婁云天,何盛宇,陳旭東,錢鴻昌,張達威
海洋鉆采設(shè)施的腐蝕及防護
海洋環(huán)境中油氣管道的微生物腐蝕研究進展
婁云天1,2,何盛宇1,2,陳旭東1,2,錢鴻昌1,張達威1,2
(1.北京科技大學(xué) 新材料技術(shù)研究院,北京 100083;2.北京科技大學(xué)順德研究生院,廣東 佛山 528399)
海上油氣集輸管道的腐蝕能夠?qū)е聡乐氐沫h(huán)境風(fēng)險和經(jīng)濟損失,其中微生物腐蝕一直以來被認為是造成該問題的主要因素之一。針對海洋環(huán)境油氣管網(wǎng)中腐蝕性微生物的來源進行了分類,包括油藏內(nèi)源性微生物、外注海水以及微生物采油(MEOR)引入的外源性微生物。分析了海底油藏儲層中流體化學(xué)物質(zhì)特性,確認其富含甲烷、硫化物、揮發(fā)性脂肪酸等,并依據(jù)內(nèi)源微生物代謝及產(chǎn)物特征進行了分類,包括硫酸鹽還原菌(SRB)、產(chǎn)甲烷菌、發(fā)酵菌以及鐵還原菌(IRB)。同時,通過舉例分析某油田采出水中微生物群落豐度特征,闡明了外源微生物長期受到油田開采環(huán)境脅迫后微生物群落的變化規(guī)律。在此基礎(chǔ)上,進一步針對海上油氣集輸管網(wǎng)內(nèi)涉及的微生物代謝產(chǎn)物理論、電活性微生物腐蝕理論以及腐蝕性微生物之間的協(xié)同與拮抗作用進行了全面的歸納總結(jié)。最后,對目前以純培養(yǎng)或模式菌株混合培養(yǎng)為主要方式的微生物腐蝕研究中存在的問題進行了討論,并對基于生物技術(shù)的新型防腐手段進行了展望。
油氣集輸管道;微生物腐蝕;油藏微生物;腐蝕機理
隨著我國海洋能源產(chǎn)業(yè)不斷發(fā)展,加速海洋油氣資源的合理開發(fā)利用以及油氣開采裝置的升級勢在必行[1]。海上油氣占全世界油氣資源總量的30%以上,并且開采量逐年增加[2]。海底管道作為海上油氣集輸系統(tǒng)中重要的組成部分,被譽為海洋油氣生產(chǎn)系統(tǒng)的“動脈”[3]。據(jù)統(tǒng)計,由腐蝕造成的海底管道事故占比高達37%[4]。在長時間服役過程中海底管線不可避免地遭受腐蝕甚至失效的危害,一旦出現(xiàn)損傷或破裂將造成嚴重的安全事故和經(jīng)濟損失[5-7]。轟動一時的美國阿拉斯加Prudhoe Bay油管泄漏事件造成原油日減產(chǎn)40萬桶,導(dǎo)致國際油價一度上升,當?shù)? 000 m2的土壤和水源受到嚴重污染。同時,由于管道泄露,油氣運輸被迫中斷,27 km的管線被更換。事故調(diào)查報告顯示,管道內(nèi)部檢測到大量微生物、硫化物/氯化物沉積和腐蝕性氣體(CO2、H2S)[8-9]。
海上油氣開采過程中通常使用采出水回注的方式進行生產(chǎn),該過程極易向原油中混入微生物和沙石等腐蝕因素,加之大量來自海水中的有機物質(zhì)、無機鹽以及油藏內(nèi)可溶性碳氫化合物,為微生物的生長繁殖提供了必要的能量來源,管道腐蝕失效的風(fēng)險被進一步增加。油氣儲集層中的微生物群落結(jié)構(gòu)以及代謝方式復(fù)雜且多樣,其中不乏典型的腐蝕性微生物,油氣集輸系統(tǒng)中多相流集輸管線、回注水管線以及污水處理裝置等組成部分極易受到微生物腐蝕(Microbio-lo-gically Influenced Corrosion,MIC)的危害。相關(guān)事故分析顯示,超過20%油氣管線故障和原油泄漏直接或間接地與MIC有關(guān),其中超過70%由硫酸鹽還原菌引起[10-12]。
目前,有關(guān)MIC的研究大部分還依靠實驗室條件下選取模式菌株或者實地環(huán)境采樣的方式進行純培養(yǎng)或混菌測試[13-16]。然而,依靠純培養(yǎng)的方式幾乎無法還原油氣管線在服役過程中復(fù)雜環(huán)境微生物代謝所引起的腐蝕行為,以及與之相匹配的高腐蝕速率。長期以來,由于受限于檢測手段,MIC領(lǐng)域一直缺乏有關(guān)基于生物膜復(fù)雜性、材料與腐蝕產(chǎn)物之間關(guān)系的研究。近些年,環(huán)境微生物組學(xué)的快速興起似乎能夠用于解決以上問題,宏基因組學(xué)、轉(zhuǎn)錄組學(xué)以及代謝組學(xué)能夠?qū)崿F(xiàn)對實驗室條件下無法復(fù)制的嚴苛環(huán)境中微生物種類、分布和代謝特征,以及腐蝕性/非腐蝕性代謝產(chǎn)物等因素進行多維度的監(jiān)檢測,結(jié)合新型MIC緩解和治理方法,必然會為海上油氣管線MIC相關(guān)研究的進展提供有價值的信息。
