考慮變滲透率模量的異常高壓氣藏產(chǎn)能計(jì)算新方法

摘 要

摘 要:在異常高壓氣藏開發(fā)過程中,儲(chǔ)層巖石常常表現(xiàn)出應(yīng)力敏感特征。由于壓敏儲(chǔ)層在有效應(yīng)力變化范圍較大時(shí),滲透率模量將隨著有效應(yīng)力的變化而變化,這樣基于常滲透率模量的產(chǎn)能

摘 要:在異常高壓氣藏開發(fā)過程中,儲(chǔ)層巖石常常表現(xiàn)出應(yīng)力敏感特征。由于壓敏儲(chǔ)層在有效應(yīng)力變化范圍較大時(shí),滲透率模量將隨著有效應(yīng)力的變化而變化,這樣基于常滲透率模量的產(chǎn)能方程很難準(zhǔn)確評(píng)價(jià)異常高壓氣藏的產(chǎn)能特征。為此,針對(duì)異常高壓氣藏的滲流規(guī)律,建立了考慮變滲透率模量和高速非達(dá)西效應(yīng)的產(chǎn)能計(jì)算模型,并用Newton Raphson方法對(duì)模型進(jìn)行求解。研究及實(shí)例分析結(jié)果表明:應(yīng)力敏感作用會(huì)使異常高壓氣藏氣井的產(chǎn)量降低,尤其是在井底流壓較低時(shí),其產(chǎn)量降低得更為嚴(yán)重;在井底流壓較小時(shí),用傳統(tǒng)的常滲透率模量計(jì)算的產(chǎn)量可能會(huì)錯(cuò)誤評(píng)價(jià)應(yīng)力敏感作用對(duì)氣井產(chǎn)量的影響;而高速非達(dá)西效應(yīng)將會(huì)使異常高壓氣藏氣井的產(chǎn)量降低,隨著井底流壓的減小,高速非達(dá)西效應(yīng)使氣井產(chǎn)量降低的程度更為嚴(yán)重。結(jié)論認(rèn)為:在開發(fā)異常高壓氣藏的過程中應(yīng)考慮變滲透率模量和高速非達(dá)西效應(yīng)對(duì)氣井產(chǎn)能的影響。

關(guān)鍵詞:異常高壓氣藏  變滲透率模量  生產(chǎn)能力  應(yīng)力敏感  高速非達(dá)西流動(dòng)  數(shù)學(xué)模型

A new method for calculating the productivity of abnormally high-pressure gas reservoirs considering variable permeability modulus

AbstractIn the development of abnormally high pressure gas reservoirs,the reservoir rock generally exhibits the characteristic of stress sensitivityFor such stress sensitive reservoirs,the permeability modulus changes with the stress if the effective stress is of a large Variation rangeThereforethe productivity fornlula based on the constant permeability roodulus is unlikely to accurately evaluate the productivity of such gas reservoirsIn view of this,a productivity calculation roodel was established considering both the variable permeability model and high velocity non Darcy flow effect according to the permeability rule of abnormallv high-pressure gas reservoirs;The Newton-Raphson method was adopted for solving the modelThe study resuhs and case analysis showed that the stress sensltlvlty has the effect of reducing the production of abnormally high pressure gas reservoirsParticularlv,the production decreases heavily when the FBHP(flowing bottomhole pressure)is lowThus if the traditional calculation method based on the constant permeability modulus is adoptedthe evaluated productivity will probably provide unreliable basis for assessing the stress-sensitive effect on the gas well production.Moreover,the high velocity non-Darcy flow effect will reduce the productivity of abnormally high pressure gas reservoirs,and the reduction becomes severer with the decrease of FBHPTherefore,the effect of such factors as the variable permeability modulus and the high velocity non-Darcy flow effect on well productivity should be considered in thc development of abnormally high-pressure gas reservoirs

Key wordsoverpressured gas reservoir,variable permeability modulusproductivity,stress sensitivity,high velocity non Darcy

異常高壓氣藏作為一種特殊氣藏有著廣泛的分布,近年來隨著常規(guī)天然氣藏資源的減少以及油氣田勘探開發(fā)技術(shù)的發(fā)展,我國相繼發(fā)現(xiàn)了越來越多的異常高壓氣藏,如位于塔里木盆地的克拉2氣藏和四川盆地的磨溪嘉二段氣藏。根據(jù)異常高壓氣藏開發(fā)的調(diào)研可知[1-8],與常規(guī)氣藏相比,異常高壓氣藏具有驅(qū)動(dòng)能量大、儲(chǔ)層巖石變形對(duì)氣井產(chǎn)能影響顯著等特征。因此對(duì)于異常高壓氣藏的開發(fā)需考慮高速非達(dá)西效應(yīng)和應(yīng)力敏感作用對(duì)其產(chǎn)能的影響。目前已有不少學(xué)者針對(duì)異常高壓氣藏的特點(diǎn)研究了考慮應(yīng)力敏感的產(chǎn)能計(jì)算模型[9-14],然而在描述滲透率隨有效應(yīng)力變化關(guān)系時(shí)常常假設(shè)滲透率模量為常數(shù),這樣在氣藏開發(fā)過程中有效應(yīng)力變化范圍較大時(shí),常滲透率模量就很難準(zhǔn)確表征儲(chǔ)層的應(yīng)力敏感特征。雖然已有部分學(xué)者認(rèn)識(shí)到用變滲透率模量來描述應(yīng)力敏感特征更為合理[15-20],但是目前卻沒有見到針對(duì)異常高壓氣藏特征的考慮變滲透率模量影響的產(chǎn)能計(jì)算方法。因此筆者針對(duì)異常高壓氣藏的特征,建立了考慮變滲透率模量和高速非達(dá)西效應(yīng)影響的產(chǎn)能模型,并用Newton Raphson方法[21]對(duì)模型進(jìn)行了求解,分析了變滲透率模量對(duì)于異常高壓氣藏產(chǎn)能的影響,旨在為開發(fā)異常高壓氣藏提供一種新的產(chǎn)能評(píng)價(jià)手段,以便更加合理高效地開發(fā)異常高壓氣藏。

1 巖石變滲透率模量特征

在異常高壓氣藏的開采過程中,由于應(yīng)力敏感作用,儲(chǔ)層巖石的滲透率隨地層壓力的變化關(guān)系通常表示為[22-23]

KK0exp[-a(p0-p)]                 (1)

通常滲透率模量被視為常數(shù),但是目前國內(nèi)外不少學(xué)者通過大量實(shí)驗(yàn)發(fā)現(xiàn):當(dāng)?shù)貙訅毫ψ兓秶^大時(shí),1n(KK0)p0-p關(guān)系曲線(1)并非呈現(xiàn)單一的線性關(guān)系,而表現(xiàn)為滲透率模量隨著有效應(yīng)力的增加而逐漸變小,具有明顯的變滲透率模量特征[15-20]。為了描述這種變滲透率模量特征,通常采用分段滲透率模量[15-18]來表征滲透率隨有效應(yīng)力變化規(guī)律(1)。即假定異常高壓氣藏原始地層壓力為p0,原始滲透率為K0則從P0P的壓力變化過程可以根據(jù)滲透率模量的不同而劃分為m個(gè)階段pi-1pi(i=1,2,…,m),每個(gè)階段的滲透率模量為ai(i=1,2,m)

 

2 產(chǎn)能模型的建立及求解

假設(shè)有一個(gè)圓形外邊界的等厚異常高壓氣藏,一口氣井位于其中心并以定產(chǎn)量Qsc生產(chǎn)體在儲(chǔ)層中的流動(dòng)為平面徑向流,異常高壓氣藏原始地層壓力為P0,原始滲透率為K0,從氣藏供給壓力P0到井底流壓加(p0)的壓力變化過程可以根據(jù)滲透率模量的不同而劃分為m個(gè)階段pi-1pi(i=12,…,m),每個(gè)階段的滲透率模量保持不變,滲透率模量值為ai(pi≤ppi-1,i=12,…,m)。

