煤层气储层压力预测方法

开发试采 天 然 气 工 业 2004年5月

煤层气储层压力预测方法*

苏现波 张丽萍

(焦作工学院)

苏现波等. 煤层气储层压力预测方法. 天然气工业, 2004; 24(5) :88~90

摘 要 压力是煤层气赋存的主控因素之一, 压力的预测为含气性剖面的建立提供了途径, 为煤层气成藏动力学的研究奠定了基础。本文根据吸附态、游离态气体和水的状态方程, 结合埋藏史、热史分析, 从理论上探讨了地质历史时期不同演化阶段煤储层压力的计算方法。以德国的鲁尔煤田石炭系煤储层为例, 采用正演和反演两种方法计算了抬升前后储层压力随孔隙度和孔隙中气水比例的变化情况。指出煤储层孔隙度与孔隙中气水的比例决定着抬升或埋藏后储层的压力状态(异常低压、正常压力和异常高压) 。 主题词 煤层气 储层压力 鲁尔煤田 压力预测

储层压力预测方法在油气领域已相对成熟, 常用的有作图法112、等效深度法122、Eaton 法13~速度法讨

16, 72

18~112

52

、EH Z

等。在煤层气领域尽管有一些探

抬升前煤储层孔隙中游离态气体的体积为:

t 0P 1

V g1=(1)

t 1P 0Q 抬升前煤储层中气体的总体积为吸附态与游离态气体之和:

V 1=

V L1P 1m t 0P 1

+

P L1+P 1t 1P 0Q

(2)

, 目前还没有完善的、普遍适用的储层压力

预测方法。煤与其它沉积岩明显不同, 煤层气的赋存状态与常规天然气差异显著, 这就使得常规油气储层压力预测的方法在煤层气中的应用受到了限制。根据煤储层的地质演化史、气体的赋存状态、孔隙度、孔隙中气水比例、储层温度等, 采用理论计算方法进行压力预测是本文的出发点。虽然这种预测的假设条件较多, 但可以反映储层压力的整个动态演化过程。

抬升后孔隙中水的体积为:V w2=(1-x ) [1+A (t 2-t 1) -B P 2](3)

抬升后孔隙中游离态气体的体积为: V g2=B P 2]}

t 0P 2

{1-(1-x ) [1+A (t 2-t 1) -t 2P 0Q

(4)

煤层气储层压力理论计算

根据构造抬升前的煤储层温度、压力、储层孔隙中的气/水比例、吸附态气体量和抬升后的储层温度, 从理论上可计算出抬升后的储层压力。假设:(1) 抬升过程中及抬升后整个系统没有流体的流入或流出, 即系统处于封闭状态, 类似于油气藏中的流体封存箱; (2) 煤储层的密度和孔隙度在抬升前后没有变化; (3) 煤储层中的气体仅为甲烷; (4) 煤储层中的气体以两种状态存在:吸附态和游离态, 且游离态气体为理想气体。(5) 抬升前后储层孔隙中气水比例不变。

*河南省自然科学基金资助项目(项目号:0211060800) 。

由假设条件(1) 可知:

t 0P 1

P +P +t P Q =t P Q {1-(1-x ) [1+

1020L 11

L22

A (t 2-t 1) -B P 2]}+(5)

P L2+P 2

方程(5) 为储层压力计算的基本公式, 可计算出抬升后的储层压力P 2。

计算实例及其结果分析

由式(5) 可进行两种计算:其一是由地质历史时期某一演化阶段开始计算现今的储层压力, 称为正演计算; 另外可根据现今已知的储层压力计算地质

作者简介:苏现波, 1963年生; 1988年毕业于中国矿业大学北京校区, 长期从事煤层气地质学与勘探开发研究工作, 在国内外期刊发表论文20余篇, 其中被EI 收录6篇, SCI 收录2篇。地址:(454000) 河南省焦作市焦作工学院资源与环境工程系。电话:(0319) 3980901。

第24卷第5期 天 然 气 工 业 开发试采

历史时期某一阶段的储层压力, 称为反演计算。这里以德国的鲁尔煤田为例, 具体说明上述计算的应用。 据Littke 等和Gaschnitz, 等的资料, 计算时采用的鲁尔煤田石炭系煤层的有关参数为:t 1=323. 15K, t 2=373. 15K, P L2=4. 855M Pa, P L 1=对于正演计算, 储层压力为P 1=25. 2M Pa; 对于反演计算储层压力为P 1=8. 0MPa 。其它参数取值为:t 0=293. 15K; P 0=0. 101325MPa; A =4@10-4

