Scale And Corrosion Inhibition Technique For Wastewater In Oilfield

Watertlood development is used widely in Chinese oilfield. Usually injected water is sewage separated from oilfield produced liquid. It contains multiple constitute such as inorganic salt and solved gas, to which scale and corrosion is ascribed in wellbore, surface equipment at al. The occurrence of scale and corrosion affected usual oilfield production. An effective way to inhibit scale and corrosion is using scale inhibitor and corrosion inhibitor. In this paper , the following research is implemented: systematically evaluation of liquid scale inhibitor, improved synthesis method of polymaleic acid and maleic anhydride-vinyl acetate copolymer, optimum synthesis method of phosphonate containing polymer, preparation of solid scale inhibitor, discuss of scale inhibition mechanism based on the influence of scale inhibitor on crystal form distributatuion and zeta potential of calcium carbonate, relationship of molecular structure and corrosion inhibitor performance.15 scale inhibitors were evaluated for their calcium tolerance, thermal stability, CaCO₃ and BaSO₄ scale inhibition performance. The results indicate: (1) CaCO₃ scale inhibition ratio of phosphonic acid type inhibitor is higher than 90% and that of polymer type scale inhibitor is lower than 85% at concentration of 2mg/L. Although PASP PAA and EDTMP are three effective BaSO₄ scale inhibitors, scale inhibition ratio of single inhibitor is lower than 70%. When PASP is compounded with EDTMP with a ratio of 3:2 , scale inhibition ratio can be improved to 90% with 50mg/L mixture for the occurrence of synergetic effect between PASP and EDTMP. (2)The calcium tolerance of phosphonic acid is lower than polymer type scale inhibitors. Higher calcium tolerance can be reached for ester-containing polymer or sulfonate-containing polymer.(3)The thermal stability of polymer type scale inhibitors is better after aeration, especially for acrylic acid/acrylic ester copolymer, polyacrylic acid ,CaCO₃ scale inhibition ratio can retain 70% after being heated at 150℃ for 72h .The evaluation of thermal stability on HEDP, ATMP, PBTC, EDTMP show that performance of HEDP is best and phosphonic acid can be used at temperature lower than 90℃ .when the medium temperature is high than 100℃,scale inhibitor performance may be debilitated to diffferent extent.The synthesis method of polymaleic acid, maleic anhydride-vinyl acetate copolymer, phosphonate-containing polymer in water were studied based on scale inhibition ratio as main target: for polymaleic acid synthesized in water with a transition metal ion as catalyst and H₂O₂ as stimulant, bromine value is less than 150mg/g and inhibition ratio of calciumcarbonate scale is 93.52% at concentration of 8 mg/L; for maleic anhydride-vinyl acetate copolymer synthesized with the compounded stimulant of sodium hypophosphite and ammonium persulfate, its bromine value is less than 30mg/g , calcium tolerance is 1500mg/L and inhibition ratio of calcium carbonate scale is 91.2% at concentration of 4 mg/L; for phosponate-containing polymaielic acid and phosphonate-containing maielic acid-acrylic acid copolymer, inhibition ratio of calcium carbonate scale is 85% and 90% respectively, but they exhibit ineffective on corrosion inhibition even at concentration of 50mg/L.To inhibit scale formation in oil well, three types of solid scale inhibitor, which can be hung to oil pump in well ting at low temperature, moderate temperature and high temperature respectively, was made mainly from phosphonic type inhibitors. The experiments show that, HEDP, a wide used scale inhibitor in oilfield now, can be selected as scale inhibiting material in solid scale inhibitor for its excellent scale inhibition performance and thermal stability. Before used to prepare solid inhibitor, HEDP must be made into HEDP-Na, a powder soluble in water or Ca2HEDP, a powder springly soluble in water. By comparing melting point and solubility of different plastic in diesel, 1C10A, TN00, compound of DFDA7042 and 1C10A is selected as consolidating agent for low, moderate,and high temperature solid scale inhibitor respectively. When solid scale inhibitor was made only from consolidating agent and HEDP-Na, HEDP-Na would swiftly solve in water because it wasn't not be well consolidated by consolidating agent for their great difference of polarity. After