Identification of the Location of the Leakage Layer Under the Condition of Coexistence of Circulation Leakage and Blowout: A Case Study of a Block in the Bohai Sea
Authors: Yu Qinxin , Yan Wei , Wang Kongyang , Feng Yongcun , Ma Zelin , Luo Liming , Geng Lijun , Li Xiaobo 
Published: September 2022

ABSTRACT:
The problem of coexistence of circulation leakage and blowout in drilling is difficult in drilling technology. Rapid and accurate identification of the location of the leakage layer is the key to taking reasonable measures to solve the problem. field parameter calculation method and Logging data analysis method are used in this paper. The logging data analysis is to use the comparative analysis method combined with the geological logging data and drilling engineering data, such as gamma-ray (GR), spontaneous potential (SP), density (DEN), etc. to analyze the location of the leakage layer. The field parameter calculation method is to estimate the leakage layer according to the field parameters such as the displacement of the wellhead pump, the density of drilling fluid, and the cross-sectional area of pipe string when the loss occurs. The two methods are used to analyze and calculate a well that encountered the situation of coexistence of circulation leakage and blowout in a certain block of Bohai Sea. The field parameter calculation method can estimate the location of the leakage layer more quickly when leakage occurs, which is beneficial to the field plugging operation in time. The logging data analysis method can obtain the location of suspected leakages at an earlier time more completely. Combining the two methods, the possible locations of the leakage layer are predicted by the logging data method, and the field parameter calculation method is used to estimate the exact location and compared with the predicted locations of the leakage layer, a more accurate location is obtained, which is of great significance to solve the problem of coexistence of circulation leakage and blowout in the field quickly.

1. INTRODUCTION
The problem of lost circulation in drilling is a technical problem that puzzles the field operation for a long time. It leads to the extension of the drilling cycle and the increase in drilling cost, which brings great challenges to the drilling operation. Most studies on the mechanism and model of leakage point out that fracture is the main channel of reservoir leakage. At present, the main solution is fracture plugging to improve the formation pressure bearing capacity. A comprehensive index formula of lost circulation risk assessment was constructed to reflect the level of lost circulation risk(Lu et al., 2020). Tan et al. (2021) believe that the smaller the borehole diameter, the greater the bottom hole pressure, and the higher the risk of leakage in fractured formation. Chen et al. (2021) comprehensively analyzed the leakage of a well in the Bohai oilfield in recent years. They summarized the plugging decision-making system suitable for fault leakage and igneous rock leakage. Yuan et al. (2010) established the lost circulation accident model to classify the causes of lost circulation and put forward the evaluation technology for the risk of lost circulation. Howard et al. (1951) developed a plugging tester. After the test, it is considered that the effect of granular plugging material is better. Bychina et al. (2017) Established a comprehensive analysis model to determine the influence of mud yield stress, fracture normal stiffness, and other parameters on drilling fluid leakage. Nechaikin et al. (2020) developed a multi-component reaction system that can be combined with oil-based drilling fluid. When encountering formation fluid, solid substances will form to block the leakage channel. Razavi et al. (2017) Established the leakage model, considered the influence of fluid leakage effect of high permeability formation on drilling fluid leakage, and more accurately predicted the rate and total amount of drilling fluid leakage on site. For the above leakage analysis and plugging operation, it is necessary to preliminarily estimate the location of the leakage layer and determine the reasonable operation range (Shen et al., 2013; Wang et al., 2017; Zhao et al., 2021).