劉會(huì)虎,桑樹勛,馮清凌,胡寶林,胡友彪,徐宏杰,程 喬

(1.安徽理工大學(xué)地球與環(huán)境學(xué)院,安徽 淮南 232001;2.中國(guó)礦業(yè)大學(xué) 資源與地球科學(xué)學(xué)院,江蘇徐州 221008;3.淄博金宅測(cè)繪公司,山東淄博 255000)

沁水盆地南部煤層氣井排采儲(chǔ)層應(yīng)力敏感研究

劉會(huì)虎1,桑樹勛2,馮清凌3,胡寶林1,胡友彪1,徐宏杰1,程 喬1

(1.安徽理工大學(xué)地球與環(huán)境學(xué)院,安徽 淮南 232001;2.中國(guó)礦業(yè)大學(xué) 資源與地球科學(xué)學(xué)院,江蘇徐州 221008;3.淄博金宅測(cè)繪公司,山東淄博 255000)

為分析煤層氣排采不同階段煤儲(chǔ)層應(yīng)力敏感性及滲透率變化的影響因素,采集沁水盆地南部煤樣,開展了不同實(shí)驗(yàn)條件的應(yīng)力敏感實(shí)驗(yàn)。結(jié)果表明:有效應(yīng)力從0增加到10 MPa時(shí),煤樣滲透率減少了50%～70%;有效應(yīng)力從10 MPa增加到20 MPa時(shí),損失量?jī)H約占初始滲透率的10%;有效應(yīng)力低于2.5 MPa時(shí),應(yīng)力敏感性較強(qiáng);有效應(yīng)力增加到3.5 MPa的過程中,滲透率損害系數(shù)急劇上升,滲透率損耗為20%～30%;有效應(yīng)力從2.5 MPa增加到9 MPa時(shí),應(yīng)力敏感性最強(qiáng),有效應(yīng)力從3.5 MPa上升至9 MPa時(shí),滲透率損害系數(shù)快速下降,滲透率損耗約60%;有效應(yīng)力自9 MPa之后,滲透率損害系數(shù)緩慢下降,滲透率損耗約10%;滲透率損害率介于30%～65%,臨界應(yīng)力為7～11 MPa。有效應(yīng)力較低且不變時(shí),煤樣滲透率隨孔隙壓力增加而增加。圍壓不變時(shí),隨有效應(yīng)力下降和孔隙壓力增加,煤樣滲透率下降,這與有效應(yīng)力和孔隙壓力變化引起的煤儲(chǔ)層滲透率變化量有關(guān)。

沁水盆地南部;煤層氣井;排采;煤儲(chǔ)層滲透率;應(yīng)力敏感;有效應(yīng)力;孔隙壓力

煤層氣井開發(fā)過程中由于排采過快易引起煤儲(chǔ)層應(yīng)力敏感(壓敏),造成滲透率降低,已獲得普遍共識(shí)[1-6]。國(guó)內(nèi)外學(xué)者通過不同條件的應(yīng)力敏感實(shí)驗(yàn)和結(jié)合模型分析,開展了煤儲(chǔ)層應(yīng)力敏感研究工作,揭示了不同應(yīng)力條件煤儲(chǔ)層滲透率的變化特征[7-13],指出影響煤儲(chǔ)層應(yīng)力變化過程中滲透率的變化經(jīng)過應(yīng)力主導(dǎo)的滲透率降低過程、煤基質(zhì)收縮效應(yīng)主導(dǎo)的滲透率變化過程及氣體分子滑脫效應(yīng)主導(dǎo)的滲透率回返過程[14-15],認(rèn)為影響因素包括煤儲(chǔ)層孔隙壓力、凈應(yīng)力、煤巖含水飽和度、煤基質(zhì)收縮效應(yīng)、氣體分子滑脫效應(yīng)等[16-20]。然而,煤層氣排采過程中先后伴隨有凈應(yīng)力變化、氣水流體壓力變化、煤基質(zhì)收縮等對(duì)煤儲(chǔ)層滲透率的影響。因而研究煤儲(chǔ)層應(yīng)力敏感性時(shí)需要綜合考慮,結(jié)合煤層氣排采過程和特點(diǎn)來開展工作。

