砂泥岩间互地层破裂准则选取及在裂缝穿透性评价中的应用

针对目前存在的多种岩石破裂准则,以某油气田裂缝性砂泥岩储层为例,在岩心裂缝观察和岩石力学试验的基础上,通过数理统计和公式推导的方法,建立适用于地下低渗透砂岩和泥岩间互地层的破裂判断准则,并基于构造应力场与裂缝参数之间的定量关系模型,实现砂泥互层时裂缝穿透程度定量评价。结果表明:在压缩状态下,砂泥岩间互地层破裂判据适用两段式莫尔-库仑准则,但在拉张应力状态下,莫尔-库仑准则不适用,改用格里菲斯准则;砂、泥岩力学参数存在差异,同样受力变形条件下,裂缝先在砂岩中产生,再向泥岩中垂向延伸扩展,并发生一定产状的变化;泥岩厚度为5 m是裂缝能否穿透的临界值,多数裂缝会终止于砂泥岩界面处;相同的受力条件下砂岩越薄越容易产生裂缝,泥岩厚度达到20 m时砂岩厚度的变化对砂岩裂缝延伸进泥岩中的发育程度和裂缝密度影响很小。

Best selection among crack criterions to sandstone-mudstone formation and its application to fracture penetration ability evaluation

Based on the present rock crack criterions,cores observation and rock mechanic tests,a new effective rock failure criterion for interbedded sand and mud stone formation in a certain fractured oil and gas field was developed through the method of mathematical deduction.The penetration degree of fractures in mudstone was evaluated based on the quantitative model between tectonic stress and fracture parameters.The results show that for the compressed conditions,multibowl Koulomb-Mohr criterion will be selected to study the formation,but for the tensile conditions,Griffith criterion should be selected.With obvious difference in mechanical properties of sandstone and mudstone,fractures will form in sandstone firstly,then extend into mudstone in vertical direction with variable occurrence.The critical thickness of mudstone is 5 m whether fracture could penetrate the mudstone,and most of fractures will end at the lithologic interface.For the same conditions,the thinner sandstone is easy to generate fractures.When the sandstone thickness reaches 20 m,the thickness variation of sandstone has little influence on fracture’s development and penetration degree among mudstone.