陳　奎，任廣磊，楊文娟，于耀南

(中國石化 華北油氣分公司 勘探開發(fā)研究院，河南 鄭州 450006)

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鄂爾多斯盆地大牛地氣田盒1段儲層應(yīng)力敏感性及對水平井開發(fā)的影響

陳奎，任廣磊，楊文娟，于耀南

(中國石化 華北油氣分公司 勘探開發(fā)研究院，河南 鄭州 450006)

大牛地氣田二疊系下石盒子組1段(盒1段)是典型的低滲透儲層。為明確儲層的應(yīng)力敏感性，選取了不同類型的13塊巖心開展了應(yīng)力敏感性實驗。結(jié)果表明，盒1段巖心普遍存在應(yīng)力敏感，且存在滲透率滯后現(xiàn)象，這說明隨凈應(yīng)力的增加，巖樣發(fā)生了塑性變形。采用內(nèi)應(yīng)力敏感指數(shù)法對實驗數(shù)據(jù)進行分析，結(jié)果表明物性較好的Ⅰ類儲層應(yīng)力敏感性較弱，Ⅱ類、Ⅲ類儲層為弱～中等，13塊巖心的平均內(nèi)應(yīng)力敏感指數(shù)為0.086，整體評價應(yīng)力敏感性為弱。建立了Ⅰ-Ⅲ類儲層的多級壓裂水平井數(shù)值模擬模型，并將儲層無因次滲透率隨地層壓力的變化關(guān)系加入到模型中。在定氣井產(chǎn)量的條件下，對比應(yīng)力敏感對水平井開發(fā)的影響。數(shù)值模擬結(jié)果表明：對于Ⅰ類儲層，應(yīng)力敏感對穩(wěn)產(chǎn)時間和穩(wěn)產(chǎn)期產(chǎn)氣量影響很小，僅降低1.6%；Ⅱ類儲層影響較小，降低了11.2%；Ⅲ類儲層則影響較大，降低了23.6%。因此應(yīng)加強Ⅱ類、Ⅲ類儲層氣井的配產(chǎn)研究和生產(chǎn)管理，保證開發(fā)效果。

應(yīng)力敏感；低滲透；水平井；氣田開發(fā)；大牛地氣田；鄂爾多斯盆地

大牛地氣田位于鄂爾多斯盆地伊陜斜坡北部東段，屬于典型的大型低滲致密砂巖氣田[1-4]。經(jīng)過多年持續(xù)的產(chǎn)能建設(shè)，剩余未動用儲量主要位于二疊系下石盒子組1段(盒1段)，儲層物性差、豐度低，直井開發(fā)無效益，適宜采用水平井開發(fā)。而一般認為低滲透儲層比中高滲儲層存在更強的應(yīng)力敏感性[5-7]。國內(nèi)外針對儲層應(yīng)力敏感的研究，主要集中在實驗測試方法上。應(yīng)力敏感應(yīng)用研究方面，主要是在滲流模型加入應(yīng)力敏感因素，評價對油氣井產(chǎn)能的影響。對于具體區(qū)塊的研究，沒有針對不同類型儲層進行細分。而對于應(yīng)力敏感對于氣藏多級壓裂水平井開發(fā)指標的研究也比較少[8-17]。有鑒于此，本文在大牛地氣田儲層分類評價研究成果的基礎(chǔ)上[18-19]，選取盒1段不同類型儲層巖心開展實驗，并將外應(yīng)力敏感曲線通過有效應(yīng)力將其轉(zhuǎn)換成內(nèi)應(yīng)力敏感曲線之后，評價儲層的應(yīng)力敏感性。再將實驗結(jié)果導(dǎo)入氣藏多級壓裂水平井數(shù)值模擬模型，利用數(shù)值模擬研究應(yīng)力敏感對開發(fā)指標的影響，為盒1段的開發(fā)提供指導(dǎo)。

1　應(yīng)力敏感實驗

1.1實驗結(jié)果

實驗參照SY/T 6385—1999設(shè)計，以氮氣為氣體介質(zhì)，根據(jù)大牛地氣田儲層分類評價標準[18-19]，選取了分別代表了盒1段Ⅰ類、Ⅱ類和Ⅲ類儲層的13塊巖心進行應(yīng)力敏感性實驗。結(jié)果表明盒1段巖心普遍存在應(yīng)力敏感。典型實驗結(jié)果如圖1和圖2所示，在凈應(yīng)力增加過程中，滲透率隨凈應(yīng)力的增加而減小，初期下降很快，當達到一定值后，下降幅度逐漸減小，趨于變緩。凈應(yīng)力降低過程中，滲透率逐漸恢復(fù)，但是并不能恢復(fù)到原來的大小，且凈應(yīng)力增加過程中的滲透率都大于對應(yīng)的凈應(yīng)力降低過程中的滲透率值，這是滲透率滯后現(xiàn)象。滲透率滯后現(xiàn)象說明隨凈應(yīng)力的增加，巖樣發(fā)生了塑性變形。