早在1926年,Bastin等[17]從Illinois盆地的油田中成功分離出硫酸鹽還原菌,提出了油氣儲集層中可能存在微生物的假設(shè)。在海洋環(huán)境中,沉積物1 m以下的生態(tài)系統(tǒng)被稱為“深部生物圈”,極限深度可達4 000 m以上,其中就包含了大量的海底油氣資源[18]。海底沉積層覆蓋了地球表面近70%的面積,還棲息著超過90%的海洋微生物,為海底油氣儲集層中微生物的多樣性提供了可能[19-20]。Spark等[21]在歐洲北海油田中位于井下深度約4.5 km的巖心中發(fā)現(xiàn)了微生物群落。通過16s RNA序列比對,巖心和鉆井泥漿中的微生物種類完全不同,證明了油藏極端環(huán)境下內(nèi)源微生物的存在。
海底油藏環(huán)境嚴苛,根據(jù)儲層中流體化學(xué)物質(zhì)特性(如甲烷、硫化物、揮發(fā)性脂肪酸等)以及微生物群落分析,表明油氣儲集層主要為無氧或低氧環(huán)境,油藏微生物群落為了適應(yīng)惡劣的生存環(huán)境衍生出了復(fù)雜的代謝方式。值得注意的是,許多儲集層中的微生物能夠進行鐵還原以及硫酸鹽還原呼吸。在高溫儲集層中,產(chǎn)甲烷菌通常占據(jù)生態(tài)位成為優(yōu)勢群落,協(xié)同乙酸氧化過程在油藏內(nèi)物質(zhì)循環(huán)中發(fā)揮著重要的作用。當環(huán)境中硫酸鹽含量較低時,微生物群落以發(fā)酵反應(yīng)為主要驅(qū)動力,H2、CO2和乙酸等發(fā)酵菌代謝產(chǎn)物作為底物,為產(chǎn)甲烷菌提供能量來源,通過共代謝和互養(yǎng)作用維持生長,微生物群落之間保持一定程度的代謝多樣性是一種重要的生存機制[22-23]。依據(jù)細菌代謝方式及產(chǎn)物的不同可以分為以下幾類。
1.1.1 硫酸鹽還原菌
硫酸鹽還原菌(Sulfate-Reducing Bacteria,SRB)在自然界的分布十分廣泛,并且在石油生態(tài)系統(tǒng)中扮演著重要的角色[24]。SRB是一類能夠通過將硫酸鹽、亞硫酸鹽和硫代硫酸鹽作為最終電子受體還原成H2S從而獲得能量的原核微生物[25]。SRB能夠利用糖類、氨基酸、脂肪酸等百余種化合物作為電子供體,通過還原多種價態(tài)的含硫化合物最終完成新陳代謝過程。油藏中分離得到的SRB屬于嗜溫菌或嗜熱菌,甚至在一些油井中分離得到了超嗜熱SRB,其最適生長溫度超過80 ℃[26]。同時,部分SRB還能夠耐受較高濃度的NaCl,其濃度最高可達23%[27]。目前,在油藏中分離得到的SRB超過40個屬,主要分為四大類[28-29]:(1)變形菌門(),代表菌包括脫硫弧菌屬()和脫硫桿菌屬();(2)嗜熱脫硫桿菌屬();(3)嗜熱脫硫弧菌屬();(4)硫酸鹽還原古菌,代表菌有古生球菌屬()和暖枝菌屬()。
1.1.2 產(chǎn)甲烷菌
產(chǎn)甲烷菌是一類專性厭氧菌,能夠通過代謝氫、CO2、乙酸鹽、甲胺等低分子量物質(zhì)獲取能量,其最終產(chǎn)物為甲烷[30]。產(chǎn)甲烷菌的生長活動受到溫度、鹽含量、pH和氧含量等因素的影響,甚至10?6量級的氧濃度都會對其生長產(chǎn)生顯著的抑制作用[31]。目前,在油氣藏生態(tài)系統(tǒng)中分離得到的產(chǎn)甲烷菌主要分布于古菌域廣古菌門的5個目,包括甲烷微菌目()、甲烷桿菌目()、甲烷球菌目()、甲烷炙熱古菌目()和甲烷八疊球菌目()。產(chǎn)甲烷菌代謝底物包括3種類型:氫營養(yǎng)型、乙酸營養(yǎng)型和甲基營養(yǎng)型[30]。已發(fā)現(xiàn)的產(chǎn)甲烷菌并不完全嚴格依照以上敘述的底物進行代謝,據(jù)統(tǒng)計超過70%的產(chǎn)甲烷菌能夠利用H2作為電子供體還原CO2產(chǎn)生CH4,如、、等,同時還能夠利用CO、丙酮酸鹽或乙二醇等代替H2,但是其效率顯著下降,僅為H2作為電子供體時的1%~4%[32-33]。甲基營養(yǎng)型產(chǎn)甲烷菌如不僅代謝甲醇、甲胺等簡單甲基化合物,而且一些較為復(fù)雜的甲基胺類化合物(如膽堿、甜菜堿等)同樣可以維持其代謝需要。
1.1.3 發(fā)酵菌
發(fā)酵菌是一類能夠通過代謝糖類、多肽等底物,產(chǎn)生有機酸、CO2和H2等發(fā)酵產(chǎn)物的微生物。發(fā)酵菌在產(chǎn)能反應(yīng)過程中無需外源電子受體,通過將發(fā)酵底物的氧化過程與菌體內(nèi)次級代謝產(chǎn)物的還原過程相互耦合獲取能量。作為油藏微生物群落中重要的組成部分,發(fā)酵菌可以分為嗜熱菌和嗜鹽菌2個大類。已分離出的嗜熱發(fā)酵菌大多數(shù)屬于熱袍菌屬()、石衣菌屬()、棲熱腔菌屬(),其最適生長溫度為50~70 ℃。通過比對分析從全世界范圍內(nèi)不同油藏分離得到的嗜熱發(fā)酵菌的生存環(huán)境和代謝特點,其底物種類和生長溫度都與油氣儲集層的原生環(huán)境高度相關(guān),這一現(xiàn)象表明該類微生物為油藏原生細菌[34]。嗜熱發(fā)酵菌不但能夠在高溫條件下正常生存,在營養(yǎng)物質(zhì)缺乏的情況下依然能夠保持較好的活力。Takahata等通過檢測發(fā)現(xiàn)日本Kubiki油田的石油產(chǎn)出水中的細菌濃度高達4.6×104cells/mL,即使在饑餓狀態(tài)下也能夠保持細胞活性長達200 d以上,這一特性對其能夠在油藏環(huán)境中長期生存至關(guān)重要。在有些含鹽量較高的油藏環(huán)境中,嗜鹽發(fā)酵菌能夠通過積累可溶性有機質(zhì)維持細胞與環(huán)境之間的滲透壓平衡,而并非僅僅使用Na+、K+等離子[35-36]。
