酒基鉆井完井液對頁巖儲層保護能力評價

摘 要

摘 要:在中國,頁巖氣儲層長水平井段鉆進中普遍采用油基鉆井完井液,鉆進過程中時常發(fā)生井壁垮塌和漏失,不得不頻繁處理井下事故,導致頁巖段鉆井速度慢,浸泡時間長,引發(fā)嚴重的儲層損

摘 要:在中國,頁巖氣儲層長水平井段鉆進中普遍采用油基鉆井完井液,鉆進過程中時常發(fā)生井壁垮塌和漏失,不得不頻繁處理井下事故,導致頁巖段鉆井速度慢,浸泡時間長,引發(fā)嚴重的儲層損害問題。為了探究油基鉆井完井液對頁巖儲層的損害機理及防治策略,以四川盆地志留系龍馬溪組和寒武系牛蹄塘組頁巖為研究對象,開展了頁巖儲層敏感性評價、油基鉆井完井液靜態(tài)和動態(tài)損害評價系列實驗。結(jié)果表明,油基鉆井完井液濾液對裂縫巖樣的損害率為63.5%~99.9%,體系動態(tài)損害率為49.0%~87.2%,綜合損害程度為中等——強。結(jié)論認為:固相侵入、油相圈閉損害、堿敏損害、流  固耦合強化的應(yīng)力敏感損害是主要的損害方式;及時高效封堵裂縫、降低濾失量、控制合理的pH值和正壓差,并與井眼軌跡優(yōu)化設(shè)計相結(jié)合既是強化頁巖井壁穩(wěn)定的技術(shù)對策,也是提高油基鉆井完井液保護頁巖氣儲層能力的重要途徑。

關(guān)鍵詞:頁巖  裂縫(巖石)  巖樣  油基  鉆井完井液  儲集層損害  保護措施

Damage evaluation of oil-based drill-in fluids to shale reservoirs

AbstractCurrentlyoil-based drill-in fluids have been widely used in China to prevent the frequent collapse and leakage of well borehole during long lateral horizontal drilling targeting organic-rich shale reservoirsHowever,wellbore instability still happens frequently during drilling processWhen such complicated downhole drilling accidents are dealt with,drilling speeds and trips will beslowed down and the reservoir rocks will be soaked for a long timeresulting in significantly serious formation damageTherefore,this paper aims to explore the damage mechanism of oil-based drill in fluids to shale reservoirs and to acquire prevention strategiesCollecting gas shale cores from the Silurian Longmaxi and Cambrian Niutitang Fins in the Sichuan Basin,we made an evaluation analysis of shale formation sensitivity and conducted experiments of oil based drill in fluids’static and dynamic damage to shale formationsResults showed that the static damage rate ranged from 63.5 to 99.9%,while the system dynamic damage rate from 49.0 to 87.2%,which means a moderate to strong damage as a wholeIt was considered that major damage factors include solid phase intrusion,alkali sensitivity damage,oil phase trapping damageand stress sensitivity damage enhanced by fluid solid interactionOn this basis,we presented that combining efficient fracture plugging,pH value optimizing,filtration volume and positive pressure differential control with an optimal well traj ectory design can not only strengthen the shale wellbore stabilitybut provide an effective way to mitigate the formation damage of oil based drill in fluids

Keywordsshale,fracture(rock)core,oil-based drill-in fluids,reservoir damage,reservoir protection measure

