固井水泥浆侵入对近井壁水合物稳定的不利影响

中国青藏高原多年冻土区蕴藏有丰富的天然气水合物资源，在此区域进行常规油气固井时产生水合物的几率较大。在井内压差作用下，固井水泥浆容易侵入含水合物地层内部并贴近水合物水化放热，改变水合物相平衡条件并诱发水合物分解，导致固井质量降低甚至失效。以青藏高原木里地区含水合物多年冻土层为研究对象，采用室内模拟实验和数值模拟计算相结合的方法，研究了固井水泥浆侵入对近井壁地层中水合物的影响。研究结果表明，在多年冻土区水合物地层进行常规油气固井作业时，渗入井周地层的固井水泥浆只需22 min即可导致其渗透距离内的水合物发生不同程度的分解，且距离井壁约1倍井径的范围内的水合物将全部分解，直接导致井壁失稳和固井质量低劣。

Negative effect of cementing slurry invasion on gas hydrate stability around borehole wall

The permafrost region in Qinghai-Tibet Plateau (China)is abundant in natural gas hydrate resources, where there is a great probability of encountering gas hydrate during conventional oil and gas well cementation operations. Under the intra-well pressure difference, it is easy for cementing slurry to invade into the hydrate-bearing formation and adjoin the hydrate for hydration exothermic action, so as to change the hydrate phase balance conditions and induce hydrate decomposition, thus leading to cementing quality reduction or even failure. In this paper, the permafrost region of Muli area in the Qinghai-Tibet Plateau is taken as the study object. Using indoor simulation experiment in combination with numerical simulation, this study analyzes the influence of cementing slurry invasion on the hydrate in the formation near the borehole wall. The results show that during the conventional oil and gas cementing operations in the permafrost region of the Qinghai-Tibet Plateau, it takes only 22 minutes for the cementing slurry invading into the formation near borehole to cause different degrees of hydrate decomposition; within the range of well diameter, the hydrate will be fully decomposed, leading to the instability of borehole wall and the poor quality of cementation.