what is Under Balanced Drilling ?
When the effective circulating downhole pressure of the drilling fluid – which is equal to the hydrostatic pressure of the fluid column, plus pump pressure, plus associated friction pressures – is less than the effective near bore formation pore pressure. (Definition)
Conventionally, wells are drilled overbalanced, which provides the primary well control mechanism. Imposed wellbore pressure arises from three different mechanisms:
1. Hydrostatic pressure of materials in the wellbore due to the density of the fluid used (mud) and the density contribution of any drilled cuttings (passive).
2. Dynamic pressure from fluid movement due to circulating friction of the fluid used and the relative fluid motion caused by surge/swab of the drillpipe (dynamic).
3. Imposed pressure, with occurs due to the pipe being sealed at surface resulting in an area with pressure differential (e.g., a rotating head or stripper element) (confining or active).
Under balanced drilling is defined as drilling with the hydrostatic head of the drilling fluid intentionally designed to be lower than the pressure of the formations being drilled. The hydrostatic head of the fluid may naturally be less than the formation pressure or it can be induced. The induced state may be created by adding natural gas, nitrogen or air to the liquid phase of the drilling fluid.
Whether the underbalanced status is induced or natural, the result may be an influx of formation fluids which must be circulated from the well and controlled at surface. Underbalanced drilling in practical terms will result in flow from one or more zones into the wellbore (this is more likely, owever, to be solely from one zone as cross-flow is likely to result) or where the potential for flow exists.
The lower hydrostatic head avoids the build-up of filter cake on the formation as well as the invasion of mud and drilling solids into the formation. This helps to improve productivity of the reservoir and reduce related drilling problems. When comparing underbalanced drilling with conventional drilling it soon becomes apparent that an influx of formation fluids must be controlled to avoid well control problems. In underbalanced drilling, the fluids from the well are returned to a closed system at surface to control the well. With the well flowing, the BOP system is kept closed while drilling, whereas in comparison to conventional drilling fluids are returned to an open system with the well open to atmosphere
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