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        陽(yáng)泉礦區(qū)松軟低透煤層底板穿層鉆孔抽采時(shí)間優(yōu)化

        2019-09-10 07:22:44郭春生徐超葛勇

        郭春生 徐超 葛勇

        摘 要:確定底板穿層鉆孔有效抽采時(shí)間對(duì)于優(yōu)化生產(chǎn)接替,提高生產(chǎn)效率有重要意義。在陽(yáng)泉某礦穿層鉆孔施工過(guò)程中,由于煤層透氣性差、瓦斯含量高、地應(yīng)力大,原有的抽采設(shè)計(jì)已不能滿足消突與接替需要。在不補(bǔ)打鉆孔條件下,延長(zhǎng)預(yù)抽時(shí)間就成為區(qū)域瓦斯防突達(dá)標(biāo)的重要途徑。文中采用實(shí)驗(yàn)室實(shí)驗(yàn)、數(shù)值模擬與工程驗(yàn)證相結(jié)合的研究方法,首先通過(guò)實(shí)驗(yàn)得到了煤層的瓦斯基礎(chǔ)參數(shù),建立了適用于陽(yáng)泉礦區(qū)煤層特點(diǎn)的數(shù)值模擬本構(gòu)模型,并對(duì)該煤層底板穿層鉆孔不同抽采時(shí)間下的抽采效果進(jìn)行了模擬研究。結(jié)果表明:隨著抽采時(shí)間的延長(zhǎng),穿層鉆孔的影響范圍在逐漸增大,瓦斯壓力在不斷降低。抽采時(shí)間為300 d時(shí),抽采鉆孔中心點(diǎn)殘余瓦斯壓力可以降至0.74 MPa以下,可以達(dá)到防突要求。最后,通過(guò)收集工程實(shí)踐過(guò)程中瓦斯抽采數(shù)據(jù),計(jì)算得出了煤層殘余瓦斯含量,獲得了實(shí)際殘余瓦斯含量與抽采天數(shù)的關(guān)系,驗(yàn)證了數(shù)值模擬結(jié)果的正確性。

        關(guān)鍵詞:底板穿層鉆孔;抽采時(shí)間;COMSOL模擬;松軟煤層;固氣耦合模型

        中圖分類號(hào):TD 712 ? ? ? ? ? 文獻(xiàn)標(biāo)志碼:A

        文章編號(hào):1672-9315(2019)05-0875-07

        Abstract:Determination of gas drainage time in floor crossing boreholes is vital to the production efficiency in coal mines.In one mine in Yangquan area,during the construction of boreholes,the original drainage design can not meet the current demand of production balance due to the low permeability,high gas content and high ground stress in coal seams.Therefore,to extend the drainage time is an important way to reduce the outburst risk level with no further supplementary boreholes drilled.This study used the methods of experiments,numerical simulation and engineering validation to study the gas drainage performance of flooring crossing boreholes with different drainage time.It firstly obtained the basic parameters of coal by experiments and then established a constitutive model to simulate the drainage process.The results showed that with the increase of drainage time,the drainage area increased and the gas pressure decreased.When the drainage time reached 300 days,the residual gas content at the center decreased below 0.74 MPa,which met the demands of outburst prevention standards.At last,by collecting the gas drainage data and calculating the residual gas content,the relationship between residual gas content and drainage time was also obtained,which provided a solid validation to the simulation results.

        Key words:floor crossing boreholes;gas drainage time;COMSOL simulation;soft coal seam;solid-gas coupled model

        0 引 言

        我國(guó)95%以上的高瓦斯礦井和突出礦井擁有高地應(yīng)力、高瓦斯含量、高瓦斯壓力和低透氣性的“三高一低”特征,其透氣性多處于0.04~0.004 m2/(MPa2·d)的范圍[1],遠(yuǎn)小于國(guó)際煤層氣井抽采透氣性指標(biāo),抽采十分困難。目前,我國(guó)常用的區(qū)域防突措施有保護(hù)層開(kāi)采和預(yù)抽瓦斯2種。底板巖巷穿層鉆孔預(yù)抽方法屬于預(yù)抽瓦斯方法的一種,指在煤層底板巖層中布置鉆場(chǎng)施工上向鉆孔預(yù)抽煤層瓦斯的方法,是我國(guó)松軟低透煤層防突的主要區(qū)域防突措施,能很好的適應(yīng)煤層頂?shù)装迤鸱^大的地質(zhì)條件,抽采橫向范圍和抽采煤層較多,同時(shí)由于有堅(jiān)硬的巖巷作為保障,可有效解決順層鉆孔噴孔塌孔的問(wèn)題,已在皖北、淮北、淮南、云南等礦區(qū)得到推廣應(yīng)用[2-6]。底板穿層鉆孔的抽采效果通常與鉆孔長(zhǎng)度、鉆孔直徑、抽采時(shí)間、抽采負(fù)壓、封孔長(zhǎng)度、巷道竣工時(shí)間、煤層原始滲透率及應(yīng)力條件等因素有關(guān)[7-8]。

