頁巖裂縫網(wǎng)絡(luò)水相自吸試驗(yàn)

游利軍, 程秋洋, 康毅力, 田 鍵, 楊 斌

(西南石油大學(xué)油氣藏地質(zhì)及開發(fā)工程國(guó)家重點(diǎn)實(shí)驗(yàn)室,四川成都 610500)

水力壓裂形成的裂縫網(wǎng)絡(luò)使頁巖氣藏經(jīng)濟(jì)開發(fā)[1-3]。壓裂過程入地液量巨大,但壓裂液的返排率普遍低于50%,有的甚至低于10%[4-6]。壓裂液滯留儲(chǔ)層會(huì)產(chǎn)生水相圈閉等一系列儲(chǔ)層損害,影響頁巖氣井壓裂改造效果[7-9]。在關(guān)井條件下頁巖自吸對(duì)壓裂液濾失和分布有重要作用[10-11]。目前頁巖自吸試驗(yàn)主要探究基塊和單一裂縫自吸行為,但仍無法合理解釋自吸與壓裂液滯留、返排率低的關(guān)系。考慮壓裂液含水率超過90%[12-13],研究頁巖裂縫網(wǎng)絡(luò)水相自吸有助于進(jìn)一步理解壓裂液濾失機(jī)制[14-19]。筆者開展頁巖基塊、單一裂縫、“T”型裂縫網(wǎng)絡(luò)水相自吸試驗(yàn),并對(duì)比致密砂巖水相自吸,揭示頁巖裂縫網(wǎng)絡(luò)水相自吸與壓裂液濾失的聯(lián)系。

1 試驗(yàn)樣品與試驗(yàn)方法

1.1 試驗(yàn)樣品

試驗(yàn)頁巖巖樣取自重慶市彭水縣龍馬溪組頁巖露頭,其石英平均含量45.7%,長(zhǎng)石平均含量8.7%,黏土礦物平均含量28.5%。黏土礦物以伊利石和伊/蒙間層礦物為主,伊利石平均含量46.0%,伊/蒙間層礦物平均含量42.7%,高嶺石平均含量11.3%。頁巖巖樣物性參數(shù)見表1。開展致密砂巖水相自吸對(duì)比試驗(yàn),巖樣物性參數(shù)見表2。試驗(yàn)流體采用3%KCl溶液。

表1 試驗(yàn)選用頁巖巖樣的物性參數(shù)

表2 試驗(yàn)選用致密砂巖物性參數(shù)Table 2 Physical property parameters of tightsandstone sample

1.2 試驗(yàn)方法

自吸試驗(yàn)如圖1所示,Ⅰ、Ⅱ表示頁巖基塊、單一裂縫水相自吸,作為對(duì)照組;Ⅲ、Ⅳ為不同組合方式的頁巖裂縫-基塊水相自吸,模擬頁巖氣藏水力壓裂形成的“T”型裂縫網(wǎng)絡(luò)[20]。對(duì)于試驗(yàn)巖樣不同組合定義表述為:裂縫-基塊組合代表裂縫巖樣在下,基塊巖樣在上;基塊-裂縫組合代表基塊巖樣在下,裂縫巖樣在上。

為增強(qiáng)試驗(yàn)可對(duì)比性,在同一巖塊上相近位置沿水平頁理鉆取巖心柱,然后將巖樣長(zhǎng)度處理約為4 cm;為減小孔滲差異,同一裂縫-基塊組合的上、下兩塊巖樣均由同一長(zhǎng)巖心柱截取;參照行業(yè)標(biāo)準(zhǔn)SY/T53589(2010),烘干過程中每間隔1 h對(duì)巖樣稱重,直至最后兩次稱重差值小于10 mg,以此判定巖樣初始含水飽和度近乎一致。利用自制的巖心造縫機(jī)統(tǒng)一沿過軸線截面造縫,保證縫面大小接近,考慮頁巖裂縫面黏土礦物粒度小,各人造巖樣縫面粗糙度差異近似忽略,利用同一型號(hào)的橡膠帶固定裂縫巖樣,以保證縫寬不變(圖2);注入適量3%KCl溶液至自吸試驗(yàn)裝置中,保持巖樣底端被液體浸沒5 mm;觀察自吸現(xiàn)象并拍照,利用電子天平(精度為0.1 mg)實(shí)時(shí)動(dòng)態(tài)對(duì)自吸巖樣稱重。頁巖縱向?qū)Ρ仍囼?yàn)自吸時(shí)間為24 h;頁巖與致密砂巖對(duì)比分析試驗(yàn)自吸時(shí)間為168 h。