1.1.4 鐵還原菌
鐵還原菌(Iron-reducing bacteria,IRB)是一類能夠?qū)2、有機物等作為電子供體,F(xiàn)e3+作為終端電子受體的一類嚴格厭氧或兼性厭氧的細菌或古菌[37-38]。研究人員在不同的油藏中分離得到了如脫鐵桿菌屬()、地芽孢桿菌屬()等典型嗜熱鐵還原菌,其能夠使用乳酸、氨基酸、醋酸鹽等作為電子供體[39]。在近中性的厭氧環(huán)境中,化學(xué)和生物過程中的三價鐵氧化物作為電子收集單元極易被還原,研究證明三價鐵氧化物在非硫化物沉積過程中主要受鐵還原菌的代謝過程控制[40]。在一些含硫化物的環(huán)境中,如油藏、海底沉積物等,較早的研究結(jié)果顯示Fe3+的還原過程是由于微生物成因的H2S導(dǎo)致的。最新的研究已經(jīng)證實了鐵還原菌能夠利用三價鐵還原酶直接進行反應(yīng),并且占總還原量的90%[41]。
在海上石油開采過程中,為了保持油氣儲集層的壓力,需要以不斷注入海水或回注水的方式驅(qū)動原油的開采。同時,為了確保長距離油氣管道的完整性以及安全運行,需要進行水壓測試或周期性的管道停輸檢修。在以上操作過程中,海水中種類豐富且組成復(fù)雜的微生物不可避免地被引入到集輸管道中,必然會在復(fù)雜的管網(wǎng)系統(tǒng)中形成生物膜且造成嚴重的微生物腐蝕[42-43]。相較于陸地,海洋環(huán)境中微生物對于高鹽、高壓、高溫等較為嚴苛的環(huán)境因素的耐受能力普遍更強,這意味著海上油氣管道內(nèi)的微生物及其生物膜的適應(yīng)性更強且難以殺滅。Zhou等[44]利用環(huán)境基因組測序分析手段對中國渤海某油田采出水中的微生物群落多樣性進行了分析。該油田由于長期注入海水或采出水回注,導(dǎo)致儲集層中被引入大量外源微生物。以該研究中樣本1基于RNA的結(jié)果分析為例,豐度及活性排在前十的菌屬包括博斯氏菌屬(,68.8%)、不動桿菌屬(,7.0%)、鞘氨醇單胞菌屬(,3.2%)、嗜氫菌屬(,4.7%)、無色桿菌屬(,3.0%)、短波單胞菌屬(,2.0%)、甲基桿菌屬(,1.9%)、埃希氏桿菌屬(,1.7%)、假單胞菌屬(,1.4%)、伯克氏菌科(,0.4%)。不難發(fā)現(xiàn),以上菌屬中包含多種如博斯氏菌屬()、甲基桿菌屬()和熱硫還原桿菌屬()等硫氧化細菌(Sulfur- oxidizing bacteria,SOB),具有將不同價態(tài)的含硫化合物氧化為硫酸鹽的能力[45-46]。同時,該油井由于長期受到SRB及其產(chǎn)生的H2S的污染,用于緩解該問題的硝酸鹽類抑制劑的注入促進了硝酸鹽還原菌(Nitrate-Reducing Bacteria,NRB)的生長,通過競爭攝取電子供體的方式與SRB形成競爭性抑制,其中代表性的菌屬有假單胞菌屬()、嗜氫菌屬()、不動桿菌屬()和無色桿菌屬()等[47-48]。
另外一種涉及油藏內(nèi)引入外源微生物的方式是微生物采油(Microbial Enhanced Oil Recovery,MEOR)。作為一種主要基于微生物學(xué)、分子生物學(xué)技術(shù)的三次采油方法,通過向油藏內(nèi)注入特定的菌種或營養(yǎng)物質(zhì),利用其自身生長代謝特性或產(chǎn)生功能性產(chǎn)物(產(chǎn)酸、產(chǎn)氣或產(chǎn)生物溶劑)來改變油氣儲集層內(nèi)環(huán)境和微生物種群結(jié)構(gòu),進而降低原油黏度或溶解巖層以增加儲層滲透率,從而達到提高采油率的目的[49-51]。MEOR相關(guān)微生物,包括醋酸桿菌屬()、芽孢桿菌屬()以及部分產(chǎn)甲烷菌()等,能夠在代謝過程中產(chǎn)生有機酸或生物表面活性劑。Kato等[52]分離得到了一株產(chǎn)乙酸菌GT1,其不僅可以利用有機物發(fā)酵產(chǎn)生乙酸,還能直接從鐵單質(zhì)中攝取電子,以上代謝特征極易引起微生物腐蝕。
