Changes of Mechanics and Characteristics of Porosity and Permeability of Set Cement at High and Ultra-High Temperatures
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摘要: 页岩油原位注热开采过程中，井筒温度处于高温或者超高温状态，容易导致水泥环密封完整性失效，开展水泥石在高温及超高温下的力学特性和孔渗特征变化规律极为重要。对此，研究了不同温度热处理条件下水泥石力学特性的变化，基于数字图像相关技术，分析了高温处理后水泥石破坏模式的变化，采用低场核磁共振技术对于热处理后水泥石孔隙度和渗透率的变化规律进行了研究，并分析了孔隙度与渗透率关系的相关性。研究结果表明，对于高温处理后的水泥石试样，随着热处理温度的不断增加，水泥石单轴压缩强度先增加、后减少。试样的孔隙度和渗透率不断增加，其规律为先缓慢增加、后快速增加，主要是因为热处理温度较低时，产生的主要是微、小孔隙且并未发生连通；随着热处理温度的不断提升，部分微、小孔隙聚合成为中、大孔，且不同级别的孔隙之间出现连通，进而导致渗透率显著升高。研究结果对于页岩油原位注热开采过程中水泥环密封完整性的保护以及水泥浆配方的优化具有重要的参考意义。Abstract: In shale oil production using in-situ thermal recovery method, the temperature in the borehole is high or even ultra-high, and it is easy to result in failure of the sealing integrity of the cement sheath. It is thus important to study the changes of the mechanical characteristics and the porosity and permeability of a set cement under high or ultra-high temperatures. Using digital image technology, changes of the failure mode of a set cement after high temperature treatment is analyzed. Using low field NMR, the changes of the porosity and permeability of a set cement after high temperature treatment are studied, and the correlation of porosity and permeability is analyzed. The study shows that for a high temperature treated set cement sample, as the temperature increases, the monoaxial compressive strength of the set cement first increases and then decreases. The porosity and the permeability of the set cement increase continuously; at first, the increase goes slowly and then goes fast. This is because at lower temperatures, there are only micro and small pores produced inside the cement and these pores are not inter-connected with each other. As the temperature increases, some micro and small pores combine into medium and large pores, and pores of different sizes begin to connect with each other, thereby increasing the permeability of the set cement. The achievements of the study have important reference significance for protecting the sealing integrity of cement sheath and optimizing cement slurry formulation in in-situ thermal recovery of shale oil.
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Key words:
- High temperature /
- Set cement /
- Low field NMR /
- Digital image
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