為分析煤層氣排采不同階段(排水降壓階段、產(chǎn)氣期、停排重啟階段)煤儲(chǔ)層應(yīng)力敏感性,筆者以沁水盆地南部為例,開展孔隙壓力不變條件下不同圍壓條件、孔隙壓力變化下圍壓變化條件、孔隙壓力條件變化相同圍壓下的應(yīng)力敏感實(shí)驗(yàn),探討煤層氣排采儲(chǔ)層應(yīng)力敏感對(duì)滲透率的影響及其發(fā)生條件。

1 實(shí)驗(yàn)材料與方法

實(shí)驗(yàn)研究區(qū)目標(biāo)地層為石炭二疊系山西組3號(hào)煤層。采集煤樣40塊,其中32塊煤樣(鄭莊區(qū)塊)采自煤層氣鉆孔取芯,8塊來采自于沁水長(zhǎng)平煤礦和天安潤(rùn)宏煤礦。所有樣品均為干樣,具有完整結(jié)構(gòu)。所有樣品在進(jìn)行實(shí)驗(yàn)前均進(jìn)行原始滲透率測(cè)試。表1為實(shí)驗(yàn)樣品的基礎(chǔ)數(shù)據(jù)。

第1部分為孔隙壓力不變條件下圍壓變化下的應(yīng)力敏感實(shí)驗(yàn),過程參照石油天然氣行業(yè)標(biāo)準(zhǔn)(SY 5358—2002)進(jìn)行。實(shí)驗(yàn)采用加煤樣的凈圍壓模擬地層有效應(yīng)力的變化(將有效應(yīng)力定義為凈圍壓,其大小等于圍壓與孔隙壓力的差),測(cè)量滲透率隨凈圍壓變化的情況,分別計(jì)算滲透率損害系數(shù)、滲透率損害率,并確定滲透率損害率發(fā)現(xiàn)明顯變化的拐點(diǎn),揭示煤層氣在排水階段快速排采時(shí)儲(chǔ)層壓敏發(fā)生的應(yīng)力變化條件。實(shí)驗(yàn)儀器為巖心流動(dòng)裝置。

表1 煤樣實(shí)驗(yàn)樣品基礎(chǔ)數(shù)據(jù)Table 1 Basic data of coal samples

第2部分為圍壓變化條件下孔隙壓力變化下的應(yīng)力敏感實(shí)驗(yàn),采用逐級(jí)增壓水驅(qū)實(shí)驗(yàn),實(shí)驗(yàn)過程:保持有效應(yīng)力不變(5 MPa),逐漸增加圍壓至8.5 MPa,分別測(cè)試不同壓力平衡狀態(tài)煤儲(chǔ)層的滲透率,揭示有效應(yīng)力趨于穩(wěn)定階段滲透率變化的特征及原因。實(shí)驗(yàn)儀器為巖心流動(dòng)裝置。

第3部分為相同圍壓條件下孔隙壓力變化下的應(yīng)力敏感實(shí)驗(yàn),采用逐級(jí)增壓氣驅(qū)實(shí)驗(yàn),實(shí)驗(yàn)過程:將巖心裝入巖心夾持器中,加圍壓至10 MPa(模擬上覆地層壓力),連接氣源,采用調(diào)壓閥設(shè)置氣驅(qū)壓力從低壓開始逐級(jí)增壓進(jìn)行氣驅(qū),每個(gè)壓力點(diǎn)氣驅(qū)時(shí)間1.0 h以上直至氣流量穩(wěn)定,保證巖樣變形達(dá)到一定的平衡狀態(tài),分別測(cè)試每個(gè)不同孔隙壓力下煤樣的滲透率,耦合分析滲透率隨孔隙壓力、有效應(yīng)力的變化特點(diǎn),揭示煤基質(zhì)體積應(yīng)變和氣體滑脫效應(yīng)對(duì)滲透率產(chǎn)生的影響。實(shí)驗(yàn)儀器為巖心驅(qū)替系統(tǒng)。

2 實(shí)驗(yàn)結(jié)果與分析

2.1 孔隙壓力不變情況下煤儲(chǔ)層滲透率與應(yīng)力的關(guān)系

依據(jù)1～32號(hào)煤樣的應(yīng)力敏感實(shí)驗(yàn)結(jié)果對(duì)煤層氣排采時(shí)煤儲(chǔ)層滲透率變化與有效應(yīng)力之間的關(guān)系進(jìn)行分析,如圖1所示。

