摘要
煤层气井排采过程中,煤储层原始地应力场发生调整,易导致储层失稳破坏,影响煤层气井抽采效率。基于单轴应变模型,考虑基质收缩效应,推导了排采过程中煤储层地应力动态模型,揭示了地应力动态变化规律。此外,建立了排采过程中煤储层稳定性分析模型,提出了煤储层峰值强度与最大主应力的差值(H)作为煤储层稳定评价基本参数,讨论了不同应力机制下地应力动态变化对煤储层稳定性的影响。研究表明:水平主应力在排水降压阶段呈线性降低,在产气阶段受基质收缩效应的影响呈非线性下降,解吸作用越强,下降速率越快。不同应力机制煤储层失稳破坏规律不同。正断层应力机制,排水降压和产气阶段,莫尔圆半径增大,圆心右移,接近破坏包络线,煤储层可能失稳破坏,解吸作用越强,莫尔圆半径增大速率越快,越容易破坏;走滑断层应力机制,排水降压和初始产气阶段,莫尔圆半径不变,向右平移,远离破坏包络线,煤储层稳定性增强,不发生破坏,而稳定产气及衰减阶段,莫尔圆向左平移,靠近破坏包络线,煤储层稳定性减弱,可能失稳破坏;逆断层应力机制,整个排采过程莫尔圆半径不断减小,远离破坏包络线,煤储层稳定性增强,不发生破坏。结合郑庄区块地应力测试井数据,对煤储层稳定性进行了动态评价。郑庄区块实测数据分析表明:排采前,郑庄区块煤储层H多在32~40 MPa,储层稳定性中等,由中部向西北和南部储层稳定性增强,而向西部储层稳定性减弱。排水降压阶段,区块各井H呈线性升高,储层稳定性好或中等;产气阶段各井H加速下降,储层稳定性减弱。当孔隙压力降为0.2 MPa时,区块煤储层稳定性由中部向西部逐渐减弱。郑庄区块排采曲线显示部分煤层气井产气峰值不稳定,持续时间较短,井底压力降到一定值时,日产气快速下降,产出大量煤粉,推测在较低井底压力条件下,煤储层可能发生了剪切破坏。煤层气井合理排采工作制度制定时应重点关注储层的稳定性。
The original in-situ stress varies during the depletion of coalbed methane(CBM)reservoir,which may lead to the failure of coal rocks and significantly reduce the CBM recover efficiency.In order to analyze the var-iation law of in-situ stress during the CBM reservoir depletion,a theoretical stress-depletion response model was derived based on a uniaxial strain model.Besides,a stability analysis model during depletion was built and an evaluation index(H)was proposed to evaluate the CBM reservoir stability under different in-situ stress circum-stances.This research indicates that:①In the drainage stage,with the drop of the pore pressure,the horizontal principal stress decreases linearly.However,in the gas production stage,under the strong matrix shrinkage effect,the decrease rate is enhanced.②The failure laws of CBM reservoirs under different in-situ stress regimes are different.For the normal faulting regime,in the drainage stage or gas production stage,the radius of Mohr circle rises and the center moves right.In this process,the Mohr circle approaches the Mohr failure envelopes and the reservoir may be damaged.With the increase of desorption effect,the rise rate of Mohr circle radius enhance and the coal reservoir is prone to destruction.For the striking-slip faulting regime,in the drainage stage and initial gas production stage,the Mohr circle radius remains constant and its center moves right,away from the Mohr failure envelopes.The reservoir remain stable.While the reservoir failure may occur in the stable gas production stage and the gas rate decay stage due to the strong matrix shrinkage effect which cause the Mohr circle moves left and approaches the Mohr failure envelopes.For the thrust faulting regime,the Mohr circle shrinks during the whole depletion process.The reservoir will not be damaged.③According to the data of in-situ stress testing wells in the Zhengzhuang Block,the reservoir stability was evaluated.Before CBM depletion,the H value mainly is between 32-40 MPa and the reservoir remains moderately stable.The reservoir stability reduce from the middle to the northwest and south region of Zhengzhuang block.In the drainage stage,the H value will rise linearly with the decrease of pore pressure and the reservoir remains highly or moderately stable.In the desorption stage,the H value declines rapidly and the reservoir stability is reduced.It is notable that as the pore pressure drops at 0.2 MPa,the H value declines gradually from the middle region to the western region of Zhengzhuang Block.④The gas production curve of some CBM wells shows that gas production rates were unstable and the duration of gas production peaks were short.When the reservoir pressure decreased to a relative low value,the gas production rate declined rapidly and coal fines can be monitored.It inferred that the CBM reservoirs may be damaged in the process of depletion.
作者
章朋
孟雅
刘超英
闫相宾
蔡利学
程喆
ZHANG Peng;MENG Ya;LIU Chaoying;YAN Xiangbin;CAI Lixue;CHENG Zhe(Research Institute of Petroleum Exploration & Development,SINOPEC,Beijing 100083,China;School of Energy Resources,China University of Geosciences(Beijing),Beijing 100083,China)
出处
《煤炭学报》
EI
CAS
CSCD
北大核心
2022年第4期1620-1628,共9页
Journal of China Coal Society
基金
国家自然科学基金资助项目(42002181)
2020年度山西省科技计划揭榜招标资助项目(20201101002-03)。
关键词
煤储层
地应力
稳定性
煤层气井排采
郑庄区块
coal reservoir
in situ stress
stability
CBM well depletion
Zhengzhuang Block
