梁 萌,袁海云,楊 英,藺江濤,楊云博
(1.俄羅斯國立古勃金石油天然氣大學,俄羅斯莫斯科 119991;2.中國石油長慶油田分公司第三采氣廠,陜西西安 710021)
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專論與綜述
CO2在驅油過程中的作用機理綜述
梁萌1,袁海云2,楊英1,藺江濤2,楊云博2
(1.俄羅斯國立古勃金石油天然氣大學,俄羅斯莫斯科119991;2.中國石油長慶油田分公司第三采氣廠,陜西西安710021)
摘要:綜述了CO2驅油過程中存在的幾種作用機理,地層條件下CO2在原油中的溶解導致了原油組成與性質的變化,具體表現(xiàn)在原油黏度降低、體積膨脹、油氣界面張力改善和瀝青質沉積等方面;CO2在地層水中的溶解為巖石的腐蝕提供了弱酸環(huán)境,水中陽離子濃度的增大和CO2的過量導致了碳酸鹽的溶解/析出平衡。受以上因素影響,注CO2過程中發(fā)生了巖石潤濕性和滲透率的改變。上述各個現(xiàn)象和作用機理并不是孤立存在,它們之間相互聯(lián)系相互影響,在不同程度上影響著驅替過程和最終的采收率,所以在油藏開發(fā)工藝制定、優(yōu)化時,必須充分比較、衡量各自的影響作用。
關鍵詞:二氧化碳;提高采收率;機理;驅替
氣體驅油在油田開發(fā)領域的應用日益廣泛,常用的有N2、CH4、石油伴生氣和CO2。CO2的特殊性質以及CO2兼具驅油、減排的雙贏效果使CO2驅儼然成為開發(fā)領域的最大熱點。針對不同地質特征的油藏及原油特性,開發(fā)出了多種工藝,如CO2非混相驅和混相驅、CO2段塞+N2驅、CO2水交替工藝以及CO2吞吐等。地層內CO2可與原油以及巖石和地層水發(fā)生作用,其結果導致促進驅油過程或者不利影響。CO2的溶解降低原油黏度、體積溶脹、改善油氣界面張力、瀝青質沉積等;在地層水中的溶解降低了地層環(huán)境的pH,碳酸鹽發(fā)生溶解,改變巖層表面潤濕性與地層滲透率等。所有上述作用無不直接或間接的影響著最終采收率,所以為了合理開發(fā)驅油工藝,必須清楚CO2/原油/巖石/水間的作用機理。
CO2作為一種活性流體在原油中具有優(yōu)良的溶解度(55m3/m3)[1]。CO2的溶解通常導致以下幾方面:原油黏度降低、原油溶脹、降低油氣界面張力、原油中瀝青質沉積等。
1.1原油黏度降低
CO2對原油具有稀釋作用,使原油內烴類分子間的內摩擦力減小,原油黏度降低。在溶解的初始階段原油黏度降低幅度非常大,溶解達到一定程度后,進一步降黏效果減弱[1]。降黏效果很大程度上取決于原油的初始黏度,初始黏度越高,溶解CO2后黏度降低幅度越大[2],艾柏迪重油飽和CO2后黏度由1 080 cp和4 900 cp分別降到47 cp和82 cp[3]。黏度降低增加了原油的流動性,優(yōu)化了油氣流度比,提高了地層中油相相對滲透率。
1.2原油溶脹
CO2的溶解還促使原油體積膨脹,幅度一般可達10%~60%[4]。通常用膨脹系數(shù)來描述CO2溶脹原油的能力。隨著溫度升高而膨脹系數(shù)增加[1],隨著壓力的增加,膨脹系數(shù)隨之升高,當壓力增加到一定值時,油中輕烴揮發(fā)的量超過CO2在油中的溶解量,導致原油體積下降,使膨脹系數(shù)減小甚至小于1[5]。Haishui等[6]發(fā)現(xiàn)正構烷烴的膨脹系數(shù)一般介于1.2~1.4,最高可達2.16,其隨著碳原子數(shù)的增加而減小。這說明,對于原油的溶脹效果高碳數(shù)烷烴的貢獻并不大,所以CO2對重質油的溶脹效應遠遠弱于輕質油[7]。剩余油飽和度與膨脹系數(shù)成反比,原油的膨脹迫使油滴從孔道中遷移出來,分散的油滴更容易發(fā)生聚集,提高驅油效果[8]。
1.3改善油氣界面張力
隨著滲透率減小,毛細管力逐漸顯著,制約著低滲油藏的開發(fā),為此最直接最有效的手段就是降低界面張力。諸多研究表明,CO2可明顯改善油氣界面張力。隨著壓力增加,CO2-原油界面張力下降,達到最小混相壓力時,界面張力維持在極低范圍或零[9,10]。關于溫度對CO2-原油界面張力的影響較復雜,一些研究[11-13]表明存在著轉折點壓力,當壓力小于該值時體系界面張力隨溫度增加而減小,當壓力高于該壓力時,體系界面張力隨溫度升高而增加。不同CO2-原油/烴類體系的轉折點壓力(見表1)。
表1 轉折點壓力*