1122

1132

K -1; B =3@10-

10

Pa -1; Q =1. 8t/m 3。计算其抬升

前后的压力变化情况。 1. 正演计算结果

在现今的压力剖面上, 鲁尔煤田石炭系煤层对应于

温度为50e 的储层压力为8. 0MPa 1122, 埋深在800m 只有在煤储层的孔隙度大于等于4%, 孔隙中气体占30%以上, 抬升后才可能出现正常或异常高压

3. 004M Pa, V L2=30. 642m 3/t, V L1=38. 390m 3/t 。左右, 即处于正常压力状态。计算结果见表1。

表1 正演计算获得的抬升后的储层压力数据表M

Pa

状态。且孔隙度越高, 达到异常高压时孔隙中气体所占的比例越低。如孔隙度为4%时, 孔隙中气体占90%以上, 抬升后出现正常到异常高压; 孔隙度为12%以上时, 气体仅需占30%就可出现高压。即抬升后要形成正常或异常高压, 储层就必须具备一定的孔隙度, 且孔隙中游离态气体占一定比例。这一正演计算对于判别煤层气在抬升过程中是否存在散失非常有效。如果已知储层的孔隙度(基质孔隙度和裂隙孔隙度) 以及储层孔隙中气水的比例, 就可计

算出储层压力。如果理论计算为异常高压, 但实际为低压异常, 说明抬升过程中存在气体的散失; 如果与实际情况一致, 说明系统没有与外界发生物质交换; 相反, 如果计算的为低压异常, 而实际为高压异常, 说明存在压力恢复事件, 有外部物质的注入。 2. 反演计算结果

表2是根据德国鲁尔煤田石炭系煤层现今的有关参数, 反演计算地质历史时期某一状态下的储层压力, 即现今的储层发生沉降, 被埋藏在更深的部位

M

Pa

表2 反演计算获得的抬升前的储层压力数据表

开发试采 天 然 气 工 业 2004年5月

部位的储层压力。由地质演化史分析可知, 在温度为100e 的深部, 储层压力为25. 2MPa

1122

参 考 文 献

1 M atthews W R, Kelly J How to predict formation pressure

and fracture gradient. Oil and Gas Jour nal, 65, 1967Febr u -ary 20:92~1066

2 M agara, K 著, 陈荷立译. 泥质岩压实作用. 北京:石油工业出版社, 1981

3 Eaton B A. A theory on effect of over bur den stress on geo -pressur e prediction from well logs. Soc Petroleum Eng,

1972; 3719:15~22

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well logs. SPE 50th Annual Fall M eeting , Dallas, T X, September 28-October 1. SP E paper #5544, 1975:115 Eaton B A. Gr aphical method pr edicts geopressures world -wide. World Oil, 1976; July:100~1046 Baker C. Primary migr at ion -the importance of water -organic -mineral matter inter actions in the source rock. AAPG Stud -ies in Geology, 1980:10

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and clay content in sandstone. Geophysics, 1989; 54(1) :82

~89

8 孙茂远, 黄盛初等著. 煤层气开发利用手册. 北京:煤炭工

业出版社, 19989 李国富, 雷崇利. 潞安矿区煤储层压力低的原因分析. 煤

田地质与勘探, 2002; 30(4) :30~3210 苏现波, 张丽萍. 煤层气储层异常高压的形成机制. 天然

气工业, 2002; 22(4) :15~18

11 X ianbo Su, L iping Zhang , Ruilin Zhang , T he abnormal pres -sure regime of the Pennsylvanian N o. 8coalbed methane

r eservo ir in L iulin -W upu District, Eastern O rdos Basin, Ch-i na, Inter national Journal of Coal G eolog y, 2003; 53:227~239

12 Littke R , Luckge A , Buker C, Sachsenhofer R F , Welte D

H. A new evaluat ion of palaeo -heat flows and eroded thick -nesses for the Car boniferous Ruhr basin, western Germany. Int er Jour Coal Geol. 1994; 26:155~183