the polarity was conjusted by amphiprotic polymer ,a solid scale inhibitor with good slow releasing ability can be produced.To probe mechanism of scale inhibition, calcium carbonate scale was collected before and after the addition of scale inhibitors and analysized by SEM,FT-IR,XRD qualitatively or quantitately. The results confirm that, after addition of inhibitors, both morphology and content of vaterite is changed. Scale inhibition ratio is higher, change of morphology and increase of vaterite content is more great. It can be inferred that change of morphology is ascribed to the change of crystal form.There are three stage in crastallizing process including occurrence and disappearing of unstable phase , occurrence and disappearing of metastable phase, development of stable phase. When scale inhibitors are added, the second stage and the first stage can be controlled preceedingly to different content according to scale inhibition efficency, so crystal form is changed.To verify the effect of HEDP on surface electrical properity of calcium carbonate scale, which is one of classical scale inhibiting mechanism, zeta potential of freshly CaCC>3 originated from admixture of equimolar of Ca2+ and CO32" was measured duringprecipitating process with time. The results show that the produced CaCO3 is negatively charged and IIEDP had no obvious effect on zeta potential of CaCO3 if having been added during precipitation. In addition, zeta potential of CaCO3 originated from admixture of equimolar ot Cai4 and CO3"" was also measured during dissolution. With the preceding of dissolution , zeta potentials of CaCO3 and the values of pH continuously decreased. Whether the Scale inhibitor was added or not during CaCO3 prepartation,the zeta potential had no obvious difference and whether the dispersion contained HEDP or not, zeta potential also had no obvious difference -To investigate annnelation during synthesis of imidazoline, resultants from oleic acid and diethyienetriamine at 160°C for different time were analyzed by IR and UV, produced water rate and acid value was measured, and corrosion inhibitor performance was investigated by electrochemical polarization curves. The results indicate that cyclization and amidation is developed at the same time and performance of corrosion inhibitor with cyclized structurehigher is better.For series imidazoline with different lipophilic tail and hydrophilic group ,the corrosion inhibitor performance for A3 steel in synthetic brine saturated with CO2 was investigated, and the relationship between molecular structure and performance was suggested. Results show that, in solution containing less than 5X 104mg/L salt, imidazoline with longer lipophilic tail exhibit more efficient, especially when there is double bond in the hydrophobic chain. In solution containing more than 5 X 104mg/L salt, performance of imidazoline with long hydrophobic chain, such as l-aminoethyl-2-pentadecylimidazoline,l-aminoethyl-2-septadecylimidazoline and l-aminoethyl-2-septadecylene imidazoline ,is weaken with the increase of salt content in solution, while that is slightly enhanced for short hydrophobic chain imidazolines such as 1-aminoethyl-2-unadecyl imidazoline and l-aminoethyl-2-tridecyl imidazoline.Also the performance affected by salty change, l-aminoethyl-2-septadecylene imidazoline is always most efficient in mentioned imidazoline. Aong another four imidazoline from resultants of septadecylene imidazoline with ehloroacetic acid, acetic acid and sulfaminic acid respectively, performance of l-aminoethyl-l-carboxymethyl-2-septadecylene imidazoline is best and performance of aminoethyl imidazoline is better than that of hydroxylimidazoline.Also corrosion inhibitor performance was evaluated for a series of octyl-phenol-polyoxythylene ether phosphates with different oxythylene chain length and monoester content. The results indicate that length of oxythylene chain have substantial influence on corrosion inhibitor performance,which can be improved with the change of oxythylene chain length from 10 to 4. A corrosion inhibition ratio over 90% can beobtained at the concentration of 10 mg/L when oxythylene chain is 4(OPP4). While monoester content has slightly influence on corrosion inhibitor performance . Corrosion inhibition ratio increases by no more than 4% when mono-ester content of the product increases from 29.78% to 91.55%.