生物膜的形成是微生物抵御外界環(huán)境變化維持群落內(nèi)穩(wěn)態(tài)的基本生存機制,如圖1所示,處于懸浮狀態(tài)的微生物通過附著、聚集等步驟逐漸成為復(fù)雜且穩(wěn)定的混合微生物群落。相比于懸浮狀態(tài),微生物嵌入由胞外聚合物(EPS)構(gòu)成的基質(zhì)后不僅能夠提高代謝過程的穩(wěn)定性,還能加強互養(yǎng)微生物種群之間的協(xié)同作用,這一特性使得腐蝕性微生物的危害進一步增加[53-54]。海上油氣集輸系統(tǒng)由復(fù)雜的管道網(wǎng)絡(luò)構(gòu)成,多相流的傳輸形式以及部分管網(wǎng)中較低的流速加快了腐蝕性生物膜以及沉積物在彎頭、焊縫和閥門等腐蝕敏感區(qū)域的形成和堆積,進一步減緩了管道內(nèi)物料的流速,最終導(dǎo)致油氣運輸停滯。
圖1 生物膜形成的一般過程示意圖[53]
在實際工況下,腐蝕性生物膜由微生物胞外聚合物(蛋白、多糖、核酸等)和腐蝕產(chǎn)物(FeS、FePO4和FeCO3等)共同構(gòu)成。由于EPS所構(gòu)成的三維網(wǎng)狀結(jié)構(gòu)(宏觀上多呈現(xiàn)出黏液狀),強化了細菌之間以及細菌與腐蝕產(chǎn)物之間的黏附性。同時,細菌生物膜中往往含有如氨基酸、糖醛酸等含有大量負電荷基團的有機物,能夠通過螯合、吸附等方式沉淀金屬陽離子,進一步刺激腐蝕性微生物與金屬離子之間的相互作用以及電子傳遞過程[55]。由于環(huán)境微生物種類復(fù)雜,所構(gòu)成的微生物群落以及代謝產(chǎn)物多樣,在生物膜中往往包含不同化學(xué)濃度梯度以及氧化還原電位的微環(huán)境。從微生物代謝多樣性的層面分析,這種結(jié)構(gòu)使生物膜內(nèi)的不同代謝類型的微生物之間建立了更有利的共生條件,促進了共代謝和互養(yǎng)作用。但是從微生物腐蝕的角度分析,濃差電池的產(chǎn)生極易造成金屬表面局部陰陽極的形成,是引起油氣管線局部腐蝕主要的原因之一。腐蝕性微生物對于油氣管網(wǎng)的影響可以總結(jié)為以下3點:(1)微生物及其分泌的EPS作為有機沉積物率先附著并沉積,使管道內(nèi)環(huán)境的理化性質(zhì)發(fā)生改變;(2)腐蝕性微生物的代謝活動及其產(chǎn)物會加速管道內(nèi)腐蝕產(chǎn)物的堆積;(3)腐蝕性生物膜的沉積會改變原有沉積物的性質(zhì),從而進一步加速腐蝕。
SRB、發(fā)酵菌等微生物能夠產(chǎn)生具有腐蝕性的代謝產(chǎn)物,如硫化物或有機酸等,該過程被稱為化學(xué)微生物腐蝕(Chemical Microbiologically Influenced Corrosion,CMIC)。這些腐蝕性代謝產(chǎn)物與金屬材料發(fā)生反應(yīng)后極易在管網(wǎng)內(nèi)形成沉積物,在促進內(nèi)腐蝕進一步發(fā)展的同時還會造成管網(wǎng)堵塞。以SRB為例,其產(chǎn)生的H2S微溶于水后產(chǎn)生HS?使局部環(huán)境呈酸性,造成管網(wǎng)內(nèi)部的局部腐蝕穿孔,解離出的氫也會富集在材料的缺陷處,造成氫滲透或開裂。同時,H2S擴散到金屬表面發(fā)生反應(yīng)生成具有導(dǎo)電性的無定形FeS產(chǎn)物層,隨后經(jīng)過反復(fù)溶解Fe(HS)+/HS?再沉積,腐蝕產(chǎn)物層中積累了更多的HS?,進一步加速陽極溶解速率[56-58]。有機酸的產(chǎn)生同樣對油氣管網(wǎng)具有很強的腐蝕性,發(fā)酵菌及其代謝產(chǎn)物在腐蝕性生物膜中的作用近些年得到了廣泛關(guān)注。研究發(fā)現(xiàn),醋酸菌能夠在厭氧條件下借助Wood–Ljungdahl通路中的金屬蛋白/金屬酶以H2和CO2為底物生成乙酸,即使生物膜中有機酸的濃度很低,也能增加金屬腐蝕的風(fēng)險[59]。
具有電活性代謝能力的微生物通過胞外電子傳遞(Extracellular Electron Transfer,EET)的方式從金屬氧化過程中提取電子,或者將細胞內(nèi)有機物徹底氧化后釋放的電子傳遞到細胞外的電子受體(如硫酸鹽、硝酸鹽或金屬難溶物等),以上過程能夠誘發(fā)或加速腐蝕[60-62],被稱為電化學(xué)微生物腐蝕(Electro-chemical Microbially Influenced Corrosion,EMIC)。目前,胞外電子傳遞主要有3種機制:直接電子傳遞(Direct Electron Transfer,DET)、間接電子傳遞(Me-dia-ted Electron Transfer,MET)和電運動機制(Elec-trokinesis)[63-65]。直接電子傳遞是指細菌通過外膜的細胞色素C直接與電子受體接觸,其電子傳遞效率較高,但生物膜與電子受體之間的接觸面積直接決定了電子傳遞效率的上限,且無法進行較遠距離的電子傳輸[66]。間接電子傳遞是指細菌通過內(nèi)源或外源的電子穿梭體,在細菌與電子受體之間通過往復(fù)的氧化還原反應(yīng)實現(xiàn)較長距離的電子轉(zhuǎn)移。電運動機制是指細菌通過將電子傳遞到細胞膜表面,然后依靠布朗運動或鞭毛驅(qū)使細胞撞擊電子受體表面,撞擊瞬間完成電子傳遞過程[67]。