溫度升高,分子平均動能增加,分子更加活躍,界面間分子交換更加頻繁,但是恒壓下溫度的升高同樣導致氣相(或者超臨界)體積的增大,導致分子濃度降低,所以就出現(xiàn)了兩種競爭情形:分子平均動能的增加與氣相體積的增大,兩種效應導致界面張力向相反的方向發(fā)展。所以這也就出現(xiàn)了上述轉折點壓力。低壓下溫度升高導致體積的膨脹效應要弱于溫度升高帶來的分子平均動能的升高,所以低壓下隨著溫度增加界面張力減下;高壓下,超臨界相的CO2體積受溫度的變化要強于常規(guī)的氣態(tài)CO2,體積效應增強,隨著溫度升高界面張力增大。
原油組成也影響著CO2-原油界面張力,隨著原油中C1組分的增加,界面張力增大;隨著C2~C10含量的增加,界面張力減小[10]。而瀝青質含量增加時,體系界面張力變大[12]。
1.4瀝青質沉積
瀝青質是一類極性多環(huán)高分子化合物。原油中CO2的溶解打破了原有溶解平衡,破壞了分散體系的穩(wěn)定性,重組分凝聚析出,最先有可能沉積的為瀝青質、膠質以及大的直鏈烷烴[15,16]。
研究發(fā)現(xiàn)只有當CO2在原油中達到一定溶解量時瀝青質才開始沉積,也就出現(xiàn)了瀝青質沉積的氣體初始含量[17,18]。CO2、石油伴生氣、N2的瀝青質沉積的氣體初始含量分別為0.25、0.28、0.5[19],可見CO2最易析出瀝青質。瀝青質沉積的CO2初始含量受原油組成影響,瀝青質含量4wt%和4.9wt%的原油,對應的初始沉積含量分別為41 mol%和39 mol%(59°C,16MPa)[20]。
隨著CO2含量增加瀝青質沉積達到最大值后又減?。ㄒ妶D1),該現(xiàn)象在其他研究中也得到證實[21-24]。這是因為當CO2在油中含量較高時,二者的傳質形式主要是CO2對原油中輕組分的抽提,原油逐漸重質化,發(fā)生瀝青質返溶[22]。
圖1 瀝青質沉積量與CO2在原油中含量的關系[25]
CO2壓力的增加導致瀝青質沉積量的增大[26],類似地存在瀝青質沉積的初始壓力[27]??挛钠娴龋?8]得到了Weyburn輕質油的瀝青質沉積初始壓力為4.7MPa~4.8MPa。Wang等[27]得出Alberta輕質油的瀝青質沉積初始壓力約為4.8MPa。
瀝青質沉積還受巖性等因素影響,相比砂巖瀝青質更容易在碳酸巖沉積[29]。滲透率越高瀝青質沉積量越大,造成的地層破壞越嚴重[30],而巖石非均質性越大,瀝青質越易沉積[31]。瀝青質的沉積導致地層孔道堵塞,滲透率和孔隙度下降[32]。此外瀝青質的沉積也是巖石表面潤濕性向親油性轉變的重要影響因素[33]。
2.1碳酸巖溶解
CO2的溶解一般能使地層水的pH降至3.3~3.7[34],足以使碳酸巖溶解,從而使地層結構發(fā)生破壞,孔隙度和滲透率增加[35],但當?shù)貙铀嘘栯x子的濃度增加到一定值時,會以鹽的形式重新析出,沉積在孔道中,堵塞孔道,使?jié)B透率降低。所以孔隙度和滲透率的變化是受碳酸鹽溶解和二次析出雙重控制的[30,36]。Shedid等[37]發(fā)現(xiàn)CO2與碳酸巖接觸時間為7d時,孔隙度和滲透率下降達20%;當延長至150d發(fā)現(xiàn)孔隙度和滲透率的增加。Ibrahim等[38]發(fā)現(xiàn)CO2注入速率對碳酸巖滲透率的影響明顯,低注入速率(2cm3/min)時滲透率減小,高注入速率(5cm3/min)導致滲透率增加。
2.2巖層潤濕性的改變
潤濕性是衡量巖石表面被流體潤濕的能力,其直接影響著流體在地層的分布以及流動。親油環(huán)境中,潤濕相油占據(jù)小的孔道,導致了較大的殘余油飽和度;隨著潤濕性向親水性轉變,水逐漸占據(jù)被原油占據(jù)的小孔道,從而降低了殘余油飽和度[39]。同樣在注CO2過程中發(fā)現(xiàn)了巖石潤濕性的改變。
Saad等[40]發(fā)現(xiàn)隨著巖石與CO2接觸時間增長,接觸角減小。通常認為巖石親水性的增強主要是緣于CO2對地層水酸化和地層溶解[41]。而Robert等[42]研究了CO2驅替實驗,發(fā)現(xiàn)石英和方解石的水相接觸角分別由46°和51°增加到134°和129°。這主要是因為瀝青質在巖石表面的沉積增加了其親油性。Hamed等[33]模擬了瀝青質沉積對巖石潤濕性的影響,發(fā)現(xiàn)隨著瀝青質沉積發(fā)生親水性向親油性的反轉??梢姡虢忉孋O2對巖石潤濕性的作用,必須同時考慮瀝青質沉積和巖層溶解兩因素。
2.3滲透率改變
