Knowing the type of formation being acidized and details of its composition (mineralogy) is critical to achieving positive results. In carbonate formations, the acid job design is typically based on the use of hydrochloric acid (HCl). The objective when acidizing carbonate formations is to dissolve carbonate based materials to create new or clean existing pathways or channels that allow the formation fluids (oil, gas, and water) to flow more freely into the well. In sandstone formations, the acid job design is typically based on the use of hydrofluoric acid (HF), also known as mud acid, in combination with HCl. Sandstone minerals are not appreciably soluble in HCl alone but are much more so in mixtures containing HF. The objective when acidizing sandstone formations is to dissolve fine sand (quartz), feldspar, and clay particles that are blocking or restricting flow through pore spaces, thereby allowing the formation fluids to move more freely into the well. If a sandstone formation contains appreciable carbonate minerals, then HCl may be added to the treatment. Geologic formations are rarely completely homogeneous. They contain impurities and can be highly variable in their composition. As a result, designing an effective acid job can be complex. Most simple acid job designs use blends of HCl and HF to respond to the heterogeneous nature of geologic formations. The strengths of the acids and their volumetric ratios (HCl:HF) are based on the detailed mineralogy of the formation being treated. Other additives that are commonly used in an acid job include a corrosion inhibitor to protect the well tubulars and related equipment that is exposed to the acid, an emulsion blocker (surfactant) to prevent formation of oil-water emulsions, and an “iron-control agent” to retain any dissolved iron (e.g., rust) in solution. Other more specialized additives and different types of acids may also be used based on the case specific conditions or needs.
Formation permeability determines the pumping pressure required to place the acid into the formation. In general, the lower the permeability, the higher the pumping pressure. In high permeability formations the acid can be pumped into the matrix of the formation at relatively low pumping pressures. If the pumping pressure is below the formation fracture pressure, the treatment is called “matrix acidizing.”
In lower permeability formations the acid cannot be pumped into the formation matrix as readily, but is pumped through existing or induced fractures at higher pumping pressures. If the pumping pressure is above that which will part or fracture the formation, the treatment is called “fracture acidizing” (or “acid fracturing”). There is not an absolute value of formation permeability that separates matrix and fracture acidizing, however the range of values where this may typically occur is between 0.1 millidarcies and 10 millidarcies, depending on the case specific situation. There are two subsets of fracture acidizing. The first type is performed as a preliminary step in a hydraulic fracturing operation, such as in shale or extremely low permeability sandstone or carbonate formations. In this case, acid (HCl or HCl/HF blend) is pumped ahead of the fluid carrying the proppant that will hold the fractures open once the pump pressure is released. The purpose of the acid job in this case is to provide the cleanest possible formation face to enable easier fracture creation and maximize the performance of the proppant once it is placed. The second type is a fracture acid job, primarily applicable in carbonate formations, where the acid is pumped alone or following a fracturing fluid stage – with the intent of creating new or opening existing fractures, and dissolving formation material to create irregular facture surfaces that create new flow paths or enhance existing flow paths into the wellbore when the fractures close.