與水基鉆井液相比,油基鉆井液在抑制頁巖水化膨脹,潤滑防卡,維持井壁穩(wěn)定以及快速鉆進等方面具有明顯的優(yōu)勢,已成為鉆探高溫深井、海上鉆井、大斜度定向井、水平井、各種復雜井段和保護儲層的重要手段[1-2]。近年來隨著國內(nèi)頁巖氣資源勘探開發(fā)的持續(xù)升溫,針對頁巖氣儲層的地質(zhì)特征和水平井井身結(jié)構(gòu)特點,油基鉆井完井液以其獨特的優(yōu)勢,成為水平井段鉆進的首選鉆井完井液體系[3-6]。但從現(xiàn)場實施效果來看,頁巖層段的井塌井漏現(xiàn)象仍較為嚴重[5-7]。井漏、井壁失穩(wěn)等問題通常會嚴重損害近井地帶儲層,后期的壓裂改造還可能將侵入的固相和液相推向儲層深部,造成大范圍的不可恢復損害[8-9]。已有研究認為,對于致密儲層,水基工作液的損害以水相圈閉、固相侵入、黏土礦物水化膨脹、應(yīng)力敏感等方式為主[10-16]。關(guān)于油基鉆井完井液對頁巖儲層的保護能力,目前仍缺乏深入系統(tǒng)的分析與評價。筆者以四川盆地志留系龍馬溪組和寒武系牛蹄塘組頁巖為對象,開展了頁巖儲層敏感性評價、油基鉆井完井液靜態(tài)和動態(tài)損害評價系列實驗,力求通過實驗分析,探索潛在的損害因素與機理,提出針對性的儲層保護措施。

1 巖樣及實驗方法

11 樣品選取

實驗巖樣為四川盆地龍馬溪組和牛蹄塘組頁巖,基礎(chǔ)物性參數(shù)見表l。該2組地層均是古生代海相沉積形成的富有機質(zhì)頁巖,后期經(jīng)歷了大幅度的構(gòu)造抬升和強烈的地質(zhì)改造作用[3]。地層硬脆性特征明顯,微裂縫發(fā)育,常見于石英、長石等骨架顆粒間以及云母片中。裂縫面溶蝕作用明顯,常見黏土礦物呈片狀、絮狀充填以及方解石充填。

 

鑒于評價裂縫巖樣的實際意義更大,實驗均采用平行于巖樣軸向的單條人工裂縫巖樣。實驗所用油基鉆井完井液取自四川某頁巖氣井,實驗室條件下測得其塑性黏度為l36mPa·s,表觀黏度為88mPa·s,pH值為ll.5,與地層水不互溶。

12 頁巖儲層敏感性評價方法

根據(jù)頁巖儲層典型特征,按照《儲層敏感性流動實驗評價方法》(SYT 53582010)要求的實驗方法,開展速敏、水敏、堿敏和應(yīng)力敏感實驗。

13 鉆井完井液對頁巖儲層的損害評價

131靜態(tài)損害實驗方法

靜態(tài)損害實驗的濾液為TGl6-WS高速離心機將原漿在6000rrain下離心分離固相并過濾后得到。實驗通過巖樣在濾液損害前(Kf1)與損害后(Kf2)的克氏滲透率變化,評價損害程度,具體實驗步驟請參閱本文參考文獻[8]

132動態(tài)損害實驗方法

動態(tài)損害實驗采用油基鉆井完井液原漿,實驗儀器為MFC-Ⅰ型多功能損害評價儀,在模擬地層條件(圍壓為7MPa,工作壓差為3.5MPa,溫度為70℃、剪切速率為150s-1)對巖樣進行損害,測量損害前后地層水滲透率KwKwi,根據(jù)返排恢復率(KwiKw)和動態(tài)損害率(1-KwiKw)評價損害程度。

2 實驗結(jié)果

21 儲層敏感性評價結(jié)果

龍馬溪組頁巖的流體儲層敏感性為中等偏強強,其中堿敏臨界pH值儀為7,損害率可達90(2)。儲層應(yīng)力敏感性系數(shù)為0.480.62,應(yīng)力敏感程度為中等。

 

22 鉆井完井液對儲層的損害評價結(jié)果

221靜態(tài)損害實驗

油基鉆井完井液濾液對巖樣的損害率介于63.5%~99.9(3),損害程度為中等偏強——強。損害前后同一壓力點下巖樣的氣測滲透率普遍降低且超過了60(1)。在縫寬相近時,損害程度與返排壓差存在一定的相關(guān)性,返排差壓低,損害率也相對較低;反之,損害率相對更高。

 

 