圖1 頁巖裂縫網(wǎng)絡(luò)水相自吸示意圖Fig.1 Schematic of water phase spontaneous imbibition of shale fracture network

圖2 頁巖和致密砂巖巖樣人工造縫縫面Fig.2 Artificial joint surface of shale and tight sandstone samples

2 試驗(yàn)結(jié)果分析

2.1 頁巖裂縫網(wǎng)絡(luò)水相自吸

圖3 頁巖自吸量與時(shí)間的關(guān)系Fig.3 Relationship between shale imbibition amount and time

從頁巖單塊及組合巖樣自吸試驗(yàn)結(jié)果(圖3)看,在自吸初始3 h內(nèi),Ⅰ號(hào)基塊平均自吸速率0.064 g/h,累積自吸量0.192 g;Ⅱ號(hào)裂縫平均自吸速率0.091 g/h,累積自吸量0.273 g。裂縫平均自吸速率相比基塊提高42.19%,累積自吸量增加0.081 g。表明裂縫會(huì)誘使水相沿裂縫面快速自吸,提高水相侵入深度,增加濾失量[21]。Ⅲ號(hào)基塊-裂縫組合的平均自吸速率0.049 g/h,累積自吸量1.178 g,而Ⅳ號(hào)裂縫-基塊組合平均自吸速率0.204 g/h,累積自吸量1.683 g;結(jié)合試驗(yàn)現(xiàn)象觀察發(fā)現(xiàn),Ⅲ號(hào)組合約在13 h時(shí)基塊-裂縫接觸端見水,水潤(rùn)濕接觸面積約為1/10,由于下端基塊為上端裂縫自吸供液量小,整體自吸速率提高不明顯;Ⅳ號(hào)組合約在0.7 h時(shí)裂縫-基塊接觸端見水;0.5～0.7 h平均自吸速率0.145 g/h,水迅速潤(rùn)濕整個(gè)接觸面,0.7～1.2 h平均自吸速率提高到0.172 g/h。對(duì)比分析認(rèn)為,Ⅳ號(hào)下端裂縫為水相提供快速自吸通道,使裂縫-基塊接觸端見水早,促使上端基塊自吸相對(duì)提前,加速了水相向基塊滲吸擴(kuò)散,也加快了整體自吸進(jìn)程;在毛管力作用下,水可能進(jìn)入基塊深部。

圖4 頁巖自吸量與時(shí)間開平方根的關(guān)系Fig.4 Relationship between shale imbibition amount and square root of time

2.2 頁巖與致密砂巖裂縫網(wǎng)絡(luò)水相自吸對(duì)比

頁巖和致密砂巖自吸量與時(shí)間關(guān)系曲線如圖5所示。致密砂巖整體自吸效率[23]均高于頁巖:致密砂巖Ⅳ號(hào)組合自吸量為4.287 g,頁巖Ⅳ號(hào)組合自吸量為1.454 g,前者自吸量是后者的3倍;致密砂巖Ⅲ號(hào)組合自吸量為5.691 g,頁巖Ⅲ號(hào)組合自吸量為1.825 g,兩者自吸量之比也接近3。因致密砂巖巖樣孔隙度平均為15.42%,頁巖巖樣平均孔隙度為5.75%,前者與后者數(shù)值比為2.7。分析認(rèn)為,致密砂巖更好的孔滲條件不僅提供水更大的賦存空間也促進(jìn)水快速擴(kuò)散分布,有效降低了自吸前緣含水飽和度,為自吸提供了動(dòng)力,表現(xiàn)出更高的自吸效率。

圖5 頁巖和致密砂巖自吸量與時(shí)間的關(guān)系Fig.5 Relationship between imbibition amount and time of shale and sandstone