13 G aschnitz R, Krooss B M , Litt ke R. Coalbed methane:ad -sor ptive gas storage capacity of coal seams in the U pper

Carboniferous of the Ruhr Basin, Germany. Procedur es

A mer Assoc. Petr. Geol Eastern Sectio n and the Soc. of O rg. Petrology Joint M eeting, Lex ing ton, Kentucky , 1997; 42~44(ISSN 1060-7250)

(收稿日期 2003-10-20 编辑 黄君权)

, 埋深在

2500m 左右, 处于正常压力状态。计算结果与正演计算基本一致。首先正常压力状态对应的孔隙度和气水比例二者基本一致; 其次正演中的异常高压对应于反演中的异常低压, 这是由于反演计算时采用了8. 0MPa 这一初始压力, 对于正演中的高压情形, 该值较低, 反演计算后当然为异常低压状态; 相反对于正演计算中的异常低压情形, 8. 0MPa 的初始压力就较高, 反演计算后出现异常高压状态。

讨 论

本文根据吸附态、游离态气体和水的状态方程, 结合埋藏史、热史分析, 提出了一种地质历史时期不同演化阶段煤储层压力的计算方法, 并且以德国的鲁尔煤田石炭系煤储层为例, 采用正演和反演两种方法计算了抬升前后储层压力随孔隙度和孔隙中气水比例的变化情况。发现煤储层孔隙度与孔隙中气水的比例决定着抬升或埋藏后储层的压力状态。煤层气储层压力的预测方法目前仍处于探索阶段。本文仅仅是一种尝试, 在许多方面需要改进。如假设条件过多、没有考虑溶解气、没有把甲烷视为真实气体等。结合其它手段的压力测定, 如包裹体测温压, 加上含气量测定, 该方法有望成为压力与含气性动态剖面建立的途径之一, 也是煤层气藏成藏动力学研究的途径之一。

符 号 说 明

Q ) 煤储层密度, t /m 3; U ) 孔隙度, %;

A ) 水的膨胀系数, 取值4@10-4K -1; B ) 水的压缩系数, 取值为3@10-10

Pa -1;

V L1、V L2) 抬升前、后煤储层兰氏体积, m 3/t; P L1、P L2) 抬升前、后煤储层气兰氏压力, M Pa; P 1、P 2) 抬升前、后煤储层压力, M Pa; t 0、P 0) 标准状态下温度(K) , 压力(M Pa) ; t 1、t 2) 抬升前、后煤储层温度, K;

x ) 煤储层孔隙中游离态气体所占比例, %; m ) 煤储层质量, t 。

NA T URA L GAS IND UST R Y /May, 2004

w ar e of numer ic simulat ion, the calculated permeability is 60%~90%smaller than the r eal one w hen the fr ee g as saturation is 0. 05%~0. 15%.In the article, a new mathematical model is established for pressur e -drop test w ith injection by the perco -lation mechanics c theor y considering the influence of free gas and Jiamin effect. Acco rding to the r esults achieved from more t han 40w ells with the new model, t he production practice and t he geo logical evaluation, it is found the results fit the real situa -tion of the coa-l bed. T he method solv es the problem that the permeability calculated w ith the conventio nal interpretation method is smaller.

SUBJEC T HEADINGS :Coa-l bed gas, W ell test, Pr es -sure -drop with injection, F ree g as, Jiamin effect

Liu Lijun (senior engineer ) w as born in 1964. Add:Dow n -hole Service Com. of N orth China Petr oleum Adminis -tration(062552) , China T el:(0317) 2732285

weig hted mean, t he /w ood barrel effect 0, the analog y evaluatio n, and the comprehensive evaluation.

SUBJEC T HEADINGS:Coal reservoir , Parameters and criteria of descript ion, Evaluation content, Evaluation pr inciple Wang Shengw ei (Doctor ) w as bor n in 1956. Add:Wuhan, Hubei(430074) , China E -mail:sww ang@cug. edu. cn

COMPREHENSIVE PREDICTION TEC HNOLOGY OF HIGH PERMEABILITY ZONE FOR C OAL -BED

GAS ) ) ) TAKING

DANING

1)

AND

HANCHENG AREAS AS EXAMPLE

M a Cailin, Li Yanxiang, Quang Haiqi, Li Ning, Cheng Jing (E &D Research Institute of Changqing Oil Field Branch, PCL ). N AT UR . GA S IND. v. 24, no. 5, pp. 85~87, 5/25/2004. (ISSN 1000-0976; In C hinese)

