When formation ballooning or breathing happens, drilling fluid losses occur in proportion to pump rates and then return when pumps are turned off. A frequent challenge with compressible non-aqueous fluids through weak formations, ballooning begins when Equivalent Circulating Density (ECD) exceeds the fracture point of the formation. Small fractures open allowing the drilling fluid into the formation. ECD includes both the hydrostatic pressure of the fluid (mud weight) and the pressure on the annulus caused by circulation. When circulation stops, the annular pressure losses are eliminated. In the case of ballooning, when the pumps are turned off, stopping circulation, the fluid density is reduced to the mud weight, which is not enough to hold the fractured formation open. The lost drilling fluid is pushed out of the formation as the fractures close. Returns can initially be misinterpreted as a kick and can be contaminated with formation fluids and gases. Once ballooning occurs, rig operations must assume a kick and employ well control operations until the ballooning is stable and benchmarked. Then, the volumes and pressures must be diligently monitored to ensure the formation is only returning the volumes lost.
ECD management is key to the prevention of formation ballooning. For the deepwater environment, Newpark offers two solutions, the DeepDrill High-Performance Water-based Drilling Fluid (HPWBDF) and the Kronos Low-ECD Synthetic-based Drilling Fluid.
DeepDrill System Combats Ballooning and Losses: As a water-based solution, the DeepDrill HPWBDF presents an advantage over non-aqueous fluids in that its brine base-fluid has minimal compressibility in comparison to non-aqueous fluids. If a drilling fluid compresses under pressure, the volume of barite increases in comparison to the volume of drilling fluid, thereby increasing the mud weight of the fluid. That can be enough to fracture the formation and induce ballooning.
Kronos Low-ECD Synthetic-based System: The Kronos™ Low ECD system is a synthetic-based solution designed to reduce ECD spikes magnitude through minimization of pressure losses and optimization of flow rates. These characteristics are achieved at any given fluid density by lowering the overall rheological characteristics of the fluid while minimizing sag potential. The Kronos Low-ECD system is appropriate for deeper well sections with narrow wellbore diameters.