根據(jù)滲流力學(xué)中的穩(wěn)態(tài)平面徑向滲流理論可以知道,儲(chǔ)層中的壓力分布是以井眼為中心的同心圓環(huán)。則根據(jù)滲透率模量的不同,從井筒半徑rm(rw)到供給半徑r0(re)也可以劃分為m段,其分界點(diǎn)依次為rmrm-1,r1,r0,分界點(diǎn)處對(duì)應(yīng)的壓力和滲透率分別為pm, pm-1,,p1p0,Km,Km-1,,K1K0,其中:

 

由于異常高壓氣藏驅(qū)動(dòng)能量較大,氣體在地層中的流速較高,因此流體的流動(dòng)不再符合達(dá)西定律,而滿足Forchheimier二項(xiàng)式方程[24]

 

在平面徑向滲流過程中氣體通過徑向距離為r時(shí)的滲流速度為:

 

地層條件下的氣體密度為:

 

描述孔隙介質(zhì)的湍流系數(shù)為[25]

 

Pi≤P≤i-1內(nèi)(其對(duì)應(yīng)的徑向距離ri≤r≤ri-1),儲(chǔ)層滲透率可以表示為:

 

將式(3)、(4)、(5)(6)代入式(2)得:

 

對(duì)式(7)左右兩邊在ri≤r≤ri-1內(nèi)積分,可以得到其在法定單位下的計(jì)算公式為:

 

其中:

 

i1,2,m分別代入式(8)中,可以得到由m個(gè)方程構(gòu)成考慮變滲透率模量的異常高壓氣藏產(chǎn)能計(jì)算模型。由于該產(chǎn)能計(jì)算模型是含有柳個(gè)未知數(shù)(未知數(shù)分別為r1,r2,…,rm-1,Qsc)的非線性方程組。因此采用Newton-Raphson方法[21]對(duì)該產(chǎn)能模型進(jìn)行求解,從而可以得到給定井底流壓下氣井的產(chǎn)量,進(jìn)而分析變滲透率模量和高速非達(dá)西作用對(duì)異常高壓氣藏產(chǎn)能的影響。

3 實(shí)例分析

以某異常高壓氣藏為例,其中1口井的資料如下:j0.09K00.708mD,rel000m,rw=0.1mh=15m,m=0.02mPa·sZ1.15,T95.5℃,gg=0.57p0=65MPa。

采用擬三軸應(yīng)力巖心夾持器測定該井取樣巖心在不同有效應(yīng)力下的滲透率,發(fā)現(xiàn)在儲(chǔ)層壓力范圍為065MPa時(shí),表現(xiàn)為變滲透率模量特征,研究發(fā)現(xiàn)可以用3段滲透率模量來表征滲透率隨有效應(yīng)力變化的關(guān)系(1)。

 

2為應(yīng)力敏感作用對(duì)氣井IPR曲線的影響,從圖中可以看到,應(yīng)力敏感作用會(huì)使氣井的產(chǎn)量降低,特別是在井底流壓較低時(shí),產(chǎn)量降低得越嚴(yán)重;對(duì)于滲透率模量計(jì)算的IPR曲線,在相同的井底流壓下,常滲透率模量越大,氣井的產(chǎn)量越低;對(duì)于該氣井用變滲透率模量計(jì)算的IPR曲線與用常滲透率模量計(jì)算的IPR曲線在形態(tài)上具有相似性,但是當(dāng)井底流壓較小時(shí),用變滲透率模量計(jì)算的產(chǎn)量與用常滲透率模量計(jì)算的產(chǎn)量差異較大,由于變滲透率模量更能夠反映儲(chǔ)層的滲流特征,因此用傳統(tǒng)的常滲透率模量計(jì)算的產(chǎn)量可能會(huì)錯(cuò)誤估計(jì)應(yīng)力敏感作用對(duì)氣井產(chǎn)量的影響。