圖1　大牛地氣田盒1段Ⅰ類巖樣D28-6滲透率與 凈應(yīng)力關(guān)系Fig.1　Permeability vs. net stress of Type Ⅰ core sample(D28-6) of the 1st Member of Xiashihezi Formation in Daniudi gas field

圖2　大牛地氣田盒1段Ⅲ類巖樣D41-37滲透率與凈應(yīng)力關(guān)系Fig.2　Permeability vs. net stress of type III core sample(D41-37) of the 1st Member of Xiashihezi Formation in Daniudi gas field

1.2滲透率應(yīng)力敏感性評價與分析

目前常用的滲透率應(yīng)力敏感性評價方法主要有行業(yè)標準法、應(yīng)力敏感性系數(shù)法和應(yīng)力敏感指數(shù)法。前兩種方法實際上評價的是巖石滲透率對外應(yīng)力(圍壓)的敏感程度，而實際的油氣生產(chǎn)過程是內(nèi)壓不斷改變的過程，所以油氣藏應(yīng)力敏感性評價應(yīng)采用內(nèi)應(yīng)力敏感指數(shù)法。

應(yīng)力敏感指數(shù)的表達式為：

(1)

式中：SI為應(yīng)力敏感指數(shù)，無因次；ki為原始地層壓力下的滲透率，10-3μm2；k為某個地層壓力下的滲透率，10-3μm2。

本文在用常規(guī)應(yīng)力敏感實驗數(shù)據(jù)評價儲層巖石的內(nèi)應(yīng)力敏感性時，首先分別通過李傳亮的本體有效應(yīng)力方法計算出的有效應(yīng)力，將外應(yīng)力敏感曲線轉(zhuǎn)換成內(nèi)應(yīng)力敏感曲線，然后才進行內(nèi)應(yīng)力敏感性評價[20-22]。根據(jù)常規(guī)實驗得到滲透率與凈應(yīng)力成乘冪關(guān)系，可統(tǒng)一表示為：

(2)

用本體有效應(yīng)力法得到的有效應(yīng)力將外應(yīng)力敏感曲線轉(zhuǎn)換成內(nèi)應(yīng)力敏感曲線后，上式即可寫成：

(3)

式中：k0為初始測點對應(yīng)的滲透率，10-3μm2；σ為有效應(yīng)力，MPa；b為巖石的應(yīng)力敏感常數(shù)，MPa-1；pc為凈圍壓，MPa；pp為進口壓力值，MPa；Φ為孔隙度，%。

為了方便對比，將原始地層壓力降低10MPa時的孔隙壓力和圍壓代入轉(zhuǎn)換后的內(nèi)應(yīng)力敏感曲線關(guān)系，得到相應(yīng)條件的滲透率值，然后根據(jù)式(1)計算出內(nèi)應(yīng)力敏感指數(shù)(表1)。結(jié)果表明，物性較好的Ⅰ類儲層應(yīng)力敏感性為弱，Ⅱ類儲層應(yīng)力敏感性為中等偏弱，物性差的Ⅲ類儲層應(yīng)力敏感性大部分為中等。說明儲層越好，應(yīng)力敏感性越弱。

表1　大牛地氣田盒1段巖樣內(nèi)應(yīng)力敏感指數(shù)Table 1　Internal stress sensitivity index of core samples of the 1st Member of Xiashihezi Formation in Daniudi gas field

2　應(yīng)力敏感對水平井開發(fā)的影響

根據(jù)應(yīng)力敏感實驗結(jié)果，結(jié)合儲層實際上覆巖層壓力以及實際地層流體壓力，將實驗曲線轉(zhuǎn)換到地下儲層滲透率隨地層流體壓力的變化關(guān)系。氣藏Ⅰ類、Ⅱ類和Ⅲ類儲層無因次滲透率隨地層壓力的變化關(guān)系見表2。

根據(jù)儲層分類，建立不同類型儲層的典型數(shù)值模擬模型，將表2數(shù)據(jù)加入模型，進行應(yīng)力敏感數(shù)值模擬動態(tài)預(yù)測并對比分析，儲層參數(shù)取值見表3。水平井及分段壓裂參數(shù)為：水平段長度為1 100 m，采用局部加密網(wǎng)格模擬壓裂縫，壓裂段數(shù)11段，壓裂有效半縫長25 m，裂縫導(dǎo)流能力30×10-3μm2·m。

表2　大牛地氣田盒1段不同類型儲層無因次 滲透率隨地層壓力變化關(guān)系Table 2　Dimensionless permeability vs. formation pressure of different reservoir types of the 1st Member of Xiashihezi Formation in Daniudi gas field

表3　大牛地氣田盒1段水平井單井數(shù)值模擬儲層參數(shù)Table 3　Reservoir parameters for horizontal well numerical simulation of the 1st Member of Xiashihezi Formation in Daniudi gas field

