LA PERCEPCIÓN DEL SONIDO
4. LA MÚSICA ES VIDA
The hydrostatic pressure of the drilling fluid column exerted against the borehole wall helps prevent unconsolidated or over pressured formations from caving into the hole. This pressure also helps prevent kicks, which are the controllable flow of formation fluids into the wellbore resulting in displaced drilling mud at the surface and blowouts (Figure 36 and Figure 37), uncontrolled flow of formation fluids into the wellbore.
Figure 36: Land Rig Blowout
Figure 37: Offshore Blowout (Deepwater Horizon Rig)
4.1 UNDERBALANCE VERSUS OVERBALANCE
If the hydrostatic pressure is equal to the formation fluid pressure, the well is at balance. An overbalance exists when the mud hydrostatic pressure is greater than the formation pressure.
In permeable formations, an overbalance can result in invasion of the formation, i.e. drilling fluids enter the formation, displacing formation fluids away from the wellbore. In very permeable formations or when the overbalance is excessive, flushing can occur ahead of the bit before the formation is drilled. This may result in no show, or gas response, being seen from a potential productive formation.
An important consideration, especially in long-hole sections, is that whereas the mud hydrostatic may provide a marginal overbalance against high- pressure formations at the bottom of the hole, it may be imposing an excessive pressure against shallower, weaker formations. This may lead to formation damage, and in the worst scenario, may even fracture the formation.
When fracture has occurred, drilling fluid flows freely into the formation. Such lost circulation may lead to the loss of hydrostatic head in the annulus. This is not only costly, but may result in an underbalanced situation lower in the hole where a kick is then a very real danger.
Such a situation of lost circulation and a kick occurring simultaneously can easily lead to an underground blowout.
Underbalance occurs when the hydrostatic pressure is lower than the formation pressure. This may allow an influx of formation fluids into the wellbore which may, in turn, result in a kick. This influx will be large, or more rapid, where there is good permeability and / or high formation pressure.
Where formations are impermeable, the formation fluid is unable to flow freely. In this situation, the differential pressure results in the fracturing and caving of the formation. This not only leads to an increase of formation fluid entering the drilling mud, but also to loading of the annulus with cuttings. This can in turn lead to tight-hole or stuck pipe problems and difficulties in knowing the depth that cuttings are actually from.
Underbalanced drilling can dramatically improve penetration rates. In fact, with the appropriate surface equipment, underbalanced drilling has several benefits, including limited formation and reservoir damage, no lost circulation or differential sticking, no flushing of formations, and, in effect, a continual formation test.
4.2 PORE PRESSURE
Pore pressure is the pressure exerted by the fluid contained in the pore space of the rock and is the strict meaning of what is generally referred to as formation pressure. All rocks have porosity to some extent. If permeability also exists and formation pressure is greater than mud hydrostatic pressure, a kick occurs. In impermeable formations, excessive pore pressure is confined and produces sloughing or caving.
4.3 HYDROSTATIC PRESSURE
Hydrostatic pressure is the pressure that exists because of mud weight and vertical depth of the column of fluid. The size and shape of the fluid column have no effect.
Hydrostatic Pressure (psi) = 0.0519 x MW (lbs/gal) x TVD (feet)
OrHydrostatic Pressure (bars) = 0.0981 x MW (g/cc) x TVD (meters)
OrHydrostatic Pressure (kPa) = 0.0098 x MW (Kg/m
3) x TVD (meters)
Where:MW = Mud Density TVD = True Vertical Depth
4.4 PRESSURE GRADIENT
Pressure gradient is the rate of change of hydrostatic pressure with depth for a unit of fluid weight. That is,
Pressure Gradient = P
TVD = 0.0519 x MW
The pressure gradient for fresh water (MW= 8.33 lbs/gal):Pressure Gradient (fresh water) = 8.33 x 0.0519 = 0.432 psi/ft
The pressure gradient for typical formation water (MW = 8.6 lbs/gal):Pressure Gradient (formation water) = 8.6 x 0.0519 = 0.446 psi/ft
The value 8.6 lbs / gal (ppg) is an average used worldwide, but may not fit local conditions. However, this value should be used until a local value is determined.
4.5 APPARENT AND EFFECTIVE MUD WEIGHT
The mud weight measured at the pits is the apparent mud weight going into the hole, and exerts a hydrostatic pressure equal to the hydrostatic pressure. This is a static pressure.
But if the mud is circulated, additional pressure is placed against the formation because of frictional effects in the mud. This additional pressure, called effective mud weight (EMW) or effective circulating density (ECD), can be estimated by calculating the pressure loss in the annulus using Bingham’s power law, or modified power law formulae.
Bingham’s formula for calculating ECD is:
Pressure Loss (psi) = PV x V x L
(dh-dp) = hole diameter minus pipe outside diameter (inches) PV = plastic viscosity (centipoise)
V = annular velocity in laminar flow (ft / min)
In calculating the ECD, each section of the annulus should be considered separately and losses summed to total loss. Weatherford mud logging software calculates the ECD using the power law formula.
The effective mud weight, EMW, or effective circulating density, ECD, is the equivalent mud weight for the sum of the hydrostatic pressure plus the pressure loss in the annulus.
ECD = MW + Total annular pressure loss
(0.0519 x TVD)
Therefore, the Bottomhole Circulating Pressure (BHCP) is:
BHCP = ECD x 0.0519 x TVD
When pipe is pulled out of the hole, the pressure on the formation is reduced by an amount of similar magnitude to the pressure loss in the annulus. That is:
MWe = MW − pressure loss (0.0519 x TVD)
Where:Mwe = equivalent weight during POOH
A safe rule of thumb is to use an actual mud weight required to balance the formation pressure and add to this a mud weight equivalent to twice the annular pressure loss. That is: