Abstract:

Rich in natural gas resources，Misui Block is a key strategic area for accelerating the production of Changqing Oilfield. However，the frequent occurrence of lost circulation in recent years has seriously restricted improvement of cost-efficiency. Therefore，to curb the occurrence of well lost circulation，the lost circulation mechanism in this area has been studied through multi-scale quantitative experiment on rock physical property，microelectrode imaging logging observation and geo-mechanical profile analysis of wells. The results show that the thief zones in Misui Block are mainly in Liujiagou and Shiqianfeng Formations，which have large numbers of micro- and nano-scale fractures. The average calculated dynamic Yang's modulus of the cores from these formations are 17.94 GPa and 8.47 GPa，respectively，and the average dynamic Poisson's ratios are 0.394 and 0.064，respectively. The clay mineral content of Liujiagou Formation is more than 50%，and the brittle mineral content of Shiqianfeng Formation is as high as 72%. The imaging logging map shows that the area contains tensile fractures. The single well geo-mechanical profile reveals a low-pressure area in the thief zone，where the density of drilling fluid is 1.18 g/cm³. Finally，analysis shows that the high contents of clay and brittle minerals in the formation caused both hydration fractures and structural fractures，with low formation pressure causing drilling fluid to flow into natural fractures，eventually resulting in lost circulation. The research results not only reveal the lost circulation mechanism at Misui Block，but also provide technical guidance for selecting plugging methods and reducing well lost circulations at Misui Block.

Key words:

lost circulation mechanism, rock mechanics parameter, scanning electron microscope, imaging logging, geo-mechanical analysis, drilling fluid

摘要：

米绥新区天然气资源丰富，是长庆油田快速上产的重要战略接替区，但近几年井漏问题的频繁出现已经严重制约效益开发的进程。为了遏制井漏的发生，通过多尺度定量岩石物性实验、微电极成像测井观测和单井四压力剖面分析对该区域的漏失机理展开了研究。结果发现米绥新区的漏层主要在刘家沟组和石千峰组，漏层存在大量微纳米级裂缝，岩心平均动态杨氏模量分别为17.94 GPa、8.47 GPa，平均动态泊松比分别为0.394、0.064；刘家沟组地层的黏土矿物含量超过一半，石千峰组的脆性矿物含量高达72%，成像测井图显示该区域含有张性缝；单井四压力剖面揭示漏层存在低压区，而钻井液密度为1.18 g/cm³。最终分析得出高含量的黏土矿物与脆性矿物造成地层内部容易形成水化裂缝和构造裂缝，同时地层压力较低，导致钻井液流向地层中的天然裂缝，最终发生漏失。研究成果不仅揭示了米绥新区的漏失机理，也为后续选择堵漏方法及减少米绥新区井漏事故提供技术指导。

关键词:

漏失机理, 岩石力学参数, 扫描电镜, 成像测井, 四压力剖面, 钻井液