為了便于對比，氣井生產(chǎn)制度統(tǒng)一設(shè)定為2.2×104m3/d。計算結(jié)果表明，Ⅰ類儲層不考慮應(yīng)力敏感可穩(wěn)產(chǎn)6.22年，而考慮應(yīng)力敏感后穩(wěn)產(chǎn)時間降低到6.12年，降低0.1年，降低幅度只有1.6%，應(yīng)力敏感對開發(fā)的影響很小。Ⅱ類儲層不考慮應(yīng)力敏感可穩(wěn)產(chǎn)3.39年，而考慮應(yīng)力敏感后穩(wěn)產(chǎn)時間降低到3.01年，降低0.38年，降低幅度11.2%，應(yīng)力敏感對開發(fā)的影響也比較小。Ⅲ類儲層不考慮應(yīng)力敏感可穩(wěn)產(chǎn)0.89年，而考慮應(yīng)力敏感后穩(wěn)產(chǎn)時間降低到0.68年，降低0.21年，降低幅度23.6%，應(yīng)力敏感對開發(fā)的影響較大，如表4、圖3、圖4和圖5。因此Ⅲ類儲層氣井的產(chǎn)量不宜過高，以免較大的應(yīng)力敏感影響穩(wěn)產(chǎn)期累產(chǎn)氣量。

表4　大牛地氣田盒1段應(yīng)力敏感對不同類型儲層影響Table 4　Influences of stress sensitivity on different reservoir types of the 1st Member of Xiashihezi Formation in Daniudi gas field

圖3　大牛地氣田盒1段應(yīng)力敏感對Ⅰ類儲層影響Fig.3　Influences of stress sensitivity on type Ⅰ reservoirs of the 1st Member of Xiashihezi Formation in Daniudi gas field

圖4　大牛地氣田盒1段應(yīng)力敏感對Ⅱ類儲層影響Fig.4　Influences of stress sensitivity on type Ⅱ reservoirs of the 1st Member of Xiashihezi Formation in Daniudi gas field

圖5　大牛地氣田盒1段應(yīng)力敏感對Ⅲ類儲層影響Fig.5　Influences of stress sensitivity on type Ⅲ reservoirs of the 1st Member of Xiashihezi Formation in Daniudi gas field

3　結(jié)論

1) 實驗結(jié)果表明盒1段巖心普遍存在應(yīng)力敏感。物性較好的Ⅰ類儲層，應(yīng)力敏感性為弱，Ⅱ類儲層應(yīng)力敏感性為中等偏弱，物性差的Ⅲ類儲層應(yīng)力敏感性大部分為中等。

2) 考慮了應(yīng)力敏感的水平井開發(fā)數(shù)值模擬結(jié)果表明，Ⅰ類儲層應(yīng)力敏感對穩(wěn)產(chǎn)期等指標的影響很小，Ⅱ類儲層影響較小，Ⅲ類儲層影響較大。

3) 盒1段Ⅲ類儲層氣井配產(chǎn)不宜過高，避免因應(yīng)力敏感效應(yīng)，對氣井穩(wěn)產(chǎn)期及累產(chǎn)氣量產(chǎn)生較大影響。

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(編輯張亞雄)

Stress sensitivity of the 1stMember of Xiashihezi Formation and its influences on horizontal well development of Daniudi gas field,Ordos Basin

Chen Kui,Ren Guanglei,Yang Wenjuan,Yu Yaonan

(ExplorationandDevelopmentResearchInstitute,SINOPECNorthChinaCompany,Zhengzhou,Henan450006,China)

The 1stMember of Xiashihezi Formation(H1)of Daniudi gas field is a typical low permeability reservoir in Ordos Basin.In order to clearly understand the stress sensitivity of H1,13 core samples of different types were chosen for stress sensitivity tests.The experimental results show that the core samples from H1 commonly show stress sensitivity and permeability hysteresis,indicating that the cores experience plastic deformation with the increasing of net stress.Analysis of the experimental data with internal stress sensitivity index method shows that the type Ⅰ reservoirs with better physical properties has a relatively low stress sensitivity,while the type Ⅱ and Ⅲ reservoirs have low to moderate stress sensitivity.The averaged internal stress sensitivity index of these 13 core samples is 0.086,indicating that the overall stress sensitivity is low.We built numerical simulation models of multi-stage fracturing of horizontal wells for Ⅰ-Ⅲ type reservoirs,into which the relationship between dimensionless permeability and the formation pressure was incorporated.Under a constant gas production rate scheme in the numerical model,the effects of stress sensitivity on horizontal well performance were investigated.The numerical simulation results show that the stress sensitivity has very little effect on gas rate during stable production period for type Ⅰ reservoir,which only decreased by 1.6% in comparison with that without stress sensitivity case.For type Ⅱ and Ⅲ reservoirs,the gas rate during stable production period decreased by 11.2% and 23.6%,respectively.Therefore,production allocation and management should be strengthened for type Ⅱ and Ⅲ reservoirs to guarantee well performance.

stress sensitivity,low permeability,horizontal well,gas field development,Daniudi gas field,Ordos Basin

2015-12-20;

2016-02-24。

陳奎(1980—)，男,碩士，工程師，氣藏工程。E-mail:1235199@qq.com。

國家科技重大專項(2011ZX05045)。

0253-9985(2016)02-0267-05

10.11743/ogg20160216

TE37

A