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渗透水化对饱和页岩吸水的影响
冷静怡,王琳琳
金沙城娱乐最新官方网站(北京) 安全与海洋工程学院,北京 102249
The osmotic effect on water imbibition in saturated shale formation
LENG Jingyi, WANG Linlin
College of Safety and Ocean Engineering, China University of Petroleum-Beijing, Beijing 102249, China

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摘要  现场统计资料显示,页岩储层压裂施工后大量压裂液无法返排至地面。这些压裂液在地层中的滞留问题 引发了环保及生产相关的争议。一些学者怀疑这些含有化学添加剂的漏失液会迁移到地下水层污染地下水。同 时由于页岩吸水膨胀,一些学者认为漏失液会引起近井地带渗透率降低从而使油气井生产一段时间后产能骤降。 但也有现场研究表明这些漏失液在焖井一段时间后会引起气井初期产气量增加。为了清楚地解释这些问题,学 者们注意到了页岩中一种独特的吸水机理——渗透水化。渗透水化作用导致页岩中的渗流过程除了受注入压差 的影响,还会受到孔隙中水溶液的离子浓度的影响,使得原有的达西公式不再适用于页岩地层中。本研究通过 对达西公式进行修正,建立了考虑渗透水化的饱和页岩吸水的数学模型,并给出了在常见初始、边界条件下的 一维模型的解析解,得到了在页岩吸水过程中,其孔隙流体压力、浓度的变化。研究发现渗透水化的存在会对 水压的变化产生影响,即两种吸水方式间存在耦合关系。也就是说,渗透水化除了通过浓度差引起页岩吸水, 也会通过影响水压进而影响吸水。这两种吸水过程作用方向相反且在很大程度上相互抵消,故对总吸水量影响 不大。此外,在页岩吸水的初期,吸水量的增长主要由水压传递引起;而在吸水后期,吸水量的变化主要由渗 透水化导致。由于浓度扩散的速度远远低于压力传递的速度,渗透水化的存在会显著延长渗流平衡时间。
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关键词 : 页岩气井压裂;渗透水化;页岩吸水量;平衡时间
Abstract
Field statistics have shown that most fracturing fluid would be lost in shale formations after hydraulic fracturing treatment. This fracturing fluid retention in formations has led to some production and environmental issues. Some scholars suspected that the water with chemical additives might migrate to drinking water formations and contaminate them. Some also assumed that this imbibed water might lead to the swelling of shales, which may damage the permeability in the near wellbore area and, in turn, harm the production of the shale gas and oil wells in the long term. But some field studies showed that with a relatively long    shut-in time period, the water leakage would benefit the production by enhancing the flow rate of shale gas at the early stage. To    further explain the phenomena observed in fields and to end those debates, many scholars have paid attention to a special water    imbibition mechanism working in shale formations: osmosis. The existence of osmosis makes the water imbibition process in    shales not only related to the injection pressure, but also the concentration distribution in the porous medium, and accordingly    Darcy’s law no longer holds in shale formations. So in this paper, in order to analyse the water flux influenced by osmosis in    saturated shale formations, a mathematical model established by adding an additional term in the conventional Darcy’s law is    employed. Ananalytical solution of the corresponding 1-D model is given under typical initial and boundary conditions, and    the evolution of pressure and concentration distribution of the pore fluid in shales during the process are obtained. According to    the solution, the existence of the osmotic effect will influence the hydraulic pressure propagation, which means that the osmotic    effect couples with the Darcy flow. That is to say, it would cause additional water flux not only because of the concentration    difference, but also because it can disturb the evolution of hydraulic pressure and thus influence the water imbibition. What    is more, since these two types of water flux occur in opposite directions and compensate for each other to a great extent, the    contribution of osmosis to the amount of water accumulation is very limited. Besides, the result shows that the early-stage water    flux is generated by the initial hydraulic pressure difference, while the late water imbibition period is dominated by the osmotic    effect. So, due to the generally low rate of the concentration diffusion compared to that of the pressure propagation, the existence    of the osmotic effect will prolong the water imbibition process significantly.  


Key words: hydraulic fracturing in shale gas wells; osmotic effect; water uptake in shale; balance time
收稿日期: 2020-12-30     
PACS:    
基金资助:国家自然科学基金青年科学基金项目“不同含水条件页岩力学变形、损伤及破坏机理多尺度研究”(51809275) 资助
通讯作者: linlin.wang@cup.edu.cn
引用本文:   
LENG Jingyi, WANG Linlin. The osmotic effect on water imbibition in saturated shale formation. Petroleum Science Bulletin, 2020, 04: 560-566.
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