Numerical Experiments of Gas Injection Pressurization for CBM Open Hole Cavity Completion
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摘要: 煤层气裸眼洞穴完井在我国的应用效果尚不理想,分析认为,这与我国对该完井技术的增产机理研究不够深入有一定关系。考虑煤岩特有的垂直裂缝系统,基于UDEC软件建立了模拟煤层气裸眼洞穴完井注气增压过程的离散元数值试验模型,模拟了不同地应力条件下,气体注入增压过程中井筒周围煤岩有效应力、孔隙压力和节点位移矢量的分布情况。研究表明:各向异性储层比各向同性储层更容易产生破裂和坍塌,从而更利于洞穴的形成;在垂直裂缝系统中,流体运移速率不同引起的有效应力差异梯度有利于剪切破裂;节点位移矢量分布图能够反映注气增压过程中张性裂缝的形成和发展趋势。所建模型在山西寿阳区块煤层气空气造穴试验井——FCC-HZ04-V井进行了应用,初步预测得到该井在空气造穴后注气增压引起井筒周围张性裂缝延伸25 m,井筒周围25 m范围内煤层平均渗透率较初始渗透率增大7倍,验证了煤层气裸眼洞穴完井注气增压可产生自支撑张性裂缝、消除储层伤害、实现增产的机理。Abstract: The stimulation of CBM cavity wells are remarkably successful in the San Juan Basin of the United States while it is not encouraging in improving CBM production in China.Considering its application prospect in the low rank coal seam,the mechanism of openhole cavity completion should be studied further.A discrete element numerical model simulating the process of gas injection for coalbed methane openhole cavity completion was built based on UDEC software.Analysis showed that: anisotropic coal reservoirs are more prone to rupture and collapse than those isotropic ones.In the vertical fracture system,the discrepancy in the fluid velocity would lead to differences in effective stress gradient and help to generate shear fractures.Tensile fractures’ generation and growing trend could be reflected by nodal displacement vector distribution.Combined with Shouyang cavity well field test,preliminary forecast showed that the tensile fracture would extend about 25 meters around the wellbore,the average permeability of this tensile fracture influenced area wa seven times lager than the initial permeability.
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