| For each oilfield, in period from the first oil well on production to theend of the oilfield life, fluid flows from reservoir to gathering systemsthrough well bores and wellheads. Changes always happen in fluidparameters such as temperature, pressure, content of oil/water/gas inthe different positions mentioned above, especially with introduction ofnew type of fluids in water injection phase of oilfield development.Scale forming which is possible whenever the fluid comprisesexceeding two types of incompatible liquids, or changes of fluidpressure and temperature parameters occur with flows of fluid, willresult in plugs in well bores and gathering lines, and result in equipmentdamages which will badly influence ordinary production. Furthermore,scale forming in formation will badly impact on oilfield developmentefficiency by resulting in poor performances of oil reserve, reduction ofproduction and water injection volume.Generally speaking, during the entire oilfield development period,calcium carbonate which is easily formed with temperature augmentand pressure reduction frequently happens in pressure reduction zonesin well bores, heating positions and separators in gathering systems.While sulphate scale which is easily formed with mixing of two types ofincompatible liquids frequently happens in gathering lines. And sulphatescale forming may also occur in formation when injected water meetwith incompatible formation water in oilfield developmented with waterinjection.Oilfield scale forming is a worldwide popular problem. For example,the scale forming problem exists in about 700 thousand productionwells and water injection wells in the US. Some of those wells have toshut down or to be disbanded ahead of schedule due to the scaleforming problem which result in reduction of productivity and waterinjection capacity, or discontinuous of production, and large amount ofwell workover. The scale forming problem also occur in China such asBaikouquan oilfield in Karamay of Xinjiang, Weiyuan gas field inSichuan, Maling oilfield in Shaaganning region, Shengli oilfield and Jilinoilfield as well as Dagang oilfield in East China, offshore oilfield in BohaiBay. West oilfield suffers the scale forming problem in well bores andgathering systems in some extent. The scale forming problem oftenexists in the gathering main lines and water-adding lines in thegathering systems, and in water injection lines, water injection wells,valves, flowrate meters, cylinders of plunger pumps in the waterinjection systems. Based on examination on tubes in water injectionwells in West oilfield, scale thickness in the internal tube wall exceeds to10 mm, the annual scale thickness of part of examined wells is in therange of 5-10 mm, which seriously impact on water injection and welllogging. In summary, in order to improve oilfield development efficiency, toincrease production efficiency of oil wells and water injection wells, toreduce operational energy consumption in gathering systems and waterinjection systems, and to lower upgrading and maintenance cost, it issignificant to conduct special studies on scale forming mechanisms andprotective technologies of gathering systems. After examination and investigation on domestic oilfield scaleprotective and removal technologies, with adoption of saturation indicesanalysis technique, oilfield water scale forming trend and its scaleforming mechanisms is studied in detail based on analysis of geologicfactors, and .on analysis of scale-forming ion changes of oilfield water.Meanwhile, the scale forming performance has been studied byconducting scale-forming tests. It has been concluded as below by this study: -firstly, hard-soluble scales in gathering systems is mainly causedby important material bases of scale forming due to high hardness andhigh scale-forming ions of formation water. -Secondly, the major contents of scales are hard-soluble bariumsulphate and Strontium sulphate which account for about 60% of totalscales. The remaining content of scales are calcium carbonate andFeCO3. It is showed from microscope that barium sulphate andStrontium sulphate are generated concomitantly. As a matter of fact,strontium sulphate is often formed where barium sulphate exists. At present, since there is no effective approach both at home andabroad to remove this kind of hard-soluble sulphate scales, protectivestrategy is dominantly used in practices, which lead to development of ahigh efficiency compounded scale inhibitor. It is demonstrated fromassessment tests that the compounded scale inhibitor is capable toprotect against and to postpone scale forming. Its scale resistantefficiency can reach to 90% and above when the recommendedchemical concentration in injected water is in the range of 5-20ppm, andthe recommended chemical concentration in produced water is in therange of 10-50ppm. It is significant to resolve scale problems resulted from hard-solublesulphate, and to improve oilfield development economics. Furthermore,scale forming monitoring should be enhanced in water injection systemsand production systems since oilfield scale protection is a kind ofsystematic project which involves dynamic process with alwayschangeable parameters of oilfield injected water and produced water. V... |
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