Simulation Study on the Effects of Drilling Fluid on Gas Cut from Fractured Formations

Gas cut caused by gravity displacement is frequently encountered when drilling fractured formations. Well kick, mud loss and well blowout will happen if gas entering the wellbore from fractured formations is not correctly handled. When gas kick is happening, pressure fluctuation around the fractures will result in borehole wall instability. In laboratory studies, based on the analyses of the characteristic parameters of the fractures in Block Shunnan located in the north slope of Tazhong, the early stage modeling and meshing were done using the modelling software Gambit, and 3D models for wellbore and fractures were established. Using ANSYS Fluent hydromechanical software, a multiphase flow model was established with which the effects of the dynamic pressure at the mouth of a fracture on the stability of the borehole wall at the same place during gravity displacement were analyzed. Drilling data of a well, such as the density, viscosity and flow rate of the drilling fluid were used to analyze their effects on gas cut caused by gravity displacement. It was found that when gravity displacement took place, high pressure differential resulted in high instant flow rate of gas, which was then reduced to a low level in just 0.1 sec. The maximum dynamic pressure was found at the upper and lower ends of the fracture, while the minimum dynamic pressure was found at the middle-upper position of the fracture. The widths of the fracture at these three points were easy to increase, causing the wellbore to destabilize. Changes in the density and viscosity of the drilling fluid caused change in the dynamic pressure at the mouth of the fracture by only 6%. The steady rate of gas cut was also less affected by the changes in the density and viscosity of the drilling fluid. On the other hand, flow rate of the drilling fluid during gas cut greatly affected the dynamic pressure at the mouth of the fracture, causing the dynamic pressure to change by 66.9%, indicating that reduced flow rate of drilling fluid during gas cut is beneficial to bringing gas cut under control.