ABSTRAC T:Besides t he restraints of geolog y and reser -voir forming conditio ns, r ich coa-l bed g as zones with high pro -ductiv ity are related to the permeability of the coa-l bed. T he coa -l bed permeability is mainly controlled by the fr actur e sys -tem. Its value can be obtained by the pressure -drop test method w ith injection of the coa-l bed gas well. But t he number of the coa -l bed gas wells is limited in t he w orking area, and the pres -sur-e drop t est method with inject ion i s not perfect now. So, w ith some indirect techniques, such as g eolo gic sect ion observa -tion, acoustical emission test, and r emote sensing interpretatio n, etc. , the 3-in -1compr ehensive prediction model of /sky, sur face and underg round 0is established for high permeability zones of coa -l bed. M ainly according to the omn-i bear ing statist ics of fracture density, it is able to predict the high permeability zones of the coa-l bed, evaluate the hig h pro duction zones of the co a-l bed g as, and select the favor able prospects of the coa-l bed g as. SUBJECT HEADINGS:Geologic inter pretation with re -mote sensing , A coustical emission, Coa-l bed cleat, Per meabil-i ty, F racture density

Ma Cailin (senior engineer ) was born in 1957. Add:Xin -glongyuan Q uarter of Changqing O il Field, Xi c an, Shaangxi (710021) , China T el:(029) 6592432 E -mail:mcl _cq@petrochina. com. cn

RESERVOIR EVALUATION FOR EXPLORATION AND DEVELOPMENT OF COAL -BED GAS 1) Wang Shengwei, Duan Lianx iu, Cheng Zhong hui and Zhang M ing (Resource College of China Geology U niversity ). NAT UR. GAS IND. v. 24, no. 5, pp. 82~84, 5/25/2004. (ISSN 1000-0976; In Ch-i nese)

ABSTRAC T:T he parameters to evaluate coal reservo irs can be obtained from the 4levels, i. e. macroscopic observation of coal reser voirs and coal rocks, sur veying under common m-i croscope, and surveying of super -microscopic pores under elec -tronic micr oscope. T he key link to obtain the evaluation param -eters of coal r eserv oirs is the coal cores fr om coa-l bed gas w ells and the coal rock fr om coal reservoir outcrop. It is necessar y to establish paramet ers and principles of coal r eservo ir description, and g ather systemic data of coal reservoir evaluation for meeting t he requirements of ex plor at ion and development for coa-l bed g as. T he majo r contents of coal reservoir evaluation as fo llows:t he g eometry of coal body and its structure, the features of the top -rock and bottom -rock and the fracture g rowing of t he coal r eserv oir, the pore -fracture system grow ing features and the per -meability of the coal reservoir , the litho -types and macer al of the coal, t he mechanics pro perty of the coal, the de -sor ption charac -teristics of the coa-l bed gas, the mo difiable proper ty of the coal r eserv oir, the metamorphosis type of the coal, and the distr ibu -tion feature of the coal rank, etc. T he basic principles of coal r eserv oir evaluation include the par ameters multiplication, the

PREDICTION OF RESERVOIR PRESSURE FOR COAL -BED GAS

1)

Su Xianbo, Zhang Liping (Jiaozuo Technolog y

NAT URA L GAS IND UST R Y /May, 2004

Institute). NAT UR. GA S IND. v. 24, no. 5, pp. 88~90, 5/25/2004. (ISSN 1000-0976; In Chinese)

ABSTRAC T:Pressure is one of the main control factors for coa-l bed gas ex ist ing. Pressure prediction pr ovides the access to establish t he g as -bear ing section and makes the base to study the r eserv oi -r forming dynamics of coa-l bed gas. According to the state equat ion of gas and water under adsorption and free states, and combining the analysis of bur ial histor y and t hermal histo -ry , the article discusses the calculatio n methods of coal r eserv oir pressure for differ ent evolution phases in t he geologic history. T aking Carboniferous coal reservoir of Lule co al field in G er -many as ex ample, with forw ard and inversion methods, the r eserv oir pressure before and after lifting is calculated as the porosit y and t he g as -water ratio in the por es change. It show s t hat the porosity and the gas -w ater ratio in the pores of t he coal r eserv oir contr ol the r eservo ir pressure before and after lifting (abnormal low pressure, no rmal pressure and abnormal high pressure) .

