Optimization of Well Completion Method and Casing Design Parameters to Delay Casing Impairment Caused by Formation Slippage

摘要

There are often plenty of horizontal planes of weakness in reservoir formations, especially in shale formations, as reported for a number of oilfields. Once the weak-plane fails, the formation will become unstable, and can easily undertake slippage across a large area along its interface. The number of casing failure caused by slippage of weak-plane has been increasing significantly in recent years. Wells with casing failure are concentrated in an increasing number of areas. However, there has been lack of research efforts on how to optimize cementing and completing parameters in order to prevent casing failure induced by formation slippage. To address the problem, a more advantage completing type has been elected by qualitative analysis. The calculation model of critical slip displacement in un-cemented conditions was established. A finite model was used to test and verify the analysis and the model. The critical slip displacement of casing shear damage was also calculated. In this study, a new cementing practice was then proposed by optimizing casing parameters according to API standards, and a new research method was also put forward by proposing new casing materials to effectively mitigate casing failure caused by formation slippage for the future. Modeling results indicate casing failure induced by formation slip is different from conventional casing damage. The slip displacement needs to be used to measure casing impairment inside of maximum stress. Casing elongation is the key parameter for controlling casing shear failure. The type that keep the weak-plane un-cemented exhibits a larger critical slippage displacement .So the casing with lower grade and smaller thickness is recommended in weak-plane if the casing could meet all other down-hole requirements. The new concept is very different from the common belief that the good quality cement and higher grade and thicker casing are safer. If the elongation of casing can be improved by 60%, the critical casing failure slippage displacement can be increased by 21.40%. In this study, a new casing design and well completion method to prevent casing failure caused by formation slippage was proposed, and some guidance was provided for manufacturing casing with new material that can effectively mitigate or delay casing damage.