The mudweight is generally designed such that the borehole pressure opposite permeable (and in particular hydrocarbon bearing sands) is around 200-300 psi greater than the formation pore pressure. This pressure differential is known as the overbalance. If the mud weight is reduced the overbalance becomes less and the risk of taking a kick becomes greater. It is therefore essential that the mudweight is continuously monitored to ensure that the mud that is being pumped into the well is the correct density. If the mudweight does fall for some reason then it must be increased to the programmed value before it is pumped downhole.
The mudweight will fall during normal operations because of the following:
1. Solids removal
2. Excessive dilution of the mud (due to watering-back)
3. Gas cutting of the mud.
a. Solids removal :
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 the weighting solids (Barite) may also be removed. The solids removal equipment must be designed such that it removes only the drilled cuttings. If Barite is removed by the solids removal equipment then it must be replaced before the mud is circulated downhole again.
b. Dilution :
When the mud is being treated to improve some property (e.g. viscosity) the first stage is to dilute the mud with water (water-back )in order to lower 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 adjusted carefully.
c. Gas cutting :
If gas seeps from the formation into the circulating mud (known as gas-cutting) it will reduce the density of the drilling fluid. When this is occurs, the mudweight measured at surface can be quite alarming. It should be appreciated however that the gas will expand as it rises up the annulus and that the reduction in borehole pressure and therefore the reduction in overbalance is not as great as indicted by the mudweight measured at surface. Although the mud weight may be drastically reduced at surface, the effect on the bottom hole pressure is not so great. This is due to the fact that most of the gas expansion occurs near the surface and the product of the mudweight measured at surface and the depth of the borehole will not give the true pressure at the bottom of the hole. For example, if a mud with a density of 0.530 psi/ft. were to be contaminated with gas, such that the density of the mud at surface is 50% of the original mud weight (i.e. measured as 0.265 psi/ft.) then the borehole pressure at 10,000ft would normally be calculated to be only 2650 psi. However, it can be seen from Figure 5 that the decrease in bottom hole pressure at 10,000 ft. is only 40-45 psi.
It should be noted however that the presence of gas in the annulus still poses a problem, which will get worse if the gas is not removed. The amount of gas in the mud should be monitored continuously by the mudloggers, and any significant increase reported immediately.
Institute of Petroleum Engineering, Heriot-Watt University