自吸進(jìn)行到第10 min時(shí),頁巖Ⅳ號(hào)與致密砂巖Ⅲ號(hào)組合自吸量曲線出現(xiàn)交點(diǎn),累積自吸量為0.160 g。相交前,頁巖Ⅳ號(hào)組合自吸量一直高于Ⅲ號(hào)致密砂巖組合,證實(shí)裂縫增加了頁巖有效滲透率,提高了自吸速率;相交后,同時(shí)間節(jié)點(diǎn)Ⅲ號(hào)致密砂巖組合自吸效率均高于Ⅳ號(hào)頁巖組合。分析認(rèn)為,頁巖孔隙結(jié)構(gòu)復(fù)雜[24]、非均質(zhì)性強(qiáng)、潤(rùn)濕不均勻[9,25],阻礙了裂縫-基塊間水相擴(kuò)散滲流,削弱了裂縫水相自吸產(chǎn)生的積極效應(yīng),降低了整體自吸效率;而致密砂巖非均質(zhì)性相對(duì)較弱,潤(rùn)濕比較均勻,水相自吸液面高度呈現(xiàn)活塞式推進(jìn),良好的孔滲條件有利于水相擴(kuò)散滲流。此外,頁巖孔隙中吸附氣含量高[26],水相逆流自吸進(jìn)入孔隙置換吸附氣發(fā)生置換,氣體排出過程可能也會(huì)阻礙水相自吸。

3 討 論

3.1 頁巖裂縫網(wǎng)絡(luò)水相自吸特征

裂縫與基塊兩種不同尺度下的自吸存在相互促進(jìn)機(jī)制:裂縫增大水相與基塊接觸面，為水相進(jìn)入基塊提供快速供液通道;自吸降低裂縫內(nèi)含水飽和度，反饋促進(jìn)裂縫持續(xù)快速的自吸。如圖6所示,頁巖Ⅳ號(hào)組合下端裂縫巖樣自吸量始終高于Ⅱ號(hào)裂縫巖樣,同時(shí)頁巖Ⅲ號(hào)組合下端基塊自吸量也一直高于Ⅰ號(hào)基塊巖樣;另一方面,Ⅳ號(hào)組合上端基塊最終自吸量0.656 g,Ⅲ號(hào)組合上端裂縫僅0.397 g,前者高于后者0.259 g(圖7)。水相未穿透下端巖樣時(shí)會(huì)優(yōu)先沿著頁巖頁理自吸[26],整體自吸高度呈現(xiàn)不均勻推進(jìn);水相自吸穿透下端巖樣后，上端巖樣為水相的進(jìn)一步自吸提供了動(dòng)力和通道,同時(shí)強(qiáng)化下端巖樣最終的水相自吸深度。

圖6 各組合下端巖樣和單巖樣自吸量與時(shí)間的關(guān)系Fig.6 Relationship between imbibition amount and time of bottom of combination and single samples

圖7 各組合上端巖樣自吸量與時(shí)間的關(guān)系Fig.7 Relationship between imbibition amount and time of top sample from different combination

在原地條件下,水相自吸存在兩個(gè)動(dòng)力:飽和度差異影響的自吸毛管力和孔滲差異引起的自吸液重新分布擴(kuò)散動(dòng)力。水力壓裂形成的頁巖裂縫網(wǎng)絡(luò)為壓裂液滯留提供了賦存空間,且裂縫網(wǎng)絡(luò)越發(fā)育,由于次級(jí)裂縫與初級(jí)壓裂縫含水飽和度的差異,自吸勢(shì)提高越明顯,最終吸入量越大[27-28];同時(shí)縫網(wǎng)面積越大,壓裂液通過裂縫向基塊滲流擴(kuò)散范圍越廣,裂縫-基塊間跨尺度的導(dǎo)流能力差異和頁巖儲(chǔ)層超低含水飽和度特征[29]有利于壓裂液重新分布以降低自吸前緣含水飽和度,提升自吸動(dòng)力。頁巖儲(chǔ)層裂縫-基塊跨尺度水相自吸行為,彼此自吸相互促進(jìn),提高頁巖整體水相自吸量。

3.2 水-巖作用與頁理縫萌生關(guān)系

頁巖儲(chǔ)層頁理結(jié)構(gòu)發(fā)育,以伊利石和伊/蒙間層為主的黏土礦物平行頁理沉積分布[30],頁理膠結(jié)程度低、滲透性好,屬于高滲透帶;同時(shí)垂直頁理方向分布親油有機(jī)質(zhì)會(huì)抑制水相滲吸[31],故水優(yōu)先沿平行頁理自吸。水迅速潤(rùn)濕頁理面后黏土礦物表面易發(fā)生水化作用,水分子滲透進(jìn)入伊/蒙間層晶層,引起晶層間距顯著膨脹擴(kuò)大,壓縮孔隙體積,但因頁巖自身孔隙度低,當(dāng)壓縮孔隙無法完全消耗水化膨脹[32]的能量時(shí),能量過剩將引發(fā)巖石局部爆裂,產(chǎn)生新裂縫。

