Sandstone acidizing removal

Wilson s process called for generation of HF in the wellbore or in the formation. In hindsight, his anticipation of add-removable formation damage due to solids plugging is remarkable. Many sandstone acidizing treatments have been pumped since, without such understanding of purpose and potential. 

Sandstone acidizing is, for practical purposes, a method for removing acid-removable damage s) only. Except in rare cases, the production rate from an imdamaged well producing from a sandstone formation could be increased up to perhaps twice the original rate, at best. Acid-removable skin, discussed in detail in chapter 6, may manifest itself in the wellbore, in the perforations, or within the formation. 

A well producing from a sandstone formation is a reliable candidate for acidizing only if acid-removable skin is present. With respect to add treatment design, it is important to determine not only whether damage (positive skin) is present but also, if it is, whether the damage can be removed with an acid treatment. 

It is the aim of sandstone acidizing treatments to reduce that portion of the total skin s) that is due to damage s). Skin damage must be present, but it must be acid removable, as manifested in the wellbore, in the perforations, and/or within the formation. In evaluating a well producing from a sandstone reservoir as a stimulation candidate, skin must be measured—or at the least, assessed as best as possible— to select the proper course of treatment (or nontreatment). 

Extremely damaged sandstones (very high acid-removable skin)... 

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. 

Damage in injection and producing wells can be removed in most cases by stimulating the well. In this case, an acid is injected to remove the damage. The type of the acid and its volume depend on the lithology of the formation (carbonate or sandstone) and the nature of the plugging material. Hydrochloric acid at 15 or 28 wt% is used to stimulate car-... 

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. 

The key point is that stimulation can very easily cause damage, rather than remove or bypass damage successfully. Damage can occur during acidizing and fracturing, in both sandstones and carbonates. 

Formation cementation may be carbonate mineral or clay. If a sandstone matrix is held together primarily by carbonate, treatments with acid solutions containing appreciable HCl or organic acids may substantially remove this mineral phase, resulting in sand production and severely impaired productivity. Expensive workover operations will likely be required in such a case. 

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... 

For sandstones with high carbonate mineral content (>15%-20%), HCl-HF mixtures should be avoided. HCl alone should be used in such cases. However, carbonates are often present in sandstones as grain cementation. Removal of carbonates with acid can diminish rock competence. Modern HF-containing buffered acid systems, which have low total acidity (relatively mild pH) and thus low reactivity with carbonates, are a good option in such cases. 

The basis of this method is that any acid is wasted when it is pumped beyond the point at which skin damage is completely removed. This is because acidizing an imdamaged sandstone formation or treatment interval cannot be expected to significantly increase production potential. 

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