Oilfield scale inhibition

Scaling in Oil/Gas production

The precipitation and accumulation of insoluble salts from a mixture of incompatible aqueous phases of produced fluids is termed scaling and represents a major challenge for flow assurance in the oil and gas industry. ‘Scale’ is a common term in the oil industry used to describe solid deposits that grow over time, blocking and hindering fluid flow through pipelines, valves, pumps etc with significant reduction in production rates and equipment damages. Examples of scales in the oil/gas industry are calcium carbonate, barium sulfate and strontium sulfate.

Background

Scale build-up effectively decreases pipeline diameter and reduces flow rate

The rate of oil production has seen a steady growth from 88 million barrels/day(mmbpd) to over 95 mmbpd since 2011. Consequently, chemical consumption in oil production has also increased significantly. Crude oil is basically a mixture of petroleum, water and natural gas extracted from accumulated fluids in oil reservoirs which. The specialty oilfield chemical market has projected a 10-year chemical consumption rate from 86.7 mmbpd to 93.3 mmbpd between 2010 to 2020, valued at $30.3 billion. However, the projected value in 2019 may be slightly lower due to the lower oil prices and drop in chemical consumption rate for 2015 and 2016. There is therefore the need for more effective and greener chemistries to meet the growing demand. The three prevailing water-related problems that upsets oil companies today are corrosion, gas hydrates and scale formation in production systems. The reservoir water has a high composition of dissolved minerals equilibrated over millions of years at constant physicochemical conditions. As the reservoir fluids are pumped from the ground, changes in temperature, pressure and chemical composition shifts the equilibria and causes precipitation and deposition of the sparingly soluble salts. Scaling can occur at all stages of oil/gas production systems (upstream, midstream and downstream) and causes blockages such as well perforations, casing, pipelines, pumps, valves etc. Severe scaling issues have been reported in Russia and certain North Sea production systems.

Types of Scales

Two main classifications of scales are known; inorganic and organic (mixed scale) and the two types are mutually inclusive. Occurrence of the two processes in one production system may result in highly complexly-structured scales that would require aggressive, severe and highly costly remediation. Paraffin wax, Asphaltenes and gas hydrates are the most encountered organic scales in the oil industry.

Inorganic scale

Inorganic scales refer to mineral deposits and results when the reservoir water is mixed with different brine such as injection water and the incompatible ions react. The mixing changes the thermodynamic and equilibrium state of the reservoir fluids and leads to supersaturation and consequent deposition of the inorganic salts. The most common types known to the oil/gas industry are carbonates and sulfates; sulfides and chlorites are also present.

Name	Chemical Formula	Mineral
Calcium Carbonate	CaCO3	Calcite, aragonite
Calcium sulfate	CaSO4	Anhydrite,gypsum (CaSO4.2H2O) hemihydrate (CaSO4.0.5H2O)
Calcium oxalate	CaC2O4	Beerstone
Barium Sulfate	BaSO4	Barite
Magnesium Hydroxide	Mg(OH)2	Brucite
Magnesium oxide	MgO	Periclase
Silicates	Me(SinOx).yH2O	Serpentine, acmite, gyrolite, gehlenite, amorphous silica, quartz, cristobalite, pectolite
Aluminium oxide hydroxides	AlO(OH)	Boehmite, gibbsite,diaspora, corundum
Aluminosilicates	AlxSiyOz	Analcite, cancrinite, noselite
Copper		Metallic copper, cuprite (Cu2O), tenorite (Cu )
Magnetite or Nickel ferrite	NiFe2O4	Trevorite
Phosphates	Ca10(PO4)6(OH)2	Hydroxyapatite

Calcium carbonate scale

Water, commonly is often referred to as a universal solvent can dissolve certain gases such as carbon(IV)oxide (CO₂) to form aqueous CO_2(aq). Under the right conditions of temperature and/or pressure a reaction between H₂O molecules and CO_2(aq) yields carbonic acid (H₂CO_3(aq)) which is only stable under low temperature and high pressure conditions. H₂CO_3(aq) dissolves in water according to equation (3) to form hydronium and bicarbonate (HCO₃^-(aq)) ions. The process can be described by the equilibrium reaction below;

1. 2HCO₃^-_(aq) ↔ CO₃^2-_(aq) + H₂O_(l) + CO_2(g)
2. Ca²⁺_(aq) + CO₃^2-_(aq) ↔ CaCO_3(s)

The two reactions (1) and (2) describe the equilibrium between bicarbonate ions (HCO₃^-), which are highly soluble in water and Calcium carbonate (CaCO₃) scales. According to Le Chartellier’s principle, drilling operations and extraction of the oil from the well bore decreases the pressure of the formation and the equilibrium shifts to the right (1) to increase the production of CO₂ to offset the change. After years of oil production, wells may experience significant pressure drop resulting in large CaCO₃ scale problems as the equilibrium shifts to the right. Further reduction in pressure along the pipelines moves the scale formation upstream in the system.

Sulfate Scales

Sulfates of Group (II) metal ions, generally decrease in solubility down the group, berylium and magnesium are exceptions.The most difficult scale to remove are Barium sulfate scales because it is highly insoluble and forms a very hard deposit. A general representation of the reaction is summarized in reaction (4)

3. M²⁺ + SO₄^2- → MSO₄ (s)

Sulphate scale usually forms when formation water and injected seawater becomes mixed[3]. The relationship between these and the degree of oversaturation is crucial to the amount of sulphate salts that will precipitate in the system[2]. It is the high concentration of sulphate in seawater which reacts with group (II) metal ions in the formation water. This will lead to the formation of sulphate scale. Severe problems with sulphate scale are most common in reservoirs where seawater has been injected to enhance oil recovery[3].

Calcium sulphate is the easiest sulphate scale to remove chemically. This is due to its relatively high solubility in water compared to strontium and barium sulphate, in water and also its solubility in several chelating solutions. Unlike calcium sulphate, barium sulphate is extremely insoluble and therefore very hard to remove chemically[3]. When scale has started to form in the production piping, it will rapidly build up if it is not treated and can end up with a full blockage. This leads to a high demand for prevention of such scale.