CMIC與EMIC在油氣管道內(nèi)腐蝕性生物膜中存在著平衡與轉(zhuǎn)化。CMIC很大程度上依靠碳氫化合物等有機碳源的降解與硫酸鹽或硝酸鹽的還原反應(yīng)耦合驅(qū)動腐蝕的發(fā)生,而EMIC則是微生物主動驅(qū)使腐蝕過程,兩者之間的轉(zhuǎn)化取決于可用的有機碳源是否充足。以典型腐蝕性微生物為例,當作為有機碳源的乳酸充足時,的CMIC過程導(dǎo)致FeS為主的腐蝕產(chǎn)物堆積,而當環(huán)境中可用的乳酸含量逐漸減少時,的代謝模式發(fā)生了轉(zhuǎn)變,利用從Fe0直接獲取電子的方式對CMIC過程中的能量缺口進行代償。從局部腐蝕形貌變化情況來看,可用有機碳源減少至10%時,其腐蝕程度最嚴重。而將有機碳源全部去除后,腐蝕程度顯著減弱,表明EMIC過程并不能維持全部的代謝需求,僅可以作為腐蝕性微生物解決環(huán)境突變的一種應(yīng)對策略[68]。
由于實際腐蝕環(huán)境中多變的物理化學(xué)因素,腐蝕性微生物膜中涉及到十分復(fù)雜的代謝過程。如圖2所示,環(huán)境微生物的多樣性結(jié)構(gòu)使得不同種類微生物之間在應(yīng)對外界環(huán)境變化時存在協(xié)同和拮抗的相互作用,最終通過群落演變、生物膜成熟直至形成具有腐蝕性的復(fù)雜微生物群落。
圖2 涉及海底油氣管道微生物腐蝕機理匯總[73]
在油氣集輸系統(tǒng)中,腐蝕性生物膜中微生物之間相互協(xié)同互補的代謝模式扮演著重要的角色。實際服役環(huán)境中,在同一區(qū)域的腐蝕產(chǎn)物沉積層中往往能夠同時分離得到發(fā)酵產(chǎn)酸菌以及具有氫代謝特征的菌種,此類微生物多出現(xiàn)在含有采出水的油氣生產(chǎn)設(shè)施中,通過發(fā)酵反應(yīng)代謝揮發(fā)性脂肪酸、醇或碳氫化合物來生長,發(fā)酵過程中產(chǎn)生的氫被生物膜的氫營養(yǎng)型微生物消耗,如產(chǎn)甲烷菌[69-70]。有機酸類代謝產(chǎn)物不僅能夠直接對金屬造成腐蝕,發(fā)酵產(chǎn)生的H2還能夠還原單質(zhì)硫,從而產(chǎn)生大量的硫化氫[71]。以上過程表明,發(fā)酵產(chǎn)酸菌能夠通過產(chǎn)生甲酸、乙酸等途徑來刺激電活性微生物的生長,從而加速金屬腐蝕進程[72]。此外,當鐵還原菌和鐵氧化菌(Iron-oxidizing bacteria,IOB)同時存在于生物膜中時,IOB能夠通過促進金屬氧化析出腐蝕產(chǎn)物(鐵氧化物和鐵硫化物),形成保護性腐蝕產(chǎn)物層,限制金屬表面與腐蝕環(huán)境直接接觸。然而,IRB利用金屬氧化沉積物作為電子受體,導(dǎo)致金屬表面再次暴露于腐蝕產(chǎn)物和腐蝕性微生物。
油氣管道中清理內(nèi)部MIC廣泛使用物理刮擦與非氧化性殺菌劑相結(jié)合的方法。傳統(tǒng)的殺菌劑有戊二醛、三氯異氰脲酸(TCCA)和四羥甲基硫酸磷(THPS)等。然而,由于使用機械破壞的方式導(dǎo)致腐蝕性微生物從破損的生物膜中擴散出來,重新彌散到管網(wǎng)中導(dǎo)致腐蝕性微生物充分地擴散,反而進一步加劇了腐蝕[74]。同時,部分管道內(nèi)長期形成的保護性鐵氧化物沉積層也會被清除,導(dǎo)致基體重新暴露在外面。長期使用殺菌劑對油氣集輸管網(wǎng)進行清理,不僅會導(dǎo)致管內(nèi)微生物群落的改變,且新的微生物群落的種群結(jié)構(gòu)不可預(yù)測,不一定具有更低的腐蝕性。此外,殺菌劑的使用對于部分頑固微生物會產(chǎn)生耐藥性,還有污染環(huán)境的風(fēng)險[75]。
為了解決海上石油設(shè)施以及油氣集輸管網(wǎng)中由微生物導(dǎo)致的H2S酸化問題,向系統(tǒng)內(nèi)注入硝酸鹽被認為是一種低成本、高效率的解決方案。外注硝酸鹽可以刺激NRB的生長,NRB和SRB都可以利用乙酸、乳酸或長鏈脂肪酸作為能源,能夠與SRB競爭電子供體。對于相同的電子供體,硝酸鹽還原過程能夠獲得更多的能量。以乙酸為例,被NRB和SRB氧化的自由能變化見式(1)—(2)[76]。
Δo= ?495 kJ/mol (1)
Δo= ?47 kJ/mol (2)