CO2驅油過程中地層滲透率的變化主要受以下因素影響:瀝青質沉積和巖石溶解[43],其中瀝青質在孔道的沉積導致滲透率下降[44]。Yoshihisa等[29]在砂巖和碳酸巖上進行CO2驅油實驗時發(fā)現(xiàn)瀝青質沉積導致巖心滲透率減小了20%,AmirMasoud等[17]甚至發(fā)現(xiàn)滲透率的減小指數(shù)與瀝青質的沉積量呈線性關系。而巖石溶解對滲透率的影響,如2.1中所述,最終結果是受多因素影響的。
CO2在原油內的溶解導致了原油降黏、溶脹和油氣界面張力改善、瀝青質沉積等效應;而CO2在地層水的溶解導致了碳酸巖的溶解,地層物化性質的改變是上述作用的綜合表現(xiàn)。在制定油藏開發(fā)工藝時為達到最大采收率,應充分考慮各作用的影響。例如對于輕質油,可忽略CO2混相驅中的瀝青質沉積;而對于重質油,CO2會促使較多的瀝青質沉積,混相驅工藝并不是最合適的選擇,建議在較少瀝青質沉積量的前提下,結合非混相和熱開采等工藝,盡可能大的降低原油黏度并使其溶脹。
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Review on the mechanisms during CO2flooding process
LIANG Meng1,YUAN Haiyun2,YANG Ying1,LIN Jiangtao2,YANG Yunbo2
(1.Gubkin Russian State Oil and Gas University,Moscow 119991,Russian;2.Gas Production Plant 3 of PetroChina Changqing Oilfield Company,Xi'an Shanxi 710021,China)
Abstract:This paper reviews several mechanisms existing in the CO2flooding,under formation conditions CO2dissolves in crude oil,which leads to changes in the compositions and properties of crude oil,in particular to reduce the viscosity of crude oil,volume expansion,reduce oil-gas interfacial tension and asphaltene deposition. CO2dissolves in the formation water and provide corrosive acidic environment for rock,however in water increasing cation concentration and excess CO2results in the balance between dissolution and precipitation of carbonate. Affected by the above factors,changes in the wettability of rock and permeability occur during CO2injection. Each of the above phenomena and mechanism does not exist in isolation,they influence each other,and in varying degrees affects the displacement process and ultimate recovery,so when a reservoir development process is designed and optimized,itis necessary to fully compare their influences.
Key words:Carbon dioxide;enhanced oil recovery;mechanism;oil displacement
中圖分類號:TE357.45
文獻標識碼:A
文章編號:1673-5285(2016)06-0001-06
DOI:10.3969/j.issn.1673-5285.2016.06.001
*收稿日期:2016-05-11修回日期:2016-05-24
作者簡介:梁萌,男(1987-),在讀博士研究生,主要從事提高采收率方面的研究工作,郵箱:liangmeng@mail.ru。