Lost circulation pattern in the vug-fractured limestone formation

Due to numerous leakage zones, such as vugs, fractures, and caves appearing in carbonate rocks, lost circulation, the main issue, restricts safe and effective drilling. The lost circulation pattern of the vug-fractured limestone formation has been extensively studied using the Mishrif formation of the H oilfield in the Middle East as the research sample. The equivalent physical models of three different types of leakage zones, including vug, natural fracture, and induced fracture, were built based on the interpretation of FMI (Formation MicroScanner Image) logging. By embedding discrete fractures into a variable permeability matrix and solving it using the finite element method (FEM), a three-dimensional fluid–solid coupling mathematical model of mud loss rate was established. The study focused on the effects of drilling fluid performance parameters on lost circulation as well as the parameters of the single leakage zone (vug, natural fracture, or induced fracture) on lost circulation. Additionally mentioned were the variable characteristics of mud loss rate and cumulative mud loss volume in various combined ways of leakage zones. The findings indicate that there are considerable differences in how various types of leakage zones affect lost circulation properties. Fractures, particularly induced fractures, can result in more severe lost circulation than those brought on by vug for the creation of a single leaking zone. Mud loss rate and cumulative mud loss volume are greatly influenced by fracture width and dissolved radius, both of which have thresholds. Due to the enhanced flow conductivity, the lost circulation is more severe the more complicated the leakage zone combination mode is. The numerical model developed in this research provides a new perspective on how to analyze the lost circulation pattern of vug-fractured limestone formations and provides theoretical direction for lost circulation control of drilling in this kind of formation.