222動態(tài)損害實驗

動態(tài)損害后,巖樣返排恢復率普遍小于50%,平均損害率為49.0%~87.2%,損害程度中等——強。切除侵入端(1cm)再進行返排測試,損害率總體有所降低(4)。損害程度主要受固相顆粒與縫寬匹配關(guān)系及固相侵入嚴重程度的影響,詳見后面“討論”一節(jié)。

 

3 討論

31 油基鉆井完井液損害機理分析

頁巖儲層的水平段鉆井普遍存在井漏等井壁失穩(wěn)問題(5),并由此引發(fā)嚴重的儲層損害問題。分析認為油基鉆井完井液對頁巖儲層的損害方式主要包括固相侵入、油相圈閉損害、堿敏和應(yīng)力敏感損害。此外,體系巾的聚合物殘渣和親油膠體的吸附滯留現(xiàn)象也能造成一定的儲層損害。

 

311固相侵入

馬爾文激光粒度儀測得實驗所用油基鉆井完井液的粒度分布為:D10l.593mm、D50l2.573mmD9033.760mm,其中粒徑小于l0mm和介于1030mm的固相顆粒含量分別約占40%和50%。該體系對縫寬小于50mm的裂縫具有一定的封堵效果,表現(xiàn)為巖樣損害率相對較低,并且切除侵入端后,損害率能進一步下降。對于縫寬大于80mm的巖樣,顆粒直徑與縫寬的匹配關(guān)系差,有效固相顆粒含量不足,無法在裂縫端面形成有效的暫堵層[17],在正壓差作用下,外來固相極易侵入裂縫深處,即使切除侵入段,損害率也無明顯降低。此外,由于體系中含有大量親油性質(zhì)的有機土,以及表面吸附有瀝青質(zhì)或油膜的固相顆粒,相比于水基鉆井完井液,侵入固相更難以返排[18]。這也是動態(tài)損害率較高的原因之一。

312油相圈閉損害

水相圈閉是制約致密砂巖氣井產(chǎn)能的重要因素,毛細管自吸和液相滯留則是產(chǎn)生水相圈閉的主要原因[19]。與致密砂巖儲層相比,貞巖儲層具有更為發(fā)育的微裂縫系統(tǒng),更高的毛細管力。并且由于頁巖富含有機質(zhì)等性質(zhì),巖石也具有明顯親油性質(zhì)(6)。多種因素的共同作用,使得頁巖對油相具有顯著的自吸及滯留作用[20-22]。油基鉆井完井液濾液的自吸實驗表明,干巖樣的自吸飽和度為38%~48(2)。油相飽和度大幅度增加,大量濾液吸附滯留在孔道之中而難以返排,勢必導致氣相滲透率降低,產(chǎn)生油相圈閉損害。

 

 

313堿敏

敏感性實驗表明頁巖具有強堿敏(2),堿液浸泡實驗進一步證實了堿液與巖石礦物間的強烈作用(3)。堿液浸泡前,巖樣宏觀和微觀上的孔隙結(jié)構(gòu)均保存完整,表面整潔;但浸泡后,巖心薄片上的微裂縫明顯擴展,見乳白色溶蝕物質(zhì)充填;SEM圖像顯示孔隙結(jié)構(gòu)已經(jīng)受到嚴重破壞,見大量溶蝕微粒堵塞原有孔隙。實驗巖樣富含堿敏礦物(1),當高pH值的油基鉆井完井液濾液侵入裂縫后,大量的溶蝕微粒以及氫氧根與某些二價陽離子結(jié)合生成不溶物,勢必堵塞滲流通道,降低裂縫滲透率[12]。

 

314應(yīng)力敏感

頁巖儲層微裂縫發(fā)育,本身已具有一定應(yīng)力敏感性,流體損害對其還有進一步的強化作用[12-13]。油基鉆井完井液濾液損害后,裂縫壁面支撐顆粒強度降低,油相的潤滑作用也會減小微凸體間錯動變形的摩擦阻力,在有效應(yīng)力作用下,裂縫更容易被壓實,從而加劇應(yīng)力敏感。

