Formation Matrix and Its Application in Drilling Risk Evaluation

Drilling safety is the foundation for the safe and efficient development of oil and gas resources. However, drilling accidents (e.g. blowout, lost circulation and borehole instability) often occur due to the complex geological conditions, especially in deep wells, ultra-deep wells and offshore oil and gas wells. To prepare a targeted drilling plan and minimize drilling risks, the key is to grasp the accurate geological parameters of the target formation. However, drilling safety is seriously threatened by the numerous complex, stochastic, and uncertain factors in the target formation, as well as the errors in the formation parameters captured by the current techniques. This paper attempts to quantify drilling risk accurately, despite the uncertainty of drilling information, and make efficient use of geological parameters. For this purpose, the concept of formation matrix was put forward, and coupled with credibility to handle the inevitable errors in formation parameters. According to the mechanical mechanism of common drilling risks, a Monte-Carlo (MC) simulation method was established to calculate the probabilities of drilling risks. The proposed method was applied to evaluate the well kick risk of a well in Kekeya area of Tuha Basin. The results show that our method can accurately predict the drilling risks that conforms to the mechanical hazard mechanism, effectively integrate various drilling information, making them complement each other, and improve the characterization accuracy of formation matrix, thereby enhancing the reliability of drilling risk evaluation. The proposed method provides theoretical support for geological modeling and drilling risk evaluation.