液氮接触页岩表面淬火传热特性实验

Experimental Study on Cryogenic Queching Characteristics of Rock Surface in Liquid Nitrogen

液氮压裂改造页岩储层技术涉及液氮与储层岩石之间的淬火传热现象,且岩石具有复杂多样的表面形貌。为研究液氮接触页岩过程中的淬火特性,分别制备不同表面结构的岩样进行淬火实验,并对页岩表面进行接触角、电镜扫描和3D表面形貌测试。结果表明,岩石经36目粗砂纸打磨后,表面具有较高的表面自由能,且粗糙度明显提高,因而产生较高的莱登佛罗斯特点温度。电镜结果表明,岩石表面呈现多尺度孔穴结构,这为气泡核化提供了大量活化核心,削弱气膜的稳定性。当壁面温度从室温降至-100℃,刻痕结构和铺砂表面与基准面相比将淬火时间分别缩短了20%和30%,显著提高淬火传热速率。刻痕和砂粒增加了岩样表面积,产生更频繁的间歇性固-液接触,因此能够在较高的过热度条件下进入过渡沸腾阶段,从而提高表面换热能力。

During shale reservoir reformation,liquid nitrogen fracturing involves quenching heat transfer between liquid nitrogen and reservoir rock which generally presents complicated surface morphology.The shale samples with different surface structures were prepared to investigate the characteristics of quenching heat transfer during contact with liquid nitrogen.The contact angle,scanning electron microscope(SEM)and 3 D surface morphology test were performed.The sample polished by 36-grit sandpaper has larger surface energy and roughness,consequently resulting in larger LFP temperature.The scanning electron microscope(SEM)results showed that numerous pores and cavities with multi-dimensions were present on rock surface.This kind of hollow structure provides plenty of nucleation sites for bubble formation and weakens the stability of bubble film.Compared with smooth surface,the surfaces with grooves and sands shorten quenching time with 20%and 30%respectively when surface temperature decreased from ambient temperature to-100°.Larger surface area through grooves and sand particles can cause intermittent solid-liquid contact more frequently and enter into transition boiling regime under higher wall superheat condition thereby accelerate quenching heat transfer rate.