Abstract:

To address the low prediction performance of drilling risks in unknown formations during oil and gas development, a drilling risk assessment approach based on grey adjacent well information was proposed. Firstly, a three-parameter interval grey number was proposed to present formation information. Secondly, the depth adjustment and weighted interpolation methods were used to develop a target well gray wellbore information matrix based on adjacent well information. Subsequently, a three-parameter grey safety drilling fluid density window was proposed. Then, a risk assessment model including overflow, well leakage, collapse, and adhesion was developed based on generalized strength-interference theory. Finally, a case study was performed on five wells in an oil field in western China. The results indicated that introduced concept of wellbore information matrix and optimization parameter description of depth adjustment and weighted interpolation effectively presented the profile of the target well's wellbore information. The grey safe drilling fluid density window developed by three-parameter interval grey numbers and wellbore information matrix significantly reduced uncertainty in drilling risk assessment. The evaluation results from drilling risk assessment model based on generalized strength-interference theory agreed well with that in real situations, providing reliable fundamental knowledge for drilling risk assessment.

Key words: adjacent well grey information, drilling risk, three-parameter interval gray number, wellbore information matrix, safety drilling fluid density window