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The formation of scale poses several serious menaces in oil and gas exploitations. The scaling phenomenon may cause technical problems such as obstruction of equipment and pipes, which causes great damages and results in economic losses; besides, scale is one of the most difficult issues to resolve.
Different parameters cause the precipitation of scale minerals in large quantities that it impairs production. Although it is very common, rather it can be tricky to suspect the exact nature of precipitation nor the composition of scale. But precise studies and simulation techniques could give an insight on the fouling environment with fair accuracy. Generally, scale induced formation damage is highly predicted at the following conditions
Production data, produced water analysis and reservoir conditions combined data could be used to assume or predict the damage effect, also can be used to calculate skin factor which is used to express the impairment to the formation permeability. Common practices to treat skin damage include hydraulic fracture and chemical treatment. While hydraulic fracture seems effective for severely damaged wells where injectivity tests failed to reach minimum injection rate requirements, chemical treatment is more favorable for ease of job implementation and handling and lower costs. While its results may be beneficial, chemical treatment could have disastrous effect in case of poor job design and incompatibility between treatment fluids and the system.
Different parameters cause the precipitation of scale minerals in large quantities that it impairs production. Although it is very common, rather it can be tricky to suspect the exact nature of precipitation nor the composition of scale. But precise studies and simulation techniques could give an insight on the fouling environment with fair accuracy. Generally, scale induced formation damage is highly predicted at the following conditions
- Formations with high saline waters
- Where water injection techniques are applied with high volumes of waters
- When producing from multiple zones with different waters’ characteristics
- Near-well history including similar problems
- Previous encounter to scale buildup on ESP or downhole equipment
Production data, produced water analysis and reservoir conditions combined data could be used to assume or predict the damage effect, also can be used to calculate skin factor which is used to express the impairment to the formation permeability. Common practices to treat skin damage include hydraulic fracture and chemical treatment. While hydraulic fracture seems effective for severely damaged wells where injectivity tests failed to reach minimum injection rate requirements, chemical treatment is more favorable for ease of job implementation and handling and lower costs. While its results may be beneficial, chemical treatment could have disastrous effect in case of poor job design and incompatibility between treatment fluids and the system.
The main factors affecting scale formation:
- Temperature
- Pressure
- pH
- Souring
- Ionic strength
The chief source of oil field scale is mixing of incompatible waters. Two waters are called incompatible if they interact chemically and precipitate minerals when mixed. A typical example of incompatible waters is sea water with high concentration of SO4 –2 and low concentrations of Ca+2, Ba+2/Sr+2, and formation waters with very low concentrations of SO4-2 but high concentrations of Ca+2, Ba+2 and Sr+2. Mixing of these waters, therefore, causes precipitation of CaSO4, BaSO4, and/or SrSO4. Field produced water (disposal water) can also be incompatible with seawater. In cases where disposal water is mixed with seawater for re-injection, scale deposition is possible.
At the first glance, scaling and corrosion are separate issues, but in fact, they can influence each other. Corrosion products such as iron carbonate, any forms of iron sulfides and/or iron oxides are directly affected by the magnitude of corrosion rate. On the other hand, formation of scales on the surface of tubulars either decrease or increase the corrosion rate depending on the physiochemical characteristics of the surface layers.
Scale deposition is one of the most serious oil field problems that inflict water injection systems primarily when two incompatible waters are involved. Scale deposition can occur from one type of water because of super-saturation with scale-forming salts attributable to changes in the physical conditions under which the water exists. Scale also deposited in down-hole pumps, tubing, casing flow-lines, heater-treaters, tanks and other production equipment and facilities. Scale can occur near the downstream of any point in the production system where super-saturation is generated. Super saturation can be generated in water by changing the pressure and temperature conditions or by mixing two incompatible waters. The most common oil field scales deposited are calcium carbonate, calcium sulfate, strontium sulfate and barium sulfate. Scale also can deposit when two incompatible waters are mixed and super-saturation is reached.
Solubility is defined as the limiting amount of solute that can dissolve in a solvent under a given set of physical conditions. The chemical properties of interest to us are present in aqueous solutions as ions. Certain combinations of these ions lead to compounds which have low solubility. Once the solubility capacity is exceeded the compounds precipitate from solution as solids. Therefore, precipitation of solid materials, which may form scale, will occur if:
A solution that contains less solute than required for saturation is called an unsaturated solution.
A solution, whose concentration is higher than that of a saturated solution due to any reason, such as change in other species concentration, temperature, etc., is said to be supersaturated. When the temperature or concentration of a solvent is increased, the solubility may increase, decrease, or remain constant depending on the nature of the system. For example, if the dissolution process is exothermic, the solubility decreases with increased temperature; if endothermic, the solubility increases with temperature.
- The water contains ions which are capable of forming compounds of limited solubility.
- There are changes in the physical conditions or water compositions which are lowering the solubility.
A solution that contains less solute than required for saturation is called an unsaturated solution.
A solution, whose concentration is higher than that of a saturated solution due to any reason, such as change in other species concentration, temperature, etc., is said to be supersaturated. When the temperature or concentration of a solvent is increased, the solubility may increase, decrease, or remain constant depending on the nature of the system. For example, if the dissolution process is exothermic, the solubility decreases with increased temperature; if endothermic, the solubility increases with temperature.