馬東民,伋雨松,陳 躍,鄭 超,滕金祥,馬卓遠(yuǎn),肖嘉隆
基于煤層氣井排采數(shù)據(jù)的儲(chǔ)層含氣量動(dòng)態(tài)反演研究
馬東民1,2,伋雨松1,陳 躍1,鄭 超1,滕金祥1,馬卓遠(yuǎn)1,肖嘉隆1
(1. 西安科技大學(xué) 地質(zhì)與環(huán)境學(xué)院,陜西 西安 710054;2. 煤與煤層氣共采國(guó)家重點(diǎn)實(shí)驗(yàn)室,山西 晉城 048012)
煤儲(chǔ)層含氣量是煤層氣開(kāi)發(fā)的核心參數(shù),但實(shí)測(cè)煤儲(chǔ)層含氣量與煤儲(chǔ)層的真實(shí)含氣量之間往往存在誤差?;诟G街礦區(qū)海石灣井田煤層氣井不同時(shí)段的產(chǎn)氣量,以煤儲(chǔ)層含氣量“定體積”降低為基礎(chǔ),反演煤儲(chǔ)層實(shí)時(shí)含氣量,研究煤層氣井排采過(guò)程煤儲(chǔ)層實(shí)時(shí)含氣量的變化規(guī)律。結(jié)果表明:煤儲(chǔ)層含氣量隨排采時(shí)間呈線性下降趨勢(shì),不同步長(zhǎng)煤層氣井產(chǎn)氣量與煤儲(chǔ)層含氣量降低幅度一致,遵循“定體積”產(chǎn)氣特征,即煤層氣單井產(chǎn)氣量是煤基質(zhì)“定體積”產(chǎn)出;煤層氣井的產(chǎn)氣量與含氣量降低速率有關(guān),而與煤儲(chǔ)層原始含氣量無(wú)關(guān)。煤儲(chǔ)層為隔水層,水力壓裂難以改變煤基微孔隙通道的結(jié)合水狀態(tài),CH4產(chǎn)出過(guò)程受水–煤界面作用控制,煤層氣產(chǎn)出是“CH4·煤·水”三相界面?zhèn)髻|(zhì)作用的結(jié)果,水–煤界面作用中水的湍動(dòng)提供并傳遞能量,激勵(lì)塊煤中CH4解吸與產(chǎn)出。
窯街礦區(qū);海石灣煤礦;煤層氣;定體積;排采數(shù)據(jù)反演
煤儲(chǔ)層地質(zhì)條件的復(fù)雜性、鉆井/壓裂工程的流程化作業(yè)、排采管理的粗放性等主客觀因素并存,造成頻繁修井與排采間斷,導(dǎo)致一些煤層氣井產(chǎn)氣量未達(dá)到預(yù)期[1-10]。但是,排采數(shù)據(jù)是儲(chǔ)層物性動(dòng)態(tài)變化的實(shí)際反饋,即煤儲(chǔ)層含氣量變化對(duì)煤層氣井產(chǎn)氣量反應(yīng)最為敏感。在煤層氣勘探開(kāi)發(fā)過(guò)程中,僅有少數(shù)井進(jìn)行儲(chǔ)層含氣量測(cè)試,由于絕大多數(shù)煤儲(chǔ)層的含氣量無(wú)法準(zhǔn)確獲得,少數(shù)依據(jù)參數(shù)井取心測(cè)試,但獲得的氣含量數(shù)據(jù)也由于損失氣量、殘余氣量等不穩(wěn)定因素而與實(shí)際含氣量存在偏差,尤其是低階煤煤層氣解吸快,測(cè)試誤差則更大[11-17]。目前,從煤層氣井排采數(shù)據(jù)反演儲(chǔ)層含氣量動(dòng)態(tài)變化,從而獲得煤儲(chǔ)層含氣量的相關(guān)研究較少,且儲(chǔ)層含氣量的研究集中于實(shí)驗(yàn)室塊煤,與實(shí)際開(kāi)采的煤層存在明顯差異[18-21]?;诟G街礦區(qū)海石灣井田煤層氣井基礎(chǔ)排采數(shù)據(jù),以“定體積法”為基礎(chǔ),計(jì)算不同煤層氣井煤儲(chǔ)層的實(shí)時(shí)含氣量,對(duì)塊煤的含氣量動(dòng)態(tài)變化進(jìn)行分析研究,分析其產(chǎn)氣機(jī)理,為研究區(qū)后續(xù)煤層氣勘探開(kāi)發(fā)提供重要的理論依據(jù)。
窯街礦區(qū)海石灣井田位于民和盆地中央隆起帶的西南端,其主要含煤地層為侏羅系中統(tǒng)窯街組(圖1)。侏羅系在井田西部,走向NE,傾向SE,傾角10°~20°,越靠近超覆邊界或隆起邊緣,傾角逐漸加大。井田西南部煤二層中煤層氣組分以CH4為主,地質(zhì)歷史時(shí)期井田東部邊界發(fā)生F19深大斷裂,導(dǎo)致煤中甲烷沿煤層走向由東向西擴(kuò)散運(yùn)移和富集。煤二層平均厚21 m,傾角為3°~25°,大部分為可采的較穩(wěn)定煤層,埋深508~1 365 m,整體由北至南逐漸加深。煤二層CH4含量為0.011~6.22 m3/t,平均1.87 m3/t。
圖1 窯街礦區(qū)地理位置[22]
