Dynamic response mechanism of borehole breathing in fractured formations

Abstract

The borehole breathing observed when drilling into natural fractured formations is often erroneously identified as leakage or kick. The main reason for this phenomenon is that the fractures around the well have opened and closed after being affected by the wellbore pressure fluctuations. In this paper, a three-dimensional geological model is established for simulating fractured borehole breathing based on the Cohesive unit method, and the full three-dimensional simulation of fractured borehole breathing is realized for the first time. Based on this model, this paper studies the dynamic response characteristics of the fractured borehole breathing mechanism in detail, and analyzes a variety of parameters that may affect borehole breathing. The research results show that: increasing the viscosity of drilling fluid can effectively inhibit the occurrence of borehole breathing and extend its duration; formation elastic modulus is inversely correlated with borehole breathing; borehole breathing is not sensitive to changes in formation Poisson’s ratio and porosity; horizontal geostress affects the severity of borehole breathing by determining the reopening pressure of the fracture; changes in the original width of the fracture will not affect the total volume of backflow, but will affect the amount of mud lost in the early stage.

Keywords

• Borehole breathing
• Fractured formation
• Numerical Simulation
• Wellbore pressure
• Re-opening pressure of fractures