In the middle and late stage of oilfield exploitation, the amount of water injection increased. The water content of crude oil increased, and the water quality became increasingly complex. This makes the quality of crude oil worse and worse, and the composition of scale changes from single to complex. Crude oil processing conditions have also become worse.
In addition to CaCO3, CaSO4, SrSO4, and BaSO4 scales, there are also FeS, MgCO3, MgSO4, Mg(OH)2, Ca3(PO4)2, SiO2 and the like. The scaling problem of oilfield equipment and pipelines has become increasingly prominent, which has seriously affected the normal operation of the plant and the economic benefits of the enterprise.
When the scale is severe, it will also cause blockage of pipes or equipment, forcing the enterprise to stop production and to descale. This not only shortens the operating cycle of the device but also may bring hidden dangers to the safe production of the device.
Therefore, the scaling problem of oilfield equipment and pipelines has become a major bottleneck affecting the safe, stable, long-term, full-load production operation of the equipment. It is of great significance to study the normal production of oil fields by studying scale inhibitors that can effectively prevent or reduce scaling.
Threshold effects are also known as low dose effects or solubility effects. A simple understanding is that low-dose scale inhibitors have a good scale inhibition effect. When the concentration of the scale inhibitor is greater than a certain value, the increase in scale inhibition effect is not obvious.
The threshold effect is a macroscopic representation of the scale inhibition effect. It reflects the scale inhibition mechanism to a certain extent. The stylization theory of crystal growth suggests that crystal growth is carried out by the development of a relatively small number of active growth points.
These small amounts of active growth points are the kink locations of the crystal lattice. Therefore, as long as the scale inhibitor is adsorbed at a small amount of active growth point, it is difficult for the small crystal of the scale to continue to grow.
When the tiny crystal nucleus of the scale is generated in the water, the scale inhibitor is adsorbed at the interface of the crystal or doped in the lattice of the crystal lattice. The scale inhibitor makes the crystals not grow normally in strict accordance with the lattice arrangement, the crystals are distorted, the lattice is distorted, and the aggregation between the crystal grains is difficult.
The deposits formed by these lattice distortion crystals are difficult to form a dense and strong scale layer. They can only exist in the form of loose, soft scales. This allows the deposit to be easily washed away by the water stream, preventing it from depositing into hard scale.
The anionic or non-ionic polymer adsorbs on the grains of the scale in the water, changing the original charge state of the grain surface. This causes the microcrystals to carry the same electric charge, and the electrostatic repulsion prevents the microcrystals from growing up and colliding with each other to grow and deposit. Or the polymer macromolecule adsorbs the microcrystals therein, “wrapping” the grains that may be scaled, and avoiding its aggregation and deposition.
These scaled grains can be stable in a dispersed state. They can flow with water, thus avoiding deposits and scaling.
The polyacrylic scale inhibitor can form a film co-precipitated with the inorganic crystal particles on the metal heat transfer surface. When the film is increased to a certain thickness, it is broken on the heat transfer surface, and a certain size of the scale layer leaves the heat transfer surface. The growth of the scale layer is suppressed due to the continuous formation and rupture of such a film.
The scale inhibitor is enriched in the diffusion boundary layer near the nucleation growth to form an electric double layer and hinder the coalescence of scale-forming ions or molecular clusters on the metal surface. Moreover, the bond between the scale inhibitor and the nucleus (or cluster of scaly molecules) is unstable.
An organic phosphonate refers to a phosphorus atom directly attached to a carbon atom. The C-P bond in the phosphonic acid is strong. Therefore, it has high chemical stability and thermal stability. It is also difficult to hydrolyze under high temperature and high pH conditions.
Most organic phosphonates are non-toxic or low in toxicity. Commonly used are HEDP, ATMP, PBTCA, EDTMP and the like.
Organic phosphonates also have a threshold effect as polyphosphates. That is, only a few milligrams of organic phosphonate can be added to 1 L of water to prevent the precipitation of hundreds of milligrams of calcium carbonate. Its scale inhibition performance is better than polyphosphate.
Organic phosphonates are cathodic corrosion inhibitors and a class of non-stoichiometric scale inhibitors. It has a significant solubility effect. When used in combination with other water treatment agents, it also exhibits an ideal synergistic effect.
It has excellent coupling ability for many metal ions (such as Ca2+, Mg2+, Cu2+, Zn2+, etc.). Even for inorganic salts of these metals such as calcium sulfate, calcium carbonate, magnesium silicate, etc., organic phosphonates also have good deactivation.
ATMP, which forms a stable complex with metal ions. Good chemical stability, not easy to hydrolyze. It has good thermal stability, good scale inhibition at 200 °C, and corrosion inhibition. ATMP is often used in combination with polycarboxylic acids and is an excellent corrosion and scale inhibitor for treating oilfield scale.
DETPMP has a strong inhibitory effect on CaSO4•1/2 H2O scale. By using the seed growth method, the concentration of CaSO4•1/2 H2O can be completely inhibited when the concentration is about 10-7 mol/L.
Most of these scale inhibitors mentioned above can be used in oil field operations. They are an ideal oil field scale inhibitor for better corrosion and scaling of oilfield equipment and pipelines.
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