Thermal Damage of Set Silicate Cement in Ultra-High Temperature Xerothermic Environment
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摘要: 煤炭气化与油页岩原位开采时井下为超高温干热环境，对油井水泥环的热稳定性提出了挑战。对此，探索了600℃长期干热环境下硅酸盐水泥石性能劣化规律，分析了微观结构特征与水化产物变化。研究结果表明，常规硅酸盐水泥石的抗压强度显著下降，孔隙度和渗透率增大，微观结构由凝胶结构转变为粒状结构，氢氧化钙和C—S—H凝胶消失，转变为硅酸二钙-γ、斜硅钙石和钙铁石等相态。加砂水泥石孔隙度与渗透率均随养护时间呈现上升趋势，凝胶结构基本消失，以粒状和微小絮状、针状晶体结构为主，氢氧化钙和C—S—H凝胶消失，转变为大量斜硅钙石，石英参与水化反应程度低，抑制损伤效果不明显，硅酸盐水泥石无法满足原位开采高温干热环境的密封要求。初步探索了SCKL改性硅酸盐水泥与铝酸盐水泥2种体系600℃长期干热环境适应性，铝酸盐水泥有望成为原位开采超高温干热环境固井材料，但仍需进一步研究提高其综合性能。研究结果为基于油页岩与煤炭气化等原位开采工况下封固段地层特性优选水泥浆、改善水泥石综合性能和开发新型耐高温高压固井材料提供参考。Abstract: In coal gasification and shale gas in-situ development, the bottoms of the wellbores are in an ultra-high temperature xerothermic environment, which is of great challenge to the thermal stability of the cement sheaths. To deal with this challenge, the deterioration of set silicate cement long exposed to 600℃ xerothermic environment was studied, and the microstructure features and hydration products were analyzed. It was found in the study that the compressive strength of the common set silicate cement in this environment decreased significantly, and the porosity and permeability of the set cement increased, the microstructure of the set cement turned from gel structure to granular structure, and the calcium hydroxide and C—S—H gel disappeared and changed into dicalcium silicate-γ, larnite and brownmillerite. The porosity and permeability of set sanded cement increased with time of aging, and the gel structure almost all disappeared and the structure of the set cement was finally mainly granular, cotton-like and needle-like crystal. Meanwhile, calcium hydroxide and C—S—H gel disappeared and changed into a large amount of larnite. Quartz, on the other hand, took part in the hydration reaction less intensively and didn’t have obvious effects on inhibiting the damage of set cement. These results show that silicate cement cannot satisfy the sealing requirement in in-situ development of shale gas in high temperature xerothermic environment. In this study, preliminary exploration was conducted on the adaptability of two cements, which are SCKL modified silicate cement and aluminate cement, to a long term 600℃ xerothermic environment, and it was found that aluminate cement can hopefully be used as a cementing material for in-situ development in ultra-high temperature xerothermic environment, further studies need to be conducted to improve its overall properties though. The results of the study have provided references to the selection of cement slurries suitable for cementing the formations in which in-situ development of shale gas and coal gasification are conducted, to the improvement of the overall properties of the set cement, and to the development of new cementing materials for high temperature high pressure well cementing.
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