其次,試驗(yàn)過程發(fā)現(xiàn),自吸萌生了宏觀新裂縫,白色可溶鹽晶體沿新裂縫析出;致密砂巖雖未出現(xiàn)裂縫,但因孔滲相對(duì)較好,白色可溶鹽晶體從巖樣孔隙中析出(圖8)。試驗(yàn)采用的3%KCl溶液常用于常規(guī)儲(chǔ)層巖樣自吸評(píng)價(jià)試驗(yàn),但頁巖自身可溶鹽含量高,自吸流體與巖石不配伍導(dǎo)致水-巖作用明顯。在孔隙度約為10%的巖石中滲透水化力高達(dá)30 MPa,且水化應(yīng)力有隨孔隙度降低而增加的趨勢(shì)[33],鑒于頁巖低孔低滲特性,且有機(jī)質(zhì)孔隙度更低,常規(guī)手段難以精確測(cè)量。水相自吸進(jìn)入頁巖基塊甚至有機(jī)質(zhì)孔隙,可溶鹽溶解后構(gòu)成礦化度差異,產(chǎn)生較高滲透水化力,一旦作用在頁理弱結(jié)構(gòu)面和裂縫尖端,勢(shì)必促使微米級(jí)裂縫擴(kuò)展導(dǎo)致宏觀頁理縫出現(xiàn)。

考慮頁巖工程地質(zhì)的特殊性,宏觀尺度上定義水-巖作用促使頁理縫萌生;而在微米尺度上應(yīng)客觀定義為微米縫的擴(kuò)展延伸。其作用機(jī)制為:水進(jìn)入黏土礦物晶層,水-巖作用導(dǎo)致黏土微結(jié)構(gòu)破壞和顆粒間黏結(jié)力減小,即巖石膠結(jié)強(qiáng)度降低。宏觀上表現(xiàn)為巖石內(nèi)聚力和內(nèi)摩擦角降低,導(dǎo)致巖石強(qiáng)度或巖石I型斷裂韌性下降,阻止裂紋失穩(wěn)擴(kuò)展的能力被削弱[34]。當(dāng)水化作用產(chǎn)生水化應(yīng)力、毛細(xì)管力、孔隙壓力共同作用微米縫尖端時(shí),引起裂紋尖端處應(yīng)力集中,使應(yīng)力強(qiáng)度因子增加;當(dāng)應(yīng)力強(qiáng)度因子大于斷裂韌性時(shí)裂紋擴(kuò)展或延伸。多條微裂紋匯合貫通后形成宏觀裂紋,宏觀裂紋進(jìn)一步發(fā)展形成裂縫[35]。分析認(rèn)為水相侵入觸發(fā)水-巖作用促進(jìn)了宏觀頁理縫的萌生。

圖8 頁巖與致密砂巖自吸過程實(shí)物圖Fig.8 Physical map of spontaneous imbibition process of shale and tight sandstone

3.3 壓裂液滯留對(duì)頁巖氣開發(fā)的積極作用

水力壓裂形成了由初級(jí)壓裂縫、天然裂縫和水-巖作用產(chǎn)生的次級(jí)衍生縫組成的復(fù)雜裂縫網(wǎng)絡(luò)[36]。壓后燜井期間,滑溜水和支撐劑填充初級(jí)裂縫,僅有滑溜水滲吸進(jìn)入次級(jí)裂縫。高返排壓力下,導(dǎo)流能力較強(qiáng)的初級(jí)裂縫中的壓裂液返排率高;次級(jí)裂縫因自身導(dǎo)流能力差,受到的返排壓力又比較低,返排困難導(dǎo)致大量壓裂液滯留。目前對(duì)壓裂液大量滯留誘發(fā)的儲(chǔ)層損害和產(chǎn)能削弱問題,礦場(chǎng)主要采取經(jīng)驗(yàn)燜井和快速返排措施以降低損害程度,但若燜井時(shí)間設(shè)計(jì)不合理會(huì)削弱壓裂液與頁巖作用對(duì)儲(chǔ)層改造的積極效應(yīng)。

