基于低场核磁共振技术的页岩气储层“焖井”增产机理研究

Large-scale hydraulic fracturing plays a vital role in the development of shale gas. In order to enhance the stimulation effect, a post-fracturing technological operation, called “fractured well shut-in” is utilized both at home and abroad, in which the fractured shale gas well would be shut in for several days and then start the flowback process. However, the mechanism of shale “well shut-in” still remains unknown. There is a lack of research and knowledge to several issues, such as where the fracturing fluid goes during shut-in, the changes in the shale gas property and the pore structure before and after the temporary shut-in, the relationship between such changes and the well production rates, etc. In this project, the mechanism of shale gas “fractured well shut-in” stimulation would be studied in detail by conducting series of laboratory experiments simulation and theoretical analysis based on low-field Nuclear Magnetic Resonance technology. This paper aims to figure out the law of the fracturing fluid migration into shale formation in the form of spontaneous imbibition. Then, it examines the mechanism of gas existing in shale matrix flow to the direction of hydraulic fractures by the fracturing fluid imbibition displacement and induced micro-fractures emerge. The above studies will provide great insights to find suitable shale formation for the “well shut-in” operation and relevant parameters’ design.

大规模水力压裂是页岩气储层成功开发的关键技术。目前国内外均出现页岩气井压裂施工完成后先进行关井，经过一段时间再进行返排的施工工艺（称之为“焖井”），以增强增产效果。然而，页岩气储层“焖井”增产机理尚不清楚，“焖井”期间“压裂液去哪儿了？页岩气储层内部天然气和孔隙结构发生了什么变化以及与增产的关系？”还缺乏相应的理论研究和认识。本项目基于低场核磁共振技术，采用实验和理论分析的手段，研究“焖井”期间压裂液向页岩气储层深处渗吸的规律，探索渗吸压裂液对页岩气微观渗吸置换以及诱导页岩微裂隙生成的机理，从而揭示页岩气储层“焖井”增产机理。以上研究可为页岩气储层“焖井”工艺适用储层选择和施工设计提供理论参考依据。

大规模水力压裂是页岩气储层成功开发的关键技术。目前国内外均出现页岩气井压裂施工完成后先进行“焖井”，经过一段时间再进行返排的施工工艺，以增强压裂增产效果。然而，页岩储层“焖井”增产机理尚不清楚，“焖井”制度的制定缺乏理论依据。本项目通过基于低场核磁共振技术的室内实验和相关理论分析等研究手段，①直接描述了压裂液在页岩孔隙间的运移动态，明确了渗吸压裂液的分布范围，分析了“焖井”阶段压裂液向页岩深处渗吸的规律和影响因素，研究结果表明页岩储层的压裂液渗吸是毛细管力和化学渗透压共同作用的结果，运移距离与压裂液的矿化度、表面活性剂、储层层理方向等相关；②研究了压裂液渗吸置换页岩储层基质天然气机理，研究结果表明压裂液由于与黏土矿物间具有更强的吸附作用，能够置换出页岩内的吸附态甲烷，置换效率与压裂液溶液中的阳离子含量和储层的黏土矿物成分有关；③定量分析水化作用下页岩岩心不同尺度孔隙空间的变化，研究结果表明页岩与压裂液发生水化作用时，各尺度孔隙的体积增加并不是均匀的，而是集中于某一孔隙尺度区域，且与粘土类型相关。该项目的研究阐释清楚了页岩气储层“焖井”增产机理，将为页岩气井的压裂“焖井”增产措施的适应性评价和制度的制定提供理论依据。