10.3.2.1.1 Mud Weight
The pressure exerted by the mud column at the bottom of the borehole is a function of the mud density and column height. It must be sufficient to prevent a kick. A kick is situation wherein formation fluids enter the well. If the pressure exerted by the mud column falls below that which is necessary to hold back formation pressures, then a kick will occur. If this condition is remains unchecked for even a short period of time, the mud density may be reduced (“cut”) so severely that a blowout will occur. A blowout is an uncontrolled kick.
On the other hand, it is not practical or economical to have the mud weight too high. Excessive mud weights result in:
Low rates of penetration: The rate of penetration is reduced rapidly as the
pressure differential between the mud hydrostatic pressure and the formation pressure increases.
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Fracturing of weak formations that may cause the loss of drilling fluids (lost circulation): If this situation is not addressed the hydrostatic level will
be severely reduced causing formation fluids to enter the hole and causing a blowout.
Density is also important in preventing unconsolidated formations from caving into the borehole.
The effects of mud weight on mud logging are:
Hydrostatic pressure in excess of formation pressure might cause a loss of circulation. If circulation is lost, then the cuttings, drilling mud and any formation fluids they may contain are also lost.
1. Coarse, permeable unconsolidated formations: There is normally some loss by filtration into these formations, until an impermeable filter cake if formed. If pore openings are large enough, then loss of whole mud occurs. Other than in extreme cases, this is a slow, regular seepage loss. Partial returns are maintained.
2. Cavernous and vugular formations: Loss is usually sudden and of a finite amount, after which full returns are maintained.
3. Fissured or fractured formations: Fractures may be natural or induced and opened by the hydrostatic pressure. Losses of drilling mud are large and continuous.
Formation pressures that approximate or are greater than the hydrostatic pressure may cause a kick to occur, depending on permeability. In low permeability formations, like shales, cavings may occur, making cuttings analysis difficult. • • • • • • •
10.3.2.1.2 Pressure Control Terminology
The following terms must be understood when discussing pressure-control terminology:
Hydrostatic Pressure Pressure Gradient Equivalent Mud Weight
Apparent and Effective Mud Weight Pore Pressure
Overburden Pressure
10.3.2.1.2.1 Hydrostatic Pressure
Hydrostatic pressure is the pressure that exists due to the mud weight and vertical depth of the column of fluid. The size and shape of the fluid column have no effect.
Hp or P (psi) = 0.519 x MW (lbs/gal) x TVD (feet)
Or
Hp or P (bars) = 0.0981x MW (g/cc) x TVD (meters)
Where:
MW = Mud Density TVD = True Vertical Depth
10.3.2.1.2.2 Pressure gradient
Pressure gradient is the rate of change of hydrostatic pressure with depth for any given unit of fluid weight. That is,
Pressure gradient = P/TVD = 0.0519 x MW
The pressure gradient for fresh water (MW= 8.33 ppg) is:
Pressure Gradient = 8.33 x 0.0519= 0.432 psi/ft
The pressure gradient for typical formation water (MW = 8.6 ppg) is:
Pressure Gradient = 8.6 X 0.0519 = 0.446 psi/ft
The value 8.6 ppg is an average used worldwide, but may not fit local conditions. However, this value should be used until a local value is determined.
10.3.2.1.2.3 Apparent and Effective Mud Weight or Effective Circulating Density
The mud weight measured at the pits is the apparent mud weight going into the hole, and will exert 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 due to 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, the Power Law or Modified Power Law formulae.
Bingham’s formula for calculating ECD is:
Pressure loss (psi) = (PV x V x L) / (60000 x (dh-dp)2) + (YP x L) / (200 x (dh-dp))
Where,
L = section length (ft) YP = yield point (lb/100 ft2)
(dh-dp) = hole diameter minus pipe outside diameter (inches) PV = plastic viscosity
V = annular velocity in laminar flow (ft/min)
In calculating the ECD, each section of annulus should be considered separately and losses summed to total loss. International Logging uses computers to calculate the
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 = MWo + pressure loss / (0 0.0519 x TVD)
Therefore the Bottomhole Circulating Pressure is,
Bottomhole Circulating Pressure (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 = MWo - pressure loss / (0.0519 x TVD) Where,
We = 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,
Mud Weight for safe trip = Wo + 2 x (pressure loss / (0.0519 x TVD))
10.3.2.1.2.4 Pore Pressure
Pore pressure is the pressure that is exerted by fluids 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 will occur. In impermeable formations (such as shales), excessive pore pressure is confined and will produce sloughing or caving.