基于頁巖水相自吸試驗(yàn)結(jié)果分析認(rèn)為,頁巖裂縫網(wǎng)絡(luò)水相自吸擴(kuò)大了頁巖儲(chǔ)層中壓裂液的分布范圍,拓寬了壓裂液與頁巖接觸面積,有利于增強(qiáng)水-巖作用萌生新裂縫(圖8),進(jìn)一步優(yōu)化改造儲(chǔ)層。在地層條件下,水-巖作用協(xié)同時(shí)間效應(yīng)導(dǎo)致頁巖膠結(jié)強(qiáng)度降低,裂縫尖端流體壓力與地應(yīng)力和巖石強(qiáng)度間的平衡被打破,燜井所維持的流體超壓條件還能促進(jìn)裂縫擴(kuò)展延伸和微裂縫萌生,使簡(jiǎn)單的裂縫網(wǎng)絡(luò)(圖9(a))演變成次級(jí)裂縫發(fā)育的復(fù)雜裂縫網(wǎng)絡(luò)(圖9(b))。在毛管力起主導(dǎo)作用下,次級(jí)裂縫中的水通過逆流自吸進(jìn)入基塊,促進(jìn)氣體從基塊中解吸-擴(kuò)散-滲流進(jìn)入裂縫,有助于提高頁巖儲(chǔ)層早期產(chǎn)氣量[37]。與之相對(duì)的,壓裂液返排率越高,壓裂液滯留量越少,水-巖作用對(duì)裂縫網(wǎng)絡(luò)的積極改造程度就越低,且水相逆流自吸對(duì)基塊內(nèi)氣體解吸-擴(kuò)散-滲流進(jìn)入裂縫的作用也越弱,一定程度上削弱了燜井對(duì)早期產(chǎn)氣量的積極作用。

頁巖氣井水力壓裂過程的入地液量巨大,壓后返排率低,大量壓裂液滯留儲(chǔ)層。壓后燜井的出發(fā)點(diǎn)就是在降低壓裂液返排量和污水處理成本的同時(shí),利用水-巖作用對(duì)儲(chǔ)層的積極改造的優(yōu)勢(shì),通過優(yōu)選入井壓裂液,將可能產(chǎn)生儲(chǔ)層損害的滯留壓裂液轉(zhuǎn)化為進(jìn)一步改造儲(chǔ)層的動(dòng)力[5,38]:一方面,憑借水-巖作用消耗部分滯留壓裂液,緩解水相圈閉、結(jié)垢堵塞等損害;另一方面,利用水-巖作用產(chǎn)生的微裂縫提高裂縫網(wǎng)絡(luò)密度,改善氣體滲流通道,同時(shí)促進(jìn)水相逆流自吸使基塊內(nèi)氣體的解吸-擴(kuò)散-滲流進(jìn)入裂縫,提高氣體產(chǎn)量,達(dá)到變害為利的目的。

圖9 頁巖儲(chǔ)層裂縫網(wǎng)絡(luò)示意圖Fig.9 Schematic of simple and complex fracture network of shale reservoir

4 結(jié) 論

(1)頁巖裂縫網(wǎng)絡(luò)快速自吸為基塊自吸充足供液,基塊自流擴(kuò)散為裂縫進(jìn)一步自吸提供動(dòng)力,裂縫與基塊自吸相互促進(jìn)利于壓裂液重新分布,增加了壓裂液濾失量。

(2)頁巖氣藏天然裂縫和頁理發(fā)育,水相易沿頁理自吸誘發(fā)頁理縫,提高了縫網(wǎng)密度,改善了滲吸路徑,擴(kuò)大了水-巖作用范圍。

(3)優(yōu)選壓裂液配方,利用水-巖作用消耗儲(chǔ)層滯留壓裂液,變害為利降低壓裂液滯留對(duì)儲(chǔ)層的損害;發(fā)揮水-巖反應(yīng)對(duì)儲(chǔ)層裂縫網(wǎng)絡(luò)的積極改造作用,改善氣體滲流通道,強(qiáng)化頁巖氣解吸-擴(kuò)散-滲流過程,提高產(chǎn)氣量。

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