Calcium based brine has regularly been used during workover operations on naturally completed wells without any sign of formation damage. In gravel packed wells which have seen severe losses of Ca based brine production decline was observed.
For some of the gravel packed wells the production decline occurred when bringing the well on production after completion, whilst in others, the loss in productivity occurred after sea water breakthrough.
In order to increase productivity and confirm the relationship between productivity decline and formation of CaSO4 scale due to losses of completion brine, scale dissolver treatments were carried out.
The use of dissolver chemicals resulted in mobilization and removal of several hundred kilograms of solids from the near wellbore area.
A post-evaluation strategy to provide information about the nature, origin and location of scale was successfully implemented. More importantly, the dissolver treatments resulted in increased oil production.
The findings and lessons learned have had several important implications for gravel pack completions and well treatments in Statoil.
Solutions of high density brines of calcium chloride and calcium bromide are extensively used in workover and drilling operations. Normally, this practice does not produce any deleterious effects. However, the potential for formation damage due to reaction between calcium-based brine and water present in the reservoir exists and is documented through laboratory studies and field experience.
Unfavorable mixing ratios of Ca-based brine and sea water, used as solvent for chemicals or as displacing fluid, can also produce favorable conditions for precipitation of solids and subsequent formation damage.
In addition, oilfield waters will deposit sparingly soluble salts (eg. CaCO3) due to either changes in physical conditions (e.g temperature, pressure) or change in chemical composition (loss of gas, mixing of waters).
Scale deposits are commonly calcium carbonate; barium, strontium and calcium sulphates; and various naturally occurring radioactive materials (NORM). The latter are commonly precipitated with the other sulphate scales.
Carbonate scales are easily removed by acid treatment and calcium sulphate with moderate difficulty by using converters.
In oilfield operations Sulphate scales are more commonly treated with non-acid dissolvers. These formulations are blends of chelating agents, accelerators, dispersants and other synergists. The chelants are capable of forming soluble complexes with metal ions. The stability of these complexes is known to depend on the chelating agent and metal, solution pH, temperature, etc.
For dissolution to occur, the solubility of the complex must exceed that of the scale as indicated by the equations:
Also, the scale must furnish metal ions in solution for chelation to occur.