基于气体扩散能力的页岩储层损害评价方法

发布时间：2018-04-30 16:05

  本文选题：页岩 + 页岩气　； 参考：《西南石油大学》2015年硕士论文

【摘要】：页岩气藏属烃源岩气藏,储层中含大量吸附气,孔隙尺寸跨度大,有机孔、无机孔和微裂缝构成了一个极为复杂的页岩气储集和流动空间。因而,页岩气的产出是-个包括了解吸、扩散和渗流的多尺度传质过程。页岩基块中的气体扩散作为这一过程的中间环节,研究气体扩散行为对认识页岩气的产出机理具有重要意义。同时,针对页岩气井水力压裂后压裂液返排率低、滞留严重的现象,开展压裂液对页岩中气体扩散能力的潜在损害研究亦十分必要。 以四川盆地志留系龙马溪组井下页岩为研究对象,以页岩中气体扩散能力实验描述为核心,建立了符合页岩气产出过程的扩散行为表征模型,设计了扩散系数实验测试方法；对比分析了压裂液自吸损害前后,甲烷在岩样中的扩散系数变化,并给出了损害评价指标和预防控制措施。 分析表明,龙马溪组井下页岩(15组)矿物组成以石英、长石和黏土矿物为主,岩样的平均孔隙度为3.45%,平均渗透率为0.056mD。储层有机质纳米孔发育,孔隙直径主要分布在3～100nm,其中直径小于10nm的孔隙控制着50%-80%的孔隙空间。 明确了气体在页岩中的扩散类型和扩散阶段。根据经典的Kn数气体流态划分标准,页岩基块中的气体扩散以Knudsen扩散为主。进一步,结合页岩的微孔结构特征和赋存状态,扩散过程可分为滑脱流中的Knudsen扩散、纳米级孔隙气体扩散和气体向干酪根内部的浓度扩散三个阶段。 分析了KKnudsen扩散对页岩中气体滑脱流的贡献率。推导了Knudsen扩散等效渗透率表征模型,并以甲烷低速渗流实验数据验证了模型的可靠性。通过引入气测渗透率修正系数F,定量描述了KKnudsen扩散对整个传质过程的贡献。随着甲烷压力和孔隙直径的降低,F值可从略大于1升高到10以上。F的值越大,则表明Knudsen扩散的贡献率越大。 建立了基于气体压力衰减的甲烷扩散系数实验测试和损害评价方法。根据9组实验数据,损害前阶段Ⅰ(纳米级孔隙气体扩散)中甲烷的扩散系数为3.78×10-13～7.04×10-13cm2/s,阶段Ⅱ(干酪根中气体扩散)中甲烷的扩散系数为2.89×10-17～3.29×10-16cm2/s；压裂液自吸损害后,扩散系数明显降低,阶段Ⅰ的损害指数为0.18-0.65,损害程度为弱～中等；阶段Ⅱ的损害指数为0.81-0.98,损害程度为强～极强。 讨论了压裂液致页岩中气体扩散能力损害的预防和控制措施。压裂液配方优化、氮气伴注等工艺等能从减弱压裂液自吸,促进返排等方面弱化压裂液对气体扩散能力的损害；非水基压裂和高温热处理等则能以另一种技术思路抑制压裂过程中的水相圈闭等储层损害,且具有在更小尺度上改善气体在页岩基块中扩散传质能力的作用。
[Abstract]:Shale gas reservoir belongs to source rock gas reservoir, which contains a large amount of adsorbed gas, large pore size span, organic pore, inorganic pore and micro-fracture, which constitute a very complicated shale gas reservoir and flow space. Thus, shale gas production is a multi-scale mass transfer process involving the understanding of absorption, diffusion and seepage. Gas diffusion in shale block is the intermediate part of this process. It is very important to study the gas diffusion behavior for understanding the mechanism of shale gas production. At the same time, it is necessary to study the potential damage of fracturing fluid to the gas diffusion ability of shale because of the low rate of back discharge and serious retention of fracturing fluid after hydraulic fracturing in shale gas wells. Taking the downhole shale of Silurian Longmaxi formation in Sichuan Basin as the research object and taking the experimental description of gas diffusion capacity in shale as the core, a diffusion behavior characterization model in accordance with the process of shale gas production is established, and the experimental test method of diffusion coefficient is designed. The variation of methane diffusion coefficient in rock samples before and after self-suction damage of fracturing fluid is analyzed, and the damage evaluation index and preventive measures are given. The analysis shows that the mineral composition of the downhole shales of Longmaxi formation is quartz, feldspar and clay minerals. The average porosity of rock samples is 3.45 and the average permeability is 0.056 mD. The pore diameter of the reservoir is mainly distributed in 3 ~ 100nm, and the pore size less than 10nm controls 50 to 80% of the pore space. The diffusion type and diffusion stage of gas in shale are defined. According to the classical Kn number gas flow state classification standard, the gas diffusion in shale block is dominated by Knudsen diffusion. Furthermore, the diffusion process can be divided into three stages: Knudsen diffusion, nano-pore gas diffusion and gas concentration diffusion to the interior of kerogen. The contribution of KKnudsen diffusion to gas detachment in shale is analyzed. The Knudsen diffusion equivalent permeability model is derived, and the reliability of the model is verified by the experimental data of methane percolation at low speed. The contribution of KKnudsen diffusion to the whole mass transfer process is quantitatively described by introducing the gas permeability correction coefficient F. With the decrease of methane pressure and pore diameter, the larger the value of F can be from slightly greater than 1 to more than 10, the larger the contribution of Knudsen diffusion is. An experimental test and damage evaluation method for methane diffusion coefficient based on gas pressure attenuation is established. According to the experimental data of 9 groups, the diffusion coefficient of methane in phase I (nano-pore gas diffusion) was 3.78 脳 10 ~ (-13) (7.04 脳 10 ~ (-13) cm ~ (2) / s, and that in stage 鈪，

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