Well stimulation is the practice of using chemical or mechanical methods to enhance the production rate of the well. It is classified into two categories based on the pressure rating:
Below fracture pressure which includes:
- Wellbore clean-up
- Matrix acidizing
Above fracture pressure which consists of:
- Hydraulic fracturing
- Acid fracturing
The key point is the mechanism of stimulation. In wellbore clean-up and matrix acidizing restoring the natural flow rate is important but in hydraulic and acid fracturing creating the high new flow capacity is the main goal.
HCL is used primarily in stimulation for carbonate reservoirs. The hydrochloric acid reacts with calcium carbonate and leaves brine as a reaction product which is compatible with formation.
HF is used in stimulation for sandstone reservoir, because the HCL does not react with sandstone. But the reaction between HF and carbonate minerals forms insoluble, damaging precipitates that may result in a reduction in productivity. In order to remove carbonate minerals before being contacted by HF, the HCL is used as a preflush.
Fluid is injected to the formation with an injection rate which is too high for formation to bear it without being fractured. This pressure increase continues to a certain level called break-down pressure. Based on rock mechanics the break-down pressure is equal to summation of strength and in-situ compressive stress of the formation. Once the pumping pressure exceeds breaks down pressure, a fracture is formed, and the injection fluid penetrates through it. The pad which is the free solid fluid is injected till the fracture is wide enough to accept a propping agent. Then the propping agent is pumped to keep apart the fracture surfaces. Once the pumping operation stops, the pressure in the fracture decreases below the compressive in-situ stress cause closing the fracture. In shallow reservoirs, sand is normally used to keep open the fractures while in deep reservoirs using the ceramic beads are common. There are many applications for hydraulic fracturing such as:
- Increasing the production rate of low-permeability reservoirs
- Increasing the production rate of oil/gas damaged wells
- Connecting the rock natural fractures to the wellbore
- Reducing the pressure drop to minimize sand production and asphaltine/paraffin deposition problem
Once the treatment is completed and pressure is released the fractures are going to be closed. In proppant fracturing, the proppant agent is used to keep the fractures open. In acid fracturing, acid etches the channels in the formation, so after the fractures are closed the desired conductivity is maintained. To perform the effective acid fracturing, the formation has to be partially soluble in acid. It limits the acid fracturing to carbonate reservoirs and should never be applied to other reservoirs such as, sandstone or shale.
Corrosion inhibitors are surface-active agents used to protect the pipe by reducing the reaction rate of acid with steel without affecting the reaction of acid and formation.
Iron control agent
Iron control agents are used to keep iron in solution. These materials are categorized into three types which are:
- Iron reducing agents
- Iron complexing agents
- Hydrogen sulfide scavengers
Surfactants are chemicals used to lower the surface tension (or interfacial tension) between two liquids or solid and liquid. The primary use of surfactants is to prevent the emulsion in interactions between acid and oil. Surfactants may be act in various applications such as:
- Wetting agents
- Foaming agents
- Mud dispersants
- Sludge preventers
- Emulsion breakers
- Acid Retarders