What is wellbore Ballooning? Causes, Effects, Detection and Control
Ballooning, also called ‘Wellbore Breathing,’ ‘Supercharging,’ or ‘Micro-fracturing, is a phenomenon that is typically observed in deep wells, HPHT Wells, shale, non-porous, and non-permeable formations.
Wells are drilled by maintaining the wellbore pressure above the formation pressure but below the fracture pressure. If, at any point, the fluid column pressure in the wellbore falls below the pore pressure, it could result in fluid influx or wellbore collapse. Conversely, if the fluid column pressure exceeds the strength of the formation, it could lead to fracturing the formation and losing the drilling fluid. This window between the pore and fracture pressure is known as the ‘drilling window.’
In the context of HPHT Wells, it's common to experience a very narrow operational window between pore and fracture pressures. The dynamic mud density, called ‘Equivalent Circulation Density (ECD), could exceed the formation's fracture strength during circulation. At that point, the formation opens or gives way to the hydrodynamic pressure inside the wellbore, resulting in the loss of drilling fluid. When the pumps are turned off, the dynamic friction effect disappears, and the pressure of the static mud column inside the wellbore falls below the formation’s fracture pressure. This results in the fluid returning to the wellbore from the formation. The measured return could be the same or less than the volume of fluid lost to the formation and could even be contaminated with formation fluid. A conceptual explanation of Ballooning is depicted below.
Detecting the ballooning phenomenon during drilling is challenging due to the mixed indications of simultaneous loss and kick. The well loses drilling fluid while drilling and gains while making connections or tripping.
Identifying the ballooning requires close monitoring of mud losses, observing return flows when the pump is off, observing if the flow rate decreases over time, and being cautious about contamination of mudflow back from formations that could potentially be carrying gas or reservoir water. If the contamination of the drilling fluid indicates formation gas, and it is not circulated out of the well, the accumulated gas in the annulus could cause significant underbalance, resulting in a kick. Hence, circulating the contaminated fluid out of the well and ensuring homogeneous mud inside the wellbore becomes important.
When this phenomenon is observed, it's recommended to address the borehole as a kick until it is confirmed that ballooning has occurred. Once a kick is ruled out, efforts should be made to reduce the ECD and dynamic effects to keep the wellbore pressures within the narrow formation and fracture pressure window. Some of the actions that could help achieve these objectives are listed below.
- Controlling the drilling rate to avoid the accumulation of drilled cutting in the annulus, which could result in higher ECD.
- Frequent bottoms-up circulation ensures that drilled cutting and any formation fluid are thoroughly circulated out of the well.
- Ensuring homogeneous mud with low friction and good cutting transport ability.
- Using optimum pump rate to balance the wellbore friction and cutting removal.
- Including the ‘Pressure While Drilling (PWD)’ tool in the drilling string to acquire real-time measurements of the downhole pressure helps manage it within the required limits.
- The option of enlarging the hole to increase annular clearance and reducing the ECD could be explored.
- If it becomes impossible to drill ahead, casing off the drilled section and using a drilling liner for the remaining part to the planned casing depth may be required.