掠食性微生物(and Like Organisms,BALOs)和噬菌體作為一種新型的基于微生物手段的防腐方法得到了廣泛關(guān)注[80]。當MIC系統(tǒng)引入BALOs或噬菌體時,腐蝕性微生物及其生物膜會被作為獵物而被捕獲并遭到破壞。它們有著相似的生存方式,首先通過入侵或感染進入宿主體內(nèi),然后分泌各種裂解酶分解并吸收宿主體內(nèi)的營養(yǎng)物質(zhì)完成自身的繁殖和復(fù)制,直到宿主死亡破裂后進行下一次“捕獵”[81]。Qiu等發(fā)現(xiàn)SRB的活性在與BALOs共存的條件下受到顯著抑制,浸泡60 d后,X70鋼的腐蝕速率由19.17 mg/(dm2?d)下降到3.75 mg/(dm2?d)。相比于傳統(tǒng)殺菌劑的方法,BALOs和噬菌體首先克服了微生物耐藥性的問題,BALOs可以侵入由混合細菌組成的微生物群落,破壞頑固的生物膜,從而削弱生物膜對外環(huán)境的抵抗。此外,BALOs和噬菌體還可以避免重復(fù)接種,并通過增殖在腐蝕體系中長期保持有效濃度直至目標微生物被殺滅,顯著降低了由于殺菌劑過量使用造成的環(huán)境污染[82-84]。
海上石油天然氣生產(chǎn)設(shè)備及管網(wǎng)的MIC不僅防治成本高,而且可能造成嚴重的生態(tài)環(huán)境危害。長期的研究積累使得研究者們對其中涉及的油藏內(nèi)微生物演化規(guī)律以及油氣集輸系統(tǒng)中的MIC機理有了更全面的理解。但是,目前MIC研究中面臨的主要問題是單純依靠實驗室條件下的純培養(yǎng)手段,無法準確地檢測和評估具有腐蝕性微生物及其生物膜中的化學(xué)、電化學(xué)和微生物代謝過程。
針對海上油氣管道MIC的防治,工業(yè)殺菌劑依然是最經(jīng)濟且高效的防腐手段,如季銨鹽類、有機溴類、雜環(huán)類或復(fù)配類殺菌劑等。然而,工業(yè)殺菌劑的使用面臨著如微生物耐藥性增加、難降解、降低油品等諸多問題。未來的MIC相關(guān)研究應(yīng)該利用先進的生物學(xué)檢測技術(shù),如環(huán)境基因組學(xué)、微流控以及高通量檢測技術(shù),對油氣集輸系統(tǒng)中微生物群落的結(jié)構(gòu)和多樣性進行檢測和分析,從組學(xué)的角度了解環(huán)境微生物在腐蝕過程中的潛在功能、代謝特征以及對于原位環(huán)境實時變化的響應(yīng)機制,更好地了解石油生產(chǎn)中的微生物過程。根據(jù)實際腐蝕環(huán)境制定特異性的殺菌防腐方案,有針對性地選擇殺菌劑的種類和用量,將有助于精準地預(yù)防和緩解MIC,為新型防腐技術(shù)的實施提供有價值的信息和數(shù)據(jù)支撐。
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Research Progress on Microbiologically Influenced Corrosion of Oil and Gas Pipelines in Marine Environment
1,2,1,2,1,2,1,1,2
(1. Institute for Advanced Materials and Technology, University of Science and Technology Beijing, Beijing 100083, China; 2. Shunde Graduate School of University of Science and Technology Beijing, Guangdong Foshan 528399, China)
Corrosion of offshore oil-gas gathering and transportation pipelines can lead to serious environmental risks and economic losses. With the continuous development of China's marine energy industry, it is imperative to accelerate the rational exploitation and utilization of offshore oil-gas resources as well as upgrade the oil-gas production equipment. As an important part of oil-gas gathering and transportation networks, submarine pipeline is known as the "artery" of offshore oil-gas production systems. Under actual working conditions, submarine pipelines are inevitably exposed to corrosion and failure. Microbiologically influenced corrosion (MIC) has been considered one of the main factors