膏盐岩脆塑性变形特征及封闭能力定量评价

Brittle Ductile Deformation Characteristics of Anhydrite—salt Rock and Quantitative Evaluation of Its Sealing Ability

膏盐岩盖层作为全球许多含油气盆地最重要的盖层之一，本身物性封闭能力毋庸置疑，野外观察和三轴试验证实随埋深的增加膏盐岩存在脆塑性转化的过程。为了明确其在不同脆塑性域的变形特征和封闭能力定量评价方法，本文以野外露头观测、三轴压缩试验和镜下观察为基础，从变形机制、破裂模式、力学行为和声发射等方面系统总结了不同脆塑性阶段膏盐岩的变形特征。应用莫尔～库伦准则、拜尔利摩擦定律和应力降规律厘定了膏盐岩脆塑性转化的围压和深度。研究结果表明：①针对脆性膏盐岩盖层，首先应用保持力定量评价水力破裂风险性，保持力越小，盖层发生水力破裂的风险就越大。同时利用断接厚度定量表征脆性盖层内断层垂向封闭性；②应用SSF定量评价脆一塑性膏盐岩盖层中涂抹型断裂的垂向封闭性，野外断层解剖确定出膏岩涂抹连续的临界SSF值为3．5—4；③塑性膏盐岩盖层具有流动特征，伴随断层逆冲滑动，膏盐岩沿断面流动，此阶段断层垂向是封闭的，但在差异压实作用下，盐岩流动形成缺失区——“盐窗”可以为油气沿断层向上运移提供通道。因此，膏盐岩盖层封闭性分析要综合考虑盖层脆塑性、不同脆塑性盖层内断裂变形机制及断裂带内部结构，从而完善盖层垂向封闭性评价体系。

Objectives： As one of the most important cap rocks, anhydrite--salt cap rock has excellent capillary sealing ability. The brittle ductile transition of anhydrite--salt cap rock with the increasing depth has been proved by the evidence from field observation and triaxial tests. This study is to determine the deformation characteristics of anhydrite--salt rock and evaluate its sealing ability quantitatively. Methods： Based on the field outcrop observation, triaxial compression tests and microscopic observation, the deformation characteristics of anhydrite--salt rock in different brittle ductile regions were summarized, with the aspect of the deformation mechanism, fracture pattern, mechanical behavior, acoustic emission, etc. Applying Mohr -- Coulomb criterion, Byerlee＇ s friction law and the law of stress dropping, the confining pressure and depth of brittle--ductile transition of anhydrite have been determined. Results： The studies show that ① for brittle anhydrite--salt cap rock, hydraulic fracturing risk shouhl be estimated quantitatively using retention force firstly： the smaller the retention force is, the greater the risk is. Meanwhile, juxtaposition thickness is used for making a quantitative evaluation of the vertical sealing of faults within it; ② SSF is applied to determine the vertical sealing of smearing faults deformed in brittle--ductile anhydrite--salt cap rock, and the field fault anatomy shows that SSF = 3.5 - 4 is the critical value to determine the continuity of anhydrite smearing; ③ ductile anhydrite--salt cap rock has flow characteristics which will flow along the thrust fault plane, where the faults are sealing vertically, but the ＂salt window＂ which is vacancy resulted from differential compaction will contribute to the oil and gas migration. Conclusions： Sealing analysis of anhydrite--salt cap rock must consider brittle ductile deformation of the cap rock, fault deformation mechanism and fault zone internal architecture in different brittle ductile cap rock comprehensively, thereby improving vertical sealing ability evaluation system of cap rock.