SUBJECT HEADINGS:Coa-l bed gas, Reservo ir pressure, L ule coal field, Pressure prediction

Su Xianbo w as bor n in 1963. Add:Jiaozuo T echnolo gy Institute, Jiaozuo, Henan (454000) , China 3980901

T el:(0319)

APPLIC ATION OF NUMERAL SIMULATION IN LIULIN TEST ZONE OF COAL -BED GAS

1)

Su Fuyi and Cai Yunfei (E &D Research Inst-i tute of North China Branch, Sinopec ). NA T UR. GA S IND. v. 24, no. 5, pp. 95~96, 5/25/2004. (ISSN 1000-0976; In C hinese)

ABSTRACT:L iulin test zone of coa-l bed gas is the first area to conduct the regular test of small w ell network for co a-l bed g as. T he article describes the geological model of the co a-l bed gas reservoir in t he area. With numer al simulation (soft -ware) of coa-l bed gas, based on the sensibilit y study of some main par ameters and the historical fitting of production situation of the coa-l bed g as r eservo ir, t he productivity of the coa-l bed g as reservoir is predicted. T he results show the av erage daily pro -duction o f a single well is low, and the r ecoverable reserves of the coa-l bed gas reservoir is small. But the reservoir is buried shallow ly. T here is very r ich r esource o f coa-l bed gas on the skirts of the area. T here i s g reat E &D potential for coa-l bed gas if lar ge -scale continuous development is performed. SUBJECT HEADINGS:Coa-l bed gas, Geological model, N umeral simulat ion, Sensitiv ity, Productiv ity prediction Su Fuyi (senior engineer , equiv alent p rof essor ) w as bor n in 1954. A dd:197F uniu Rd. , Zhengzhou, Henan (450006) ,

HYDRAULIC FRACTURING PRACTICE AND C OAL -BED FRAC TURE MODEL SELEC TING FOR C OAL -BED GAS WELLS IN CHINA

1)

China T el:(0371) 8629359 E -mail:sufy_163@163. com

Li Anqi, Cheng Caihong (Langfang Branch, PCL) and Jiang Hai (Jilin Oil Field Branch, PCL). NA T UR. GA S IND. v. 24, no. 5, pp. 91~93, 5/25/2004. (ISSN 1000-0976; In C hinese)

ABSTRACT:Combining theory w ith pract ice, the article descr ibes the influence of coal rock features on the fracture g rowing of coa-l bed hydraulic fracturing, analy zes the monitor -ing results of coa-l bed hydr aulic fractur ing in China, and propos -es t he model suitable to simulate the geometric size of coa-l bed hydraulic fractures. Especially, some new points of v iew about forming complicated fr actur e system in coal beds provides the r eference for the industry to study t he issue.

SUBJECT HEADINGS:Coal bed, Fracturing, F racture, M odel

Li Anqi (senior engineer ) w as born in 1955. Add:P. O. Box 44, Wanzhuang, L ang fang, Hebei(065007) , China T el:(010) 69213510 E -mail:liaq55@vip. sina. com

NUMERAL SIMULATION TECHNOLOGY AND APPLIC ATION OF COAL -BED GAS FIELDS

1)

Chen Caihong, Liu Honglin and Wang Xianhua (Langfang Branch of E &D Research Institute, PCL) . N AT UR. GA S IND. v. 24, no. 5, pp. 97~99, 5/25/2004. (ISSN 1000-0976; In Chinese)

ABSTRAC T:T he output pr ocess o f coa-l bed gas is differ -ent w ith the common natural g as. T he process includes 3steps, i. e. de -sorption, diffusion and percolation. T he numer al simula -tion of coa-l bed gas is a special technology to descr ibe t he pro -cess. T he article introduces the numer al simulation theory of coa -l bed gas and the characterist ics of Coalg as software briefly. T aking the test w ell group of coa-l bed gas in F anzhuang block of Q inshui coa-l bed gas field as ex ample, w ith the history of g as and water production in the block, a geolog ical model closing to the r eal situation is dev elo ped. Based on the model, the opt-i mization of w ell patter n and spacing for production, and the pre -