causing this problem. According to statistics, more than 20% of the oil-gas pipeline corrosion and oil leakage accidents are directly or indirectly related to MIC. The vast ocean includes a wide range of extreme environments such as high salt, high pressure, and low temperature environments. Marine environments are more diverse and complex than terrestrial environments, indicating that marine microbes are more tolerant to extreme conditions. Herein, the sources of corrosive microorganisms in offshore oil-gas pipelines, including reservoir endogenous microorganisms, exogenous microorganisms introduced by seawater injection, and microbial enhanced oil recovery, were classified. The characteristics of fluid chemical substances in submarine reservoirs were analyzed. It was confirmed that they were rich in methane, sulfides, and volatile fatty acids, and they were classified according to the characteristics of endogenous microbial metabolism and products, including sulfate-reducing bacteria, methanogens, fermentative bacteria, and iron-reducing bacteria. Moreover, the characteristics of microbial community abundance in the produced water of an oilfield were analyzed with an example, and the evolution rule of the microbial community under long-term oilfield environmental stress was clarified. Complex gathering and transport networks are particularly prone to biofilm formation and metabolite accumulation, which may cause or exacerbate corrosion problems. A corrosive biofilm, composed of various environmental microorganisms, is a general life form used by microorganisms to resist changes in the external environment and maintain homeostasis of the internal environment, which includes a complex symbiotic relationship between microorganisms with different metabolic characteristics. Accordingly, theories of metabolite-MIC, extracellular electron transfer-MIC, and synergism/antagonism among corrosive microorganisms in offshore pipeline networks were further reviewed. Pure/mixed culture in laboratory conditions can hardly represent the complexity of in situ biofilms in oil-gas pipelines; therefore, it