圖1 煤樣滲透率隨有效應(yīng)力的變化Fig.1 Variation of permeability with effective stress for coal samples

從煤樣滲透率隨有效應(yīng)力的變化結(jié)果看,煤樣滲透率的變化表現(xiàn)為隨有效應(yīng)力的增加而逐漸降低,但滲透率隨有效應(yīng)力增加過程而呈現(xiàn)的變化特點(diǎn)與以往研究結(jié)果并不完全一致,具有一定的差異,表現(xiàn)為:①當(dāng)有效應(yīng)力從0增加到10 MPa時(shí),煤樣滲透率為初始滲透率的30%～50%,減少了50%～70%;②有效應(yīng)力從10 MPa增加到20 MPa的過程中,煤樣滲透率降低趨勢(shì)明顯變緩,損失量?jī)H約占初始滲透率的10%;③在有效應(yīng)力增加到2.5 MPa以前,煤樣的滲透率變化表現(xiàn)為較快速下降,應(yīng)力敏感性較強(qiáng);④在有效應(yīng)力從2.5 MPa增加到9 MPa時(shí),煤樣的滲透率變化表現(xiàn)為快速下降,應(yīng)力敏感性最強(qiáng)。

根據(jù)石油天然氣標(biāo)準(zhǔn)(SY 5358—2002)滲透率損害系數(shù)計(jì)算公式,計(jì)算了滲透率損害系數(shù)隨有效應(yīng)力的變化結(jié)果,繪制了32個(gè)煤樣的應(yīng)力敏感曲線,對(duì)煤樣的應(yīng)力敏感進(jìn)行了評(píng)價(jià),如圖2所示。

圖2 煤樣滲透率損害系數(shù)隨有效應(yīng)力的變化Fig.2 Variation of damage coefficient of permeability with effective stress for coal samples

由圖2可知,在有效應(yīng)力增加到3.5 MPa過程中,煤樣的滲透率損害系數(shù)表現(xiàn)為急劇上升的特點(diǎn),表明煤樣在有效應(yīng)力增加到3.5 MPa的過程中煤樣滲透率損害較大(滲透率損耗為20% ～30%),從3.5～20.0 MPa過程中,滲透率損害系數(shù)呈現(xiàn)下降的特點(diǎn)。其中,有效應(yīng)力由3.5 MPa增加至9 MPa的過程中,雖然滲透率損害系數(shù)表現(xiàn)出快速下降的特點(diǎn),但該過程中煤樣滲透率損害最大(滲透率損耗約為60%),自9 MPa之后,滲透率損害系數(shù)表現(xiàn)為緩慢下降的特點(diǎn),表明煤樣滲透率損害較小(滲透率損耗約為10%)。

通過對(duì)圖2分析,發(fā)現(xiàn)研究的32煤樣在有效應(yīng)力為7,9,11 MPa時(shí),滲透率損害系數(shù)出現(xiàn)明顯的拐點(diǎn),即確定臨界應(yīng)力為7～11 MPa。同時(shí)根據(jù)32個(gè)煤樣在臨界應(yīng)力點(diǎn)對(duì)應(yīng)的滲透率值和第1個(gè)應(yīng)力點(diǎn)對(duì)應(yīng)的煤樣滲透率值,計(jì)算有效應(yīng)力增加過程中滲透率的損害率,結(jié)果如圖3所示。

圖3 煤樣滲透率損害率隨有效應(yīng)力的變化Fig.3 Variation of damage rate of permeability with effective stress for coal samples

由32個(gè)煤樣滲透率損害率隨有效應(yīng)力的變化,可以發(fā)現(xiàn):滲透率損害率介于30%～65%,絕大多數(shù)介于40%～60%,滲透率損害程度為中等偏弱到中等偏強(qiáng)。結(jié)合臨界應(yīng)力,對(duì)滲透率損害率值和對(duì)應(yīng)的臨界應(yīng)力進(jìn)行了頻數(shù)統(tǒng)計(jì),結(jié)果顯示:23個(gè)煤樣應(yīng)力敏感的臨界應(yīng)力為11 MPa,5個(gè)煤樣應(yīng)力敏感的臨界應(yīng)力為9 MPa,7個(gè)煤樣應(yīng)力敏感的臨界應(yīng)力為7 MPa。需要指出的是,其中18號(hào)煤樣、21號(hào)煤樣及25號(hào)煤樣應(yīng)力敏感的臨界應(yīng)力有2個(gè),分別為7和9 MPa,即煤樣滲透率損害系數(shù)出現(xiàn)2個(gè)明顯的拐點(diǎn)。