        關(guān)于抽采時(shí)間的合理設(shè)定,國(guó)內(nèi)外學(xué)者都將其與抽采半徑結(jié)合分析,李峰等以馬村礦為例研究了水力沖孔后鉆孔的合理抽采時(shí)間,認(rèn)為90 d后抽采基本穩(wěn)定[9]。張超林等認(rèn)為抽采時(shí)間隨鉆孔數(shù)量的增加呈冪函數(shù)降低,降低速度逐漸變緩[10]。劉清泉認(rèn)為由于基質(zhì)體平衡時(shí)間的差異會(huì)使得抽采在不同階段難易不同[11]。程遠(yuǎn)平等認(rèn)為瓦斯流動(dòng)的主控形式會(huì)隨著抽采時(shí)間而改變,因此鉆孔抽采負(fù)壓需要響應(yīng)改變[12]。Pillalamarry 等認(rèn)為擴(kuò)散是控制抽采后期效果的主要因素,也重申了抽采時(shí)間對(duì)于抽采效果的重要性[13],此結(jié)論得到了Wang 等研究的驗(yàn)證[14]。之后,通過(guò)建立引入時(shí)間常數(shù)或平衡時(shí)間常數(shù)的擴(kuò)散方程[15-16],Liu以及Dong等均明確了抽采時(shí)間以及負(fù)壓在抽采效果中的作用[17-18]。

        在陽(yáng)泉礦區(qū)某礦穿層鉆孔施工過(guò)程中,由于煤層透氣性差、瓦斯含量高、地應(yīng)力大,原有的抽采設(shè)計(jì)已不能滿足消突和工作面接替需要。在不補(bǔ)打鉆孔條件下,延長(zhǎng)預(yù)抽時(shí)間就成為區(qū)域瓦斯防突達(dá)標(biāo)的重要途徑。如何確定鉆孔抽采的合理時(shí)間,達(dá)到預(yù)定的防突效果,從而合理制定抽采計(jì)劃,已成為該礦區(qū)域防突工作亟待解決的技術(shù)難題。

        本項(xiàng)目以陽(yáng)泉某礦松軟低透煤層工作面為試驗(yàn)研究對(duì)象,通過(guò)工程試驗(yàn)與實(shí)測(cè)、理論分析與模擬實(shí)驗(yàn),得到了煤層的瓦斯基礎(chǔ)參數(shù),并基于鉆孔瓦斯抽采的固—?dú)怦詈夏P?,采用COMSOL模擬的方法,得出試驗(yàn)工作面底板巖巷穿層鉆孔的合理抽采時(shí)間,結(jié)合現(xiàn)場(chǎng)鉆孔殘余瓦斯含量的測(cè)定結(jié)果對(duì)數(shù)值模擬結(jié)果進(jìn)行了驗(yàn)證。

        1 瓦斯基礎(chǔ)參數(shù)測(cè)定

        陽(yáng)泉礦區(qū)主要的含煤地層為石炭系太原組及二疊系山西組,共含煤16層,其中主采3#,12#,15#煤層,煤層傾角一般為5°~10°,煤種為無(wú)煙煤。文中所研究工作面處于+420 m水平,開(kāi)采煤層為15#煤層,平均埋藏深度為497.5 m,采長(zhǎng)為194.56 m,走向長(zhǎng)度為935 m.工作面煤層平均厚度為625 m,煤層傾角平均為8°.根據(jù)鉆孔地質(zhì)資料,開(kāi)采煤層為復(fù)雜結(jié)構(gòu)煤層,一般含矸2~4層,巖性為泥巖,煤巖類型為半亮型-光亮型。

        參照《煤的工業(yè)分析方法》(GB/T 212—2001)及《煤的甲烷吸附量測(cè)定方法(高壓容量法)》(MT/T 752—1997)標(biāo)準(zhǔn),對(duì)研究礦井15#煤取得的煤樣進(jìn)行基本參數(shù)測(cè)定,測(cè)定結(jié)果見(jiàn)表1.

        2 數(shù)值模擬方法

        2.1 本構(gòu)方程建立

        在建立本構(gòu)型前要對(duì)方程進(jìn)行預(yù)先的假設(shè),簡(jiǎn)化計(jì)算過(guò)程如下:①研究煤體為各向同性的均質(zhì)體;②瓦斯氣體的狀態(tài)變化符合理想氣體狀態(tài)方程;③瓦斯吸附解吸過(guò)程為等溫過(guò)程。模型建立時(shí)涉及的主要模型有基于煤體孔隙率的動(dòng)態(tài)滲透率響應(yīng)模型、煤層瓦斯?jié)B流模型、煤體變形場(chǎng)的控制模型3種,3種方程耦合構(gòu)成了本次計(jì)算模擬的本構(gòu)模型[19-21]。

        2.2 幾何模型和邊界條件

        研究工作面采用底板穿層鉆孔進(jìn)行瓦斯抽采,在進(jìn)回風(fēng)巷下方各布置一條底板巖巷,鉆孔直徑為110 mm,終孔進(jìn)入15#煤層頂板0.5 m,形成3 m×6 m的矩陣,鉆孔整體控制范圍為進(jìn)回風(fēng)巷兩側(cè)15 m(鉆孔布置如圖2所示)。煤層原始瓦斯最大值為11.29 m3/t,最大瓦斯壓力為0.93 MPa.

