pore network near the wellbore. 

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