Well Control
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Well control
Introduction
The magnitude of the pressure in the pores of a formation, known as the formation pore pressure (or simply formation pressure), is an important consideration in many aspects of well planning and operations. It will influence the casing design and mud weight selection and will increase the chances of stuck pipe and well control problems. It is particularly important to be able to predict and detect high pressure zones, where there is the risk of a blow-out. In addition to predicting the pore pressure in a formation it is also very important to be able to predict the pressure at which the rocks will fracture. These fractures can result in losses of large volumes of drilling fluids and, in the case of an influx from a shallow formation, fluids flowing along the fractures all the way to surface, potentially causing a blowout. To prevent to flow an uncontrolled fluid from the formation to the wellbore and to the surface is an important issue in the drilling process. This flow occurs if the formation pressure is greater than the borehole pressure. If for some reason, the formation pressure is greater than the borehole pressure an influx of fluid, kick will occur. If this influx is not stopped the drilling mud and formation fluids will be pushed out of the borehole in an uncontrolled way and it may cause some consequences, such as fatality, losing of the rig and equipment or reservoir fluids. Thus it is important to understand the principles of well control. Every operating company has a policy to deal with pressure control problems. In my project I have researched the importance of primary well control, reasons of the primary control losing and warning indicators of the kick.
Primary well control
Primary well control over the well is held by ensuring that the pressure of the mud column in the borehole is greater than the pressure in the formations being drilled. It maintains a positive differential pressure or overbalance on the formation pressures.
Mud hydrostatic pressure > Formation pore pressure
Primary well control can be lost in two ways. The first is if the formation pressure is higher than that predicted by the reservoir engineers or geologists. The second is if the pressure due to the column of mud decreases for some reason, and bottom hole pressure drops below formation pressure. The bottom hole pressure is a product of the mud density and the height of the mud column. Therefore it can only decrease if the mud density or the height of mud column decreases. There are different reasons in which the density of the mud and the height of mud column can fall during normal drilling operations.
Reduction in mud weight
The mud weight is generally designed such that the borehole pressure is around 200-300 greater than the formation pore pressure. This pressure differential called overbalance. The overbalance becomes less and the risk of taking a kick becomes greater when the mud weight reduces, so it is essential that the mud weight must be checked continuously to ensure that it is pumped into the well in the correct density. The mud weight will fall during normal operations because of following reasons:
Solids removal
Excessive dilution of the mud
Gas cutting of the mud
The drilled cuttings must be removed from the mud when the mud returns to surface. If the solids removal equipment is not designed properly a large amount of weighting solids may also be removed. The solids removal equipment must remove only the drilled cuttings. The weighting solids must be added before the mud is circulated down hole again.
b. Dilution:
Water is the main component of the mud. The first stage of improving some property of the mud such as viscosity is to dilute the mud with water and it must be relevant to the percentage of solids. Water may also be added when drilling deep wells, where evaporation may be significant. During these operations mud weight must be monitored and regulated carefully.
c. Gas cutting:
If gas leak out from the formation and mix with the circulating mud (known as gas-cutting) it will reduce the density of the drilling fluid.