        根據(jù)所研究工作面的煤層賦存條件和瓦斯鉆孔原有的設(shè)計(jì)情況,建立模擬所需的幾何模型,如圖3所示,模型參數(shù)取值見(jiàn)表2.為了研究現(xiàn)有穿層鉆孔布置參數(shù)條件下,不同抽采時(shí)間的瓦斯抽采效果,根據(jù)研究礦井的地質(zhì)條件,分別以鉆孔時(shí)間6,8,10,12個(gè)月建立數(shù)值計(jì)算模型進(jìn)行模擬,模擬抽采負(fù)壓根據(jù)實(shí)際情況取-10 kPa.另外,為了清晰的看出不同抽采時(shí)間條件下鉆孔周圍瓦斯壓力分布的情況,選取幾何模型內(nèi)水平中心線和垂直中心線上瓦斯壓力作為研究對(duì)象,水平測(cè)線和垂直測(cè)線分別為圖中紅色的x線和y線(圖3)。

        3 底板巖巷穿層鉆孔抽采時(shí)間優(yōu)化模擬 ?利用COMSOL軟件中的后處理模塊,可得到不同抽采時(shí)間條件下,研究區(qū)域內(nèi)的瓦斯壓力分布云圖,如圖4所示。從圖中可以看出,隨著抽采時(shí)間的增加,穿層鉆孔的抽采范圍在逐漸增大,4個(gè)抽采鉆孔連線的中心點(diǎn)瓦斯壓力在不斷降低,抽采中心區(qū)域也逐漸連接在一起,由橫置的“沙漏”狀逐步過(guò)渡到近“矩形”狀。

        圖5為抽采負(fù)壓10 KPa,抽采時(shí)間分別為180,240,300,360 d時(shí),水平測(cè)線x與垂直測(cè)線y上的瓦斯壓力分布圖。從圖中可以看出,隨著抽采時(shí)間增加,抽采后的瓦斯壓力在逐漸降低。在抽采時(shí)間為t=300 d的情況下,鉆孔中心區(qū)域的瓦斯壓力已經(jīng)下降至0.74 MPa以下,因此為了保證現(xiàn)有鉆孔布置條件下區(qū)域防突效果能夠達(dá)標(biāo),穿層鉆孔的預(yù)抽時(shí)間應(yīng)設(shè)置為300 d左右為宜。

        4 工程驗(yàn)證

        根據(jù)《煤礦瓦斯抽采達(dá)標(biāo)暫行規(guī)定》對(duì)工作面抽采區(qū)域不同鉆孔的殘余瓦斯含量進(jìn)行計(jì)算。計(jì)算時(shí)需首先將鉆孔按分布區(qū)域和抽采時(shí)間分為H-1(回風(fēng)測(cè)-1)、H-2(回風(fēng)測(cè)-2)和J-1(進(jìn)風(fēng)測(cè)-1)組,然后根據(jù)原始瓦斯含量和累計(jì)抽采量得到見(jiàn)表3和圖6的計(jì)算結(jié)果。從圖中可以看出工作面原始瓦斯含量為11 m3/t,在抽采天數(shù)約為280天的情況下,殘余瓦斯含量下降到了8 m3/t以下,與第4節(jié)中模擬得出的300 d相差不大,故而有效驗(yàn)證了模擬結(jié)果的正確性。值得指出的是,在實(shí)際條件下,抽采有效時(shí)間還受到諸如封孔效果、抽采負(fù)壓、周期來(lái)壓等因素的影響,因而會(huì)產(chǎn)生波動(dòng)。但總體而言,300 d的抽采時(shí)間是有指導(dǎo)意義的。

        5 結(jié) 論

        1)數(shù)值模擬結(jié)果表明:抽采時(shí)間對(duì)穿層鉆孔的抽采效果影響較大,隨著抽采時(shí)間的延長(zhǎng),穿層鉆孔的影響范圍在逐漸增大,模型中心點(diǎn)瓦斯壓力在不斷降低。在抽采時(shí)間為300 d時(shí),鉆孔中心區(qū)域的瓦斯壓力下降至0.74 MPa以下。

        2)工程實(shí)踐表明:經(jīng)過(guò)約280 d的抽采,工作面瓦斯含量從11 m3/t降至了8 m3/t,達(dá)到防突規(guī)定的要求,同時(shí)有效驗(yàn)證了數(shù)值模擬的結(jié)果。

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