Oilfield scales refer to mineral scales that are formed during oil/gas production as a result of over-saturation of dissolved minerals in produced water. Most common oilfield scales are calcium carbonate (CaCO3), barrium sulfate (BaSO4), strontium sulfate (SrSO4), calcium sulfate (CaSO4), and sulfide scales in sour systems. These scales can form due to either disruption of mineral solubility equilibrium of the formation water itself or due to mixing of different incompatible waters. A build up of mineral scales in the production flow path can seriously compromise the oil/gas production rates. Although certain remediation methods are available to help remove scales once they form, it is highly recommended to use measures, such as scale inhibitor, to prevent them from happening in the first place because lost production due to scale formation can be irreversible, especially in near wellbore region.
Why do scales form?
Scale formation can be classified into two categories, self-scaling of formation water by itself and scaling due to mixing of incompatible waters.
These minerals are part of the formation that have been dissolved in the water. When temperature or pressure changes, or other chemical changes occur, these minerals will come out of the water in solid form, or precipitate. Scale occurs when specific combinations of these precipitated minerals combine and then attach themselves to the surfaces of vessels or tubing. Once attached, the deposits will grow as other dissolved minerals attach themselves to the deposit.