研究分析海石灣井田HSW05-1D、HSW05-2D、HSW05-3V、HSW05-4D與HSW05-5D煤層氣井排采曲線(圖2)發(fā)現(xiàn):①各井出現(xiàn)套壓時(shí),根據(jù)井下壓力計(jì)壓力折算煤二層壓力均在8~9 MPa;②各井產(chǎn)氣量穩(wěn)定升高階段皆為井底流壓均勻下降階段,與套壓變化無(wú)關(guān);③井底流壓與套壓穩(wěn)定時(shí),產(chǎn)氣量皆處于下降階段,各井產(chǎn)氣量衰減幅度近于一致(2021年7月5日開(kāi)始穩(wěn)壓后各井產(chǎn)能一致下降);④各井產(chǎn)水量大小對(duì)產(chǎn)能大小影響顯著,從而判斷在穩(wěn)壓排采后各井產(chǎn)能持續(xù)下降,分析其原因是:穩(wěn)壓表現(xiàn)為排采強(qiáng)度減小,產(chǎn)水量降低,水的湍動(dòng)是一個(gè)傳質(zhì)過(guò)程,煤儲(chǔ)層解吸需要水的湍動(dòng)提供能量。
水作為煤層氣開(kāi)采的媒介,是吸附態(tài)甲烷轉(zhuǎn)化為游離態(tài)甲烷的關(guān)鍵因素,但煤層是隔水層或弱含水層,通過(guò)壓裂強(qiáng)制外界水進(jìn)入煤層的范圍有限,即水與煤接觸面范圍一定,因而1口煤層氣井產(chǎn)氣的煤體積是定值,即確定時(shí)間范圍內(nèi)煤層氣井的產(chǎn)氣量與消耗的煤儲(chǔ)層含氣量應(yīng)該一致?;诤J癁尘锩簩託饩挪商卣髡J(rèn)識(shí),采用“定體積法”進(jìn)行排采數(shù)據(jù)分析,即假設(shè)煤層氣井排采連續(xù)階段(無(wú)修井間斷),塊煤解吸–產(chǎn)氣的煤層體積保持不變,等同于壓裂注入水后所影響的儲(chǔ)層內(nèi)微孔喉的范圍不變。
具體步驟如下:①繪制煤層氣井組的排采曲線;②劃分生產(chǎn)階段,其中,自煤層氣井開(kāi)始產(chǎn)氣到產(chǎn)氣量最大時(shí)為產(chǎn)氣上升階段(其中,HSW05-3V和HSW05-5D產(chǎn)氣階段分為緩慢上升階段與快速上升階段),自產(chǎn)氣開(kāi)始下降日至2021年7月31日為產(chǎn)氣下降階段;③以“一個(gè)時(shí)間步長(zhǎng)的產(chǎn)氣量是一定體積煤儲(chǔ)層含氣量降低結(jié)果”為指導(dǎo)思路進(jìn)行分析,假定不同原始含氣量進(jìn)行計(jì)算;④計(jì)算時(shí)間步長(zhǎng)為1 d,對(duì)煤儲(chǔ)層的實(shí)時(shí)含氣量進(jìn)行數(shù)學(xué)分析。
設(shè)產(chǎn)氣階段的任意時(shí)刻為,則時(shí)刻累積產(chǎn)氣量為Q,m3;煤儲(chǔ)層原始含氣量為ys,m3/t;設(shè)煤層氣井累計(jì)產(chǎn)氣時(shí)間為i,累積產(chǎn)氣量為Qi(研究區(qū)煤層氣井處于開(kāi)發(fā)初期階段,所以0為0),m3;時(shí)刻煤儲(chǔ)層含氣量為q,m3/t;塊煤的解吸體積為,m3;煤的視密度為,t/m3。由物質(zhì)平衡定律知0時(shí)間段產(chǎn)氣量為:
即
圖2 海石灣HSW05煤層氣井群排采曲線
筆者以“定體積”產(chǎn)氣為研究基礎(chǔ),海石灣煤二層密度為1.4 t/m3,劃分產(chǎn)氣上升和下降階段計(jì)算自產(chǎn)氣日起到某時(shí)刻的剩余含氣量,計(jì)算步長(zhǎng)為1 d。煤層氣井組附近參數(shù)井測(cè)試煤二層含氣量為6.79 m3/t,等溫吸附飽和吸附量為8.7 m3/t,據(jù)此,假設(shè)原始含氣量為5、6、7 m3/t。根據(jù)HSW05煤層氣井組的產(chǎn)氣數(shù)據(jù)、煤層厚度和產(chǎn)氣范圍壓裂監(jiān)測(cè)縫長(zhǎng),分別計(jì)算出產(chǎn)氣煤體積(表1)。
根據(jù)HSW05–2D井產(chǎn)氣數(shù)據(jù),產(chǎn)氣上升階段取第1—108天(即2021年3月1日—2021年6月16日),產(chǎn)氣下降階段取第109—153天(即2021年6月17日—2021年7月29日)。時(shí)間步長(zhǎng)以1 d計(jì)算分析,假設(shè)ys=5.0 m3/t,由式(3)可得q=4.999 922 909 7 m3/t。
表1 HSW05不同煤層氣井煤二層參數(shù)
煤儲(chǔ)層日減氣量rj:
式中:rc為煤層氣井日產(chǎn)氣量,m3。
0—任一時(shí)刻儲(chǔ)層實(shí)時(shí)含氣量為:
