In contrast to hydraulic fracturing, which aims to fracture rocks by exceeding the fracture gradient, acidizing aims to clear up formation damage by dissolving precipitated minerals that have clogged up pores around the wellbore. These minerals are most often associated with carbonate rocks and include calcium carbonate and dolomite.
Acids used in acidizing operations include hydrochloric acid and hydrofluoric acid. After the acid has done its job, it’s allowed to return to the surface under formation pressure. This process is known as back-flush.
There are two types of acid jobs: matrix acidizing and fracture acidizing.
Matrix Acidizing
In matrix acidizing, acid is pumped below the fracture gradient into the rocks immediately adjacent to the producing section of the well bore. This region, called the skin region, is where most formation damage tends to occur.
By injecting acid into the well, operators are able to dissolve materials that have precipitated in pore throats.
Fracture Acidizing
In contrast to matrix acidizing, which occurs below the fracture gradient and doesn’t result in fracturing of the rock, fracture acidizing is more closely related to hydraulic fracturing. The goal of this method is to create and clear fractures in the rocks around the wellbore, creating a path that allows hydrocarbons to bypass the damaged skin region.
Images: “Acidizing” by Top Energy Training