Sandstone acidizing purposes

The Wilson patent application indicated an understanding of sandstone acidizing and its purpose far ahead of its time. His description ofthe problem, as recounted in Acidizing Fundamentals, is as follows ... 

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. 

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. 

The carbon dioxide produced during decarboxylation of the organic matter is not the only factor responsible for the formation of secondary porosity, although on a regional scale in the typical sedimentary basins it may explain a certain portion of the secondary porosity volume. The representative petrographic data, e.g. in the Cambro-Ordo-vician sandstones of the Hassi Messaoud field, show that the dissolved feldspars here account at least for 5% in volume. The amount of acid necessary to dissolve the feldspars depends on their mineralogical composition and on the type of dissolved aluminium. For this purpose each mole of mineral dissolved requires at least 1 mole of protons. In the sandstones, carbonate cementation took place probably because of the low solubility of CO in the formation waters migrating upwards. 

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... 

Fracture acidizing can be used to bjqjass formation damage in a carbonate formation. For damage b)qjass, a long fracture may not be needed. The purpose would be similar to that of a frac-and-pack procedure in a sandstone formation. In that case, a propped fracture with length and conductivity suffident to effectively extend the wellbore radius beyond a damage zone is all that is needed— not an extensive, propped fracture. 

The purpose of this chapter is to present material balance calculations to evaluate whether there were sufficent acidic solutions available to explain observed amounts of secondary porosity in sandstones overlying the deepest portion of the Denver Basin (Colorado, USA), which has thermally mature... 

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