以1 d為統(tǒng)計(jì)單位,計(jì)算可得q隨時(shí)間變化曲線,如圖3所示。
可見(jiàn),設(shè)定原始含氣量為5 m3/t時(shí),q隨時(shí)間變呈線性遞減變化規(guī)律,擬合度較高。當(dāng)設(shè)定原始含氣量分別為6、7 m3/t,q隨時(shí)間變化仍呈線性遞減關(guān)系,且下降趨勢(shì)一致。其中,將產(chǎn)氣階段分為產(chǎn)氣上升階段與產(chǎn)氣下降階段,規(guī)律性與全過(guò)程一致。
圖3 不同原始含氣量步長(zhǎng)為1 d實(shí)時(shí)煤儲(chǔ)層含氣量變化曲線
以相同計(jì)算方法,分別得出HSW05-1D、HSW05-3V、HSW05-4D和HSW05-5D不同原始含氣量、步長(zhǎng)為1 d產(chǎn)氣時(shí)間與實(shí)時(shí)含氣量擬合關(guān)系??梢钥闯觯? d的時(shí)間步長(zhǎng),設(shè)定原始含氣量分別為5、6、7 m3/t時(shí),不同煤層氣井的排采數(shù)據(jù),在不同階段(產(chǎn)氣上升階段和產(chǎn)期下降階段)的煤儲(chǔ)層實(shí)時(shí)含氣量變化關(guān)系高度一致。
以3 d為步長(zhǎng),進(jìn)行數(shù)據(jù)統(tǒng)計(jì)。即設(shè)置第1、2、…個(gè)統(tǒng)計(jì)點(diǎn)為第1、4 d、…含氣量值,按照1 d步長(zhǎng)的計(jì)算方法,獲得實(shí)時(shí)含氣量隨時(shí)間的關(guān)系曲線。同理,得出步長(zhǎng)為5 d和7 d的關(guān)系曲線,如圖4所示。由圖中可以看出,3、5、7 d步長(zhǎng)條件下,原始含氣量ys分別為5、6、7 m3/t時(shí),煤儲(chǔ)層實(shí)時(shí)含氣量變化關(guān)系高度一致。
無(wú)論原始含氣量大小,隨著排采的進(jìn)行,不同煤層氣井其煤儲(chǔ)層實(shí)時(shí)含氣量,在產(chǎn)氣上升階段或產(chǎn)氣下降階段實(shí)時(shí)含氣量都與排采時(shí)間呈線性遞減關(guān)系,其線性擬合公式斜率見(jiàn)表2。
表2 HSW05井群不同排采階段曲線斜率
每口井2個(gè)產(chǎn)氣階段煤儲(chǔ)層實(shí)時(shí)含氣量變化斜率皆為負(fù)值,表明其實(shí)時(shí)含氣量在遞減;上升階段實(shí)時(shí)含氣量直線斜率絕對(duì)值均小于下降階段的,這可能是因?yàn)榕挪蛇^(guò)程產(chǎn)生煤粉堵塞造成儲(chǔ)層傷害,影響塊煤中CH4的解吸,能夠解吸的煤層體積縮小。在產(chǎn)氣過(guò)程中,煤層的實(shí)時(shí)含氣量隨煤層氣井產(chǎn)氣量的緩慢上升或快速上升變化速率不一。
圖4 不同原始含氣量不同步長(zhǎng)實(shí)時(shí)含氣量變化曲線
煤層含氣量一般是通過(guò)煤層氣測(cè)試井鉆取煤心進(jìn)行自然解吸測(cè)試獲得,多數(shù)煤層氣開(kāi)采井無(wú)具體數(shù)據(jù),而實(shí)測(cè)過(guò)程,含氣量包含損失氣、解吸氣、殘余氣3部分,其中,損失氣是根據(jù)美國(guó)礦業(yè)局直接法(USBM)估算得到,實(shí)踐證明,估算值較實(shí)際值偏低,尤其是構(gòu)造煤與低階煤,初始煤層氣解吸速度快,誤差更大,導(dǎo)致自然解吸氣含量較實(shí)際含氣量偏低[23]。在“定體積法”分析過(guò)程中,本文設(shè)定多個(gè)原始含氣量:5、6、7 m3/t,但t隨時(shí)間變化均呈線性遞減關(guān)系,且下降趨勢(shì)一致(圖3);實(shí)時(shí)含氣量線性變化斜率相同,表明產(chǎn)氣的連續(xù)性與原始含氣量的大小無(wú)關(guān)。
對(duì)比分析海石灣5口煤層氣井排采數(shù)據(jù)發(fā)現(xiàn):實(shí)時(shí)含氣量隨時(shí)間呈線性變化,斜率絕對(duì)值增加,煤層氣井高產(chǎn);反之,產(chǎn)量衰減很快。一方面,說(shuō)明煤儲(chǔ)層污染很難恢復(fù),另一方面,阻礙了塊煤微孔腔內(nèi)表面CH4解吸,因?yàn)闀r(shí)間延續(xù)導(dǎo)致微孔喉結(jié)合水重新恢復(fù),煤孔隙通道中結(jié)合水的氫鍵與范德華力得不到弱化,最終影響自由水的傳質(zhì)作用。因此,連續(xù)排采能激勵(lì)塊煤的解吸作用,連續(xù)排采是實(shí)現(xiàn)煤層氣井高產(chǎn)的關(guān)鍵。