結(jié)合圖1～3,煤樣滲透率在有效應(yīng)力為2.5 MPa之前下降較慢,滲透率損害系數(shù)在有效應(yīng)力為3.5 MPa之前上升急劇,且此時(shí)滲透率損耗較大;在有效應(yīng)力自2.5 MPa或3.5 MPa之后的增加過程中,煤樣滲透率下降迅速,滲透率損害系數(shù)雖然下降但滲透率損耗最高,至有效應(yīng)力到達(dá)臨界值范圍(7～11 MPa),滲透率基本降到最低值,損害系數(shù)也基本到達(dá)了拐點(diǎn)。

綜上,在煤層氣排水降壓過程中僅考慮流體壓力下降和有效應(yīng)力增加的前提下,煤儲(chǔ)層發(fā)生明顯的應(yīng)力敏感效應(yīng),尤其是當(dāng)快速排采時(shí)會(huì)加快煤儲(chǔ)層的壓縮,產(chǎn)生應(yīng)力體積應(yīng)變,造成滲透率的快速損耗。另外,在產(chǎn)氣期提高排水量,使有效應(yīng)力增加也會(huì)使煤儲(chǔ)層發(fā)生壓縮,使煤儲(chǔ)層滲透率發(fā)生二次損耗。因而排采時(shí)應(yīng)適當(dāng)有效延長(zhǎng)排水降壓期的排采降壓時(shí)間,避免大幅提高排水速率,以防止煤儲(chǔ)層滲透率的大幅損耗。

2.2 有效應(yīng)力不變情況下煤儲(chǔ)層滲透率與流體壓力的關(guān)系

排采過程中儲(chǔ)層壓力臨近解吸壓力階段,如果快速排采仍然產(chǎn)生應(yīng)力敏感。為更好地討論產(chǎn)氣期間煤儲(chǔ)層滲透與流體壓力之間的關(guān)系,消除有效應(yīng)力的影響,本次研究設(shè)置有效應(yīng)力不變(5 MPa),通過改變孔隙壓力進(jìn)行煤樣的敏感性實(shí)驗(yàn),分析儲(chǔ)層流體壓力在低值范圍內(nèi)發(fā)生改變時(shí)煤儲(chǔ)滲透率與儲(chǔ)層流體壓力之間的關(guān)系,實(shí)驗(yàn)結(jié)果如圖4所示。

圖4 煤樣滲透率隨孔隙壓力的變化Fig.4 Variation of permeability with pore pressure for coal samples

由圖4可知,在有效應(yīng)力為較低定值時(shí),隨著孔隙壓力的增加,滲透率表現(xiàn)出上升的特點(diǎn),對(duì)比表1中33～37號(hào)煤樣的原始滲透率,原始滲透率越高,滲透率增幅越大。實(shí)驗(yàn)結(jié)果表明:在有效應(yīng)力較低時(shí),隨孔隙壓力增大(最高3.6 MPa),煤儲(chǔ)層滲透率不斷得到改善,且煤儲(chǔ)層原始滲透率越高,這種改善越明顯。分析認(rèn)為煤儲(chǔ)層滲透率得到改善與煤基質(zhì)收縮效應(yīng)和氣體分子滑脫效應(yīng)有關(guān),尤其是煤基質(zhì)的收縮效應(yīng)引起滲透率的改善更加顯著,因?yàn)橹挥挟?dāng)儲(chǔ)層壓力低于1 MPa(文獻(xiàn)值為0.7和0.8 MPa[14-15,20])時(shí)氣體分子滑脫效應(yīng)引起滲透率的增量才比較顯著。

根據(jù)上文分析,煤層氣排采過程中,當(dāng)動(dòng)液面臨近煤儲(chǔ)層,儲(chǔ)層壓力接近臨界解吸壓力時(shí),應(yīng)合理控制排水量,降低動(dòng)液面下降速度,保證一定的孔隙流體壓力,發(fā)揮煤儲(chǔ)層基質(zhì)收縮效應(yīng)對(duì)滲透率的改善作用。

