薄互层型页岩油储集层水力裂缝形态与支撑剂分布特征

Hydraulic fracture geometry and proppant distribution in thin interbedded shale oil reservoirs

选用准噶尔盆地吉木萨尔凹陷二叠系芦草沟组页岩油储集层井下岩心制备薄互层状页岩岩样,开展小尺度真三轴携砂压裂实验,结合高精度CT扫描数字岩心模型重构技术,研究了薄互层型页岩油储集层水力裂缝形态与支撑剂分布特征。研究表明:薄互层型页岩油储集层中近井筒处层间岩石力学差异及界面对缝高的延伸无明显遮挡作用,但对缝高方向上缝宽的分布有显著影响,水力裂缝趋于以“阶梯”形式穿层扩展,在界面偏折处缝宽较窄,阻碍支撑剂垂向运移,穿层有效性差;如泥页岩纹层发育,则易于形成“丰”或“井”字形裂缝。射孔层段岩石强度大,破裂压力高,则主缝起裂充分,缝宽较大,整体加砂较好;射孔层段强度低且纹层较为发育,则压裂液滤失量较大,破裂压力较低,主缝起裂不充分,缝宽较窄,易出现砂堵。支撑剂主要铺置在射孔层段附近缝宽较大的人工裂缝的主缝内,分支缝、邻层缝、开启的纹层缝内仅含有少量(或不含)支撑剂,整体上支撑剂铺置范围有限;支撑剂可进入裂缝的极限宽度约为支撑剂粒径的2.7倍。

Small-scale true triaxial sand fracturing experiments are conducted on thin interbedded shale samples made from cores of Permian Lucaogou Formation shale oil reservoir in Jimsar sag,Junggar Basin.Combined with high-precision CT scanning digital core model reconstruction technology,hydraulic fracture geometry and proppant distribution in thin interbedded shale oil reservoirs are studied.The research shows that:In thin interbedded shale oil reservoir,the interlayer difference of rock mechanics and the interlayer interface near the wellbore cannot restrain the growth of fracture height effectively,but has a significant impact on the fracture width distribution in the fracture height direction.Hydraulic fractures in these reservoirs tend to penetrate into the adjacent layer in“step-like”form,but have a smaller width at the interface deflection,which hinders the transport of proppant in vertical direction,resulting in a poor effect of layer-crossing growth.In shale layers with dense laminae,hydraulic fractures tend to forming“丰”or“井”shapes.If the perforated interval is large in rock strength and high in breakdown pressure,the main fracture is fully developed initially,large in width,and supported by enough sand.In contrast,if the perforated interval is low in strength and rich in laminae,the fracturing fluid filtration loss is large,the breakdown pressure is low,the main fracture will not open wide initially,and likely to have sand plugging.Proppant is mainly concentrated in the main hydraulic fractures with large width near the perforated layer,activated laminae,branch fractures and fractures in adjacent layers contain only a small amount of(or zero)proppant.The proppant is placed in a limited range on the whole.The limit width of fracture that proppant can enter is about 2.7 times the proppant particle size.