研究區(qū)煤儲(chǔ)層為低階煤,當(dāng)在同一煤層氣井,步長(zhǎng)分別為3、5、7 d時(shí),其儲(chǔ)層實(shí)時(shí)含氣量的線性關(guān)系與步長(zhǎng)為1 d時(shí)結(jié)果一致,表明煤儲(chǔ)層含氣量變化與生產(chǎn)時(shí)間間隔無(wú)關(guān)。水的湍動(dòng)所提供并傳遞的能量是煤層氣井產(chǎn)氣量上升或穩(wěn)定的關(guān)鍵,即排采過(guò)程“CH4·煤·水”三相界面?zhèn)髻|(zhì)作用需要水的湍動(dòng)提供能量。煤層氣井產(chǎn)氣高效的必要條件是增加水–煤接觸面積,即在煤層氣井壓裂階段有充分的水和煤接觸。另一方面,泵效同樣對(duì)煤層氣井產(chǎn)氣量有重要影響,后續(xù)可繼續(xù)量化討論其對(duì)水介質(zhì)的傳質(zhì)作用,以期對(duì)煤層氣井產(chǎn)量變化進(jìn)行量變的規(guī)律認(rèn)識(shí)。
a. 煤層氣井排采數(shù)據(jù)是對(duì)煤儲(chǔ)層參數(shù)的動(dòng)態(tài)反饋,煤儲(chǔ)層含氣量隨排采時(shí)間呈線性下降趨勢(shì),實(shí)驗(yàn)發(fā)現(xiàn),不同時(shí)間步長(zhǎng)條件下,煤層氣井產(chǎn)氣量與煤儲(chǔ)層含氣量降低趨勢(shì)一致,遵循“定體積”產(chǎn)氣,即煤層氣單井產(chǎn)氣的煤體積不變。產(chǎn)氣量與含氣量消耗同步,與生產(chǎn)時(shí)間間隔無(wú)關(guān)。
b. 不同煤儲(chǔ)層原始含氣量條件下,煤儲(chǔ)層實(shí)時(shí)含氣量隨時(shí)間變化均呈線性遞減關(guān)系,且下降趨勢(shì)一致,表明煤層氣井的產(chǎn)氣量與煤儲(chǔ)層原始含氣量無(wú)直接關(guān)系,煤儲(chǔ)層含氣量降低速率決定了煤層氣井的產(chǎn)量。
c. 煤儲(chǔ)層為隔水層,水力壓裂難以改變煤基質(zhì)微孔隙通道的結(jié)合水狀態(tài),CH4產(chǎn)出過(guò)程受水–煤界面作用控制。煤層氣產(chǎn)出是“CH4·煤·水”三相界面?zhèn)髻|(zhì)作用的結(jié)果,水的湍動(dòng)提供并傳遞能量,激勵(lì)塊煤中CH4解吸與產(chǎn)出。
d. 后續(xù)可從泵效方面研究其對(duì)煤層氣井產(chǎn)量的影響,量化探討水介質(zhì)的傳質(zhì)作用,以期對(duì)煤層氣井產(chǎn)量變化進(jìn)行量變規(guī)律的認(rèn)識(shí)。
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CBM well drainage data-based dynamic inversion study of reservoir gas content
MA Dongmin1,2, JI Yusong1, CHEN Yue1, ZHENG Chao1, TENG Jinxiang1, MA Zhuoyuan1, XIAO Jialong1
(1. College of Geology and Environment, Xi’an University of Science and Technology, Xi’an 710054, China; 2. State Key Laboratory of Coal and Coalbed Methane Co-Mining Technology, Jincheng 048012, China)
The gas content of coal reservoir is the core parameter of coalbed methane production. There is an error between the measured gas content and the real gas content of coal reservoir. In this paper, based on the gas production of Haishiwan coalbed methane wells in different periods and the decrease of the “constant volume” of the gas content