is almost impossible to reconstruct the corrosion behavior of microorganisms in a real service environment. Industrial bactericides are one of the most widely used strategies for MIC in oil-gas pipeline networks. Advanced composite bactericides often possess broad-spectrum antibacterial properties, low toxicity, and sustained bactericidal activity. However, bactericides have drawbacks such as increased microbial resistance, difficulty in degradation, and deterioration of crude oil quality. Therefore, it is extremely challenging to detect the corrosive microbial community and the metabolic processes leading to corrosion accurately under actual working conditions. Advanced biological detection technologies, including environmental genomics, microfluidics, and high-throughput rapid detection technology, should be fully utilized in future research on the MIC of oil-gas pipelines. In this paper, the types of potential microbial species, types of MIC, and the corrosion mechanisms are summarized in detail, and novel anti-corrosion methods based on biotechnology are proposed.
offshore oil and gas pipeline; microbiologically influenced corrosion; reservoir microorganism; MIC mechanism
TG174
A
1001-3660(2022)05-0129-10
10.16490/j.cnki.issn.1001-3660.2022.05.014
2022–03–12;
2022–04–21
2022-03-12;
2022-04-21
國家自然科學(xué)基金面上項目(52071015)
General Program of the National Natural Science Foundation of China (52071015)
婁云天(1990—),男,博士,主要研究方向為微生物腐蝕。
LOU Yun-tian (1990-), Male, Doctor, Research focus: microbiological influenced corrosion.
張達威(1984—),男,博士,教授,主要研究方向為智能耐蝕材料、微生物腐蝕與材料腐蝕大數(shù)據(jù)預(yù)測評價。
ZHANG Da-wei (1984-), Male, Ph. D., Professor, Research focus: intelligent corrosion resistant material, microbiological influenced corrosion, prediction and evaluation of material corrosion with big data.
婁云天, 何盛宇, 陳旭東, 等. 海洋環(huán)境中油氣管道的微生物腐蝕研究進展[J]. 表面技術(shù), 2022, 51(5): 129-138.
LOU Yun-tian, HE Sheng-yu, CHEN Xu-dong, et al. Research Progress on Microbiologically Influenced Corrosion of Oil and Gas Pipelines in Marine Environment[J]. Surface Technology, 2022, 51(5): 129-138.
責(zé)任編輯:萬長清