32 儲層保護技術(shù)對策

321提高裂縫封堵能力

根據(jù)上述分析可知,提升油基鉆井完井液儲層保護能力的關(guān)鍵在于控制同相侵入和液相濾失,因而必須強化其對裂縫的封堵能力。實現(xiàn)高效封堵,首先在于準確預測儲層裂縫寬度。鉆井過程中,裂縫寬度通常處在動態(tài)變化之中,只有在室內(nèi)巖心觀測獲取裂縫參數(shù)的基礎(chǔ)上,考慮油基鉆井完井液浸泡和井筒壓力波動等因素,才能準確預測其變化范圍[23],并以此優(yōu)選剛性封堵和塑性變形材料體系。其次,注重提高封堵層的艤向承壓能力[24]。封堵層的雙向承壓能力是克服井筒壓力波動,保持其穩(wěn)定、持久封堵能力的關(guān)鍵。威遠地區(qū)威201-H3井,在充分獲取地層裂縫參數(shù)的基礎(chǔ)上,著重優(yōu)化了油基鉆井完井液的暫堵性等性能,相比于威201-H1井,有效緩解了頁巖層段的井壁失穩(wěn)問題[5]。

322控制合理的pH

頁巖儲層水平段井壁失穩(wěn)通常發(fā)生在鉆井完井液浸泡35d后。液相大量濾失侵入儲層裂縫系統(tǒng),高pH值濾液與頁巖的相互作用,導致巖石的弱結(jié)構(gòu)面強度降低是失穩(wěn)的主要原因[25]。勝利油田T2YT3WT3L井鉆井過程中井壁失穩(wěn)現(xiàn)象頻發(fā),經(jīng)分析鉆井液高pH值是誘發(fā)井壁失穩(wěn)的主要原因之一。通過降低pH值、控制濾失量等措施對鉆井液進行改進,并應(yīng)用于鄰井T3ZVT3ZS的鉆井作業(yè),穩(wěn)定井壁效果良好[26]。

323優(yōu)化井眼軌跡及鉆井液密度

威遠地區(qū)威201-H1井鉆遇龍馬溪組出現(xiàn)井壁垮塌,為了保證鉆進,油基鉆井完井液密度逐漸被提至1.85gcm3以上。然而,這不但未能解決垮塌問題,還誘發(fā)了嚴重井漏。探究原因,頁巖儲層水平段的井壁失穩(wěn)主要是應(yīng)力失穩(wěn)和層理、裂縫失穩(wěn)共同作用的結(jié)果。一旦發(fā)生應(yīng)力失穩(wěn),現(xiàn)場通常是通過增加鉆井完井液密度來應(yīng)對。但過高的正壓差常導致連通井筒裂縫的縫寬增加,當裂縫寬度超過鉆井液的封堵能力時,就會誘發(fā)層理、裂縫失穩(wěn),甚至是壓破地層,造成嚴重井漏。因此,頁巖儲層的鉆井工程設(shè)計需注重井眼軌跡優(yōu)化,改善井周應(yīng)力狀態(tài),從源頭上預防應(yīng)力失穩(wěn)。即使發(fā)生應(yīng)力失穩(wěn),也應(yīng)選擇合理的鉆井液密度進行控制,避免后續(xù)措施導致井壁應(yīng)力狀況惡化,造成又塌又漏的局面。

4 結(jié)論

1)龍馬溪組頁巖儲層敏感性損害為中等強,油基鉆井完井液濾液對裂縫巖樣的損害率為63.5%~99.9%,體系動態(tài)損害率為49.0%~87.2%,綜合損害程度均為中等——強。

2)固相侵入、油相圈閉損害、堿敏損害、流  固耦合強化的應(yīng)力敏感損害是油基鉆井完井液主要的損害方式。

3)優(yōu)化油基鉆井完井液的屏蔽暫堵性能,降低濾失量、控制合理pH值和正壓差等,并與井眼軌跡優(yōu)化設(shè)計相結(jié)合,既是強化頁巖井壁穩(wěn)定的技術(shù)對策,也是提高油基鉆井完井液保護頁巖氣層能力的重要途徑。

 

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本文作者:康毅力  楊斌  游利軍  陳強  俞楊烽

作者單位:油氣藏地質(zhì)及開發(fā)工程國家重點實驗室·西南石油大學