2.3 圍壓不變情況下煤儲(chǔ)層滲透率與流體壓力、有效應(yīng)力之間的耦合變化關(guān)系

為分析煤儲(chǔ)滲透率與儲(chǔ)層流體壓力、有效應(yīng)力之間的關(guān)系,在設(shè)置圍壓不變前提下增加孔隙壓力和降低有效應(yīng)力的條件下進(jìn)行了煤樣的應(yīng)力敏感性實(shí)驗(yàn),實(shí)驗(yàn)結(jié)果如圖5所示。

由實(shí)驗(yàn)條件可知,增加氣驅(qū)之前,圍壓增加到10 MPa的過程中,煤樣的有效應(yīng)力增加而滲透率下降,滲透率與原始滲透率相比,下降了1～2個(gè)數(shù)量級(jí),因而本次研究工作的第3部分為圍壓不變情況下回壓(有效應(yīng)力)下降的應(yīng)力敏感實(shí)驗(yàn)。

由圖5可以看出,隨著有效應(yīng)力下降和孔隙壓力增加,煤樣的滲透率表現(xiàn)出下降的特點(diǎn),與目前多數(shù)研究的結(jié)果并不一致。但有文獻(xiàn)指出滲透率隨孔隙壓力的變化存在一個(gè)臨界壓力,孔隙壓力小于臨界壓力時(shí),滲透率隨孔隙壓力的增加而減小[19-21],同時(shí)有分析認(rèn)為:在當(dāng)煤儲(chǔ)層有效應(yīng)力較低(低于4 MPa)時(shí),有效應(yīng)力為定值條件下,隨著注入氣壓(孔隙壓力)增加(從2.4 MPa增加到5.5 MPa),孔隙壓力較低時(shí)煤基質(zhì)體積應(yīng)變所產(chǎn)生的滲透率下降量遠(yuǎn)高于氣體分子的滑脫效應(yīng)所引起的煤樣滲透率增量,且這種現(xiàn)象有隨著有效應(yīng)力的增加而明顯增加的趨勢(shì)[22]。

圖5 煤樣滲透率隨孔隙壓力和有效應(yīng)力的變化Fig.5 Variation of permeability with pore pressure and effective stress for coal samples

結(jié)合以上分析和本次實(shí)驗(yàn)結(jié)果,認(rèn)為:在孔隙壓力從0.8 MPa增加到3.6 MPa的過程中,孔隙壓力低于滲透率發(fā)生變化的臨界壓力,表現(xiàn)為煤儲(chǔ)層滲透率隨孔隙壓力的增大而減小,且在此過程中煤基質(zhì)應(yīng)力體積應(yīng)變雖有下降趨勢(shì),有效應(yīng)力降低引起的煤儲(chǔ)層滲透率增加量仍低于煤基質(zhì)體積應(yīng)變所產(chǎn)生的滲透率下降量,因而導(dǎo)致煤樣的滲透率隨有效應(yīng)力的降低而降低。

由煤儲(chǔ)層滲透率與流體壓力、有效應(yīng)力之間的耦合變化關(guān)系分析可知:煤層氣井排采期間如發(fā)生修井、卡泵等停抽事故時(shí),在造成孔隙流體壓力回升、煤層有效應(yīng)力下降的條件下煤儲(chǔ)層滲透率仍然可能下降,因而排采期間應(yīng)盡量避免停抽事故,防止因排采事故引起的煤儲(chǔ)層滲透率大幅損傷。

3 結(jié) 論

(1)有效應(yīng)力從0增加到10 MPa時(shí),滲透率減少了50%～70%;有效應(yīng)力從10 MPa增加到20 MPa時(shí),滲透率降低趨勢(shì)明顯變緩,損失量?jī)H約占初始滲透率的10%;在有效應(yīng)力增加到2.5 MPa以前,應(yīng)力敏感性較強(qiáng);有效應(yīng)力從2.5 MPa增加到9 MPa時(shí),應(yīng)力敏感性最強(qiáng)。

(2)有效應(yīng)力增加到3.5 MPa過程中,煤樣的滲透率損害系數(shù)急劇上升,滲透率損耗20%～30%;有效應(yīng)力從3.5 MPa上升至9 MPa的過程中,滲透率損害系數(shù)快速下降,而煤樣滲透率損害最大,損耗約60%,自9 MPa之后,滲透率損害系數(shù)緩慢下降,煤樣滲透率損害較小,損耗約10%;滲透率損害率介于30%～65%,絕大多數(shù)介于40%～60%,滲透率損害程度為中等偏弱到中等偏強(qiáng),臨界應(yīng)力介于7～11 MPa。