 

3r1r2隨井底流壓的變化關(guān)系圖,由于實(shí)驗(yàn)發(fā)現(xiàn)在有效應(yīng)力的變化范圍內(nèi)存在著3段滲透率模量,因此從井眼半徑r3(rw)到供給半徑r0(re)之間存在著r2r1將氣藏在徑向上分為3個(gè)區(qū)域,每個(gè)區(qū)域由于有效應(yīng)力的不同而呈現(xiàn)出不同的滲透率變化規(guī)律,從圖中看到,當(dāng)井底流壓小于30MPa時(shí),r1r2隨著井底流壓的增大都會(huì)減小,當(dāng)井底流壓大于30MPa且小于56MPa時(shí),由于地層中有效應(yīng)力減小,在徑向上則只存在兩個(gè)區(qū)域,且分界處r1隨著井底流壓的增大會(huì)進(jìn)一步減小,直到當(dāng)井底流壓大于56MPa時(shí),則退化為了常滲透率模量的產(chǎn)能模型。

 

4為考慮變滲透率模量下高速非達(dá)西對(duì)氣井IPR曲線的影響,從圖中可以看到高速非達(dá)西效應(yīng)將會(huì)使異常高壓氣藏氣井的產(chǎn)量降低,并且隨著井底流壓的減小,高速非達(dá)兩效應(yīng)使得氣井產(chǎn)量降低得更嚴(yán)重。

 

4 結(jié)論

1)在異常高壓氣藏開發(fā)過程中,儲(chǔ)層巖石存在著應(yīng)力敏感效應(yīng),常滲透率模量在有效應(yīng)力變化范圍較大時(shí)難以準(zhǔn)確表征儲(chǔ)層的應(yīng)力敏感特征,因此基于常滲透率模量的產(chǎn)能計(jì)算方法不能有效評(píng)價(jià)異常高壓氣藏的產(chǎn)能特征。

2)建立了考慮變滲透率模量和高速非達(dá)西效應(yīng)的異常高壓氣藏產(chǎn)能計(jì)算模型并用Newton Raphson方法對(duì)模型進(jìn)行求解。

3)實(shí)例分析表明:應(yīng)力敏感作用會(huì)使氣井的產(chǎn)量降低,特別是在井底流壓較低時(shí),產(chǎn)量降低得越嚴(yán)重;在井底流壓較小時(shí),用傳統(tǒng)的常滲透率模量模型計(jì)算的產(chǎn)量可能會(huì)錯(cuò)誤估計(jì)應(yīng)力敏感作用對(duì)氣井產(chǎn)量的影響;高速非達(dá)西效應(yīng)將會(huì)使異常高壓氣藏氣井的產(chǎn)量降低,隨著井底流壓的減小,高速非達(dá)西效應(yīng)使氣井產(chǎn)量降低的程度越嚴(yán)重。

 

符號(hào)說明

P0為原始地層壓力,MPa;p為目前地層壓力,MPa;K0為原始滲透率,mD;K為目前滲透率,mD;a為滲透率模量,Mpa-1;r為平面徑向距離,m;m為黏度,mPa·s;u為氣體在地層中的滲流速度,m/sb為湍流系數(shù),m-1;r為地層氣體密度,kg/m3;Qsc為地面標(biāo)準(zhǔn)狀態(tài)下的產(chǎn)量,m3/d;psc為標(biāo)準(zhǔn)大氣壓,MpaZ為氣體偏差,無因次;T為地層溫度,K;h為儲(chǔ)層百度,m;Tsc為標(biāo)準(zhǔn)狀態(tài)下溫度,K;gg為地層氣體的相對(duì)密度,無因次;R為通用氣體常數(shù),Mpa·m3/(kmol·K);j為孔隙度,無因次。

 

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本文作者:任俊杰  郭平  王紹平  畢波  汪周華  趙增春

作者單位:油氣藏地質(zhì)及開發(fā)工程國家重點(diǎn)實(shí)驗(yàn)室·西南石油大學(xué)

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