of coal reservoirs, the real-time gas content of coal reservoirs is inverted, and the change rule of the real-time gas content in the process of coalbed methane well drainage is explored. The results show that: (1) The gas content of coal reservoir decreases linearly with the drainage time. the gas production of different long coalbed methane wells is consistent with the decrease of coal reservoir gas content, and follows the characteristics of “constant volume” gas production, that is, the gas production of single coalbed methane well is the “constant volume” production of coal matrix; (2) The gas production of coalbed methane wells has nothing to do with the original gas content of coal reservoir, but is related to the reduction of gas content; (3) The coal reservoir is a water-resistant layer, and hydraulic fracturing is difficult to change the combined water state of coal-based micropore channels. The CH4production process is controlled by the water-coal interface. Coalbed methane production is the result of mass transfer at the three-phase interface of “CH4, coal and water”, in which water turbulence provides and transfers energy to stimulate the desorption and production of CH4in block coal.
Yaojie mining area; Haishiwan Coal Mine; coalbed methane; constant volume; dynamic inversion of drainage data
語(yǔ)音講解
P624.7
A
1001-1986(2021)06-0067-07
2021-09-25;
2021-11-05
國(guó)家自然科學(xué)基金項(xiàng)目(41902175);山西省科技重大專項(xiàng)項(xiàng)目(20201101002);自然資源部煤炭資源勘查與綜合利用重點(diǎn)實(shí)驗(yàn)室開(kāi)放課題(KF2019-2)
馬東民,1967年生,男,陜西合陽(yáng)人,博士(后),教授,從事煤與煤層氣地質(zhì)教學(xué)與研究工作. E-mail:mdm6757@126.com
馬東民?,伋雨松,陳躍,等. 基于煤層氣井排采數(shù)據(jù)的儲(chǔ)層含氣量動(dòng)態(tài)反演研究[J]. 煤田地質(zhì)與勘探,2021,49(6):67–73. doi: 10.3969/j.issn.1001-1986.2021.06.007
MA Dongmin,JI Yusong,CHEN Yue,et al. CBM well drainage data-based dynamic inversion study of reservoir gas content[J]. Coal Geology & Exploration,2021,49(6):67–73. doi: 10.3969/ j.issn.1001-1986.2021.06.007
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