(3)有效應(yīng)力較低且不變條件下,煤樣滲透率隨孔隙壓力的增加而增加,煤基質(zhì)收縮效應(yīng)引起滲透率改善比較顯著,且煤樣原始滲透率越高,滲透率增幅越大。

(4)圍壓達(dá)到較高定值且保持不變時(shí),隨有效應(yīng)力下降和孔隙壓力增加,煤樣滲透率下降,這與有效應(yīng)力和孔隙壓力變化時(shí)引起的煤儲(chǔ)層滲透率變化量有關(guān)。

(5)煤層氣排采時(shí)應(yīng)適當(dāng)控制排水速率,發(fā)揮煤儲(chǔ)層基質(zhì)收縮效應(yīng)對(duì)滲透率的改善作用,并應(yīng)盡量避免停抽事故,有效抑制因煤層氣排采引起應(yīng)力敏感造成的儲(chǔ)層大幅損傷。

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Study on stress sensitivity of coal reservoir during drainage of coal-bed methane well in Southern Qinshui Basin

LIU Hui-hu1,SANG Shu-xun2,FENG Qing-ling3,HU Bao-lin1,HU You-biao1,XU Hong-jie1,CHENG Qiao1

(1.School of Earth and Environment,Anhui University of Science&Technology,Huainan 232001,China;2.School of Resource and Earth Science,China University of Mining&Technology,Xuzhou 221008,China;3.Jinzhai Surveying and Mapping Company,Zibo 255000,China)

Coal samples from Southern Qinshui Basin were sampled,and experimental study on stress sensitivity under different experimental condition was performed,in order to analyze stress sensitivity and affecting factors on permeability of coal reservoir at different stage of coal-bed methane drainage.The results show that permeability of coal samples decline 50%-70%when the effective stress increase from 0 to 10 MPa,and the loss of coal reservoir permeability is about 10%when the effective stress increase from 10 MPa to 20 MPa.The stress sensitivity of coal samples are relatively stronger when the effective stress is below 2.5 MPa.The damage coefficient of permeability increase quickly and the loss of permeability is about 20%-30%of the initial permeability from coal samples when the effective stress increase to 3.5 MPa.The stress sensitivity is strongest when the effective stress increase from 2.5 MPa to 9 MPa.The damage coefficients of permeability decrease quickly and the initial permeability from coal samples lose 60%when the effective stress increases from 3.5 MPa to 9 MPa.The damage coefficient of permeability decrease slowly and the lossof permeability is about 10%of the initial permeability from coal samples when the effective stress beyond 9 MPa.The damage rate of permeability from coal samples is between 30%and 65%,and the critical stress is 7 MPa to 11 MPa.The permeability of coal samples increase with increase of pore pressure,when the effective stress is relatively lower and constant.The permeability of coal samples decrease with the decline of effective stress and increase of pore pressure when the confining pressure is constant,which is related to the variation of permeability caused by the variation of effective stress and pore pressure on coal reservoir.

Southern Qinshui Basin;coal-bed methane;drainage;coal reservoir permeability;stress sensitivity;effective stress;pore pressure

P618.11

A

0253-9993(2014)09-1873-06

2014-04-15 責(zé)任編輯:常 琛

國(guó)家自然科學(xué)基金資助項(xiàng)目(41330638,41302129);山西省煤層氣聯(lián)合研究基金資助項(xiàng)目(2012012008)

劉會(huì)虎(1978—),男,江西九江人,講師。E-mail:xixiinformation@163.com

劉會(huì)虎,桑樹勛,馮清凌,等.沁水盆地南部煤層氣井排采儲(chǔ)層應(yīng)力敏感研究[J].煤炭學(xué)報(bào),2014,39(9):1873-1878.

10.13225/j.cnki.jccs.2014.8007

Liu Huihu,Sang Shuxun,Feng Qingling,et al.Study on stress sensitivity of coal reservoir during drainage of coal-bed methane well in Southern Qinshui Basin[J].Journal of China Coal Society,2014,39(9):1873-1878.doi:10.13225/j.cnki.jccs.2014.8007