Antecedentes y planteamiento del problema

Pressure drives the mudflow from the mud pumps, through the drillpipe and BHA, out through the bit and back up the annulus between the drillpipe and borehole wall to surface. The mud pressure is measured in the standpipe that carries the mud up the side of the rig derrick from where it enters the drillstring. The standpipe pressure (SPP) can be thought of as the output pressure from the mud pumps.

The mud pressure increases as the mud travels down inside the drillpipe according to the equation:

Internal Hydrostatic Pressure = MWIN. g. TVD where:

MWIN = Mud weight (density) as it is pumped into the hole.

g = earth’s gravity field strength

TVD = true vertical depth of the point where the pressure is to be determined.

When the mud is flowing the pressure will be increased by the standpipe pressure applied at surface, and decreased by frictional pressure losses along the inside of the drillstring.

Internal Circulating Pressure = Standpipe Pressure + Internal Hydrostatic Pressure – Internal Frictional Losses.

Additional pressure losses occur across the BHA where components such as the MWD mud turbine and the drilling mud motor extract power from the mud flow. There is also a significant pressure loss across the nozzles in the bit which accelerate the mud flow to clean the bit and agitate the rock cuttings into the mud flow for transport back to surface.

A strain gauge measures the mud pressure inside the tool, before the pressure loss across the mud motor and bit occurs.

As the mud flows back up the annulus between the drillstring and the borehole wall, another strain gauge measures the annular mud pressure.

The difference between the internal and annular pressures is primarily due to the pressure loss across the mud motor or rotary steerable system and the bit. Changes in this pressure difference can help in diagnosing performance issues with the mud motor, rotary steerable system and bit nozzles.

Mud returning to surface is at atmospheric, or zero gauge pressure, as it flows into the mud tanks. The annular hydrostatic pressure at a point in the well is given by the equation:

Annular Hydrostatic Pressure = MWOUT. g. TVD where:

MWOUT = Mud weight (density) that is in the annulus. This will be greater than the MWIN due to the rock cuttings that are suspended in the mud returning to surface.

g = earth’s gravity field strength

TVD = true vertical depth of the point where the pressure is to be determined.

Annular frictional forces act against the flow so to overcome them the pressure at a given depth must be increased so as to have the mud arrive at surface with zero pressure.

Annular Circulating Pressure = Annular Hydrostatic Pressure + Annular Frictional Losses

MW_IN Pressure = SPP

TVD

P

= MW

.g.TVD + ∆ P

Pressure = 0

P

= SPP + MW

.g.TVD - ∆ P

∆Pinternal friction

∆Pannular friction

Figure 4-37: Annular and internal mud pressure

The annular circulating mud pressure is often divided by gravity times TVD, to give an equivalent circulating density of the mud in the annulus that includes the effect of cuttings in circulation and frictional effects (Figure 4-37).

ECD = Annular Circulating Pressure / g.TVD where:

ECD = equivalent circulating density of the mud in the annulus g = earth’s gravity field strength

TVD = true vertical depth of the point where the pressure measurement is taken.

When the mud pumps are off and there is no mud flow, the frictional forces will drop to zero and some of the cuttings will fall out of suspension, though some of the smaller cuttings will remain in suspension. The equivalent static density is given by:

ESD = Annular Hydrostatic Pressure/ g.TVD where:

ESD = equivalent static density of the mud in the annulus g = earth’s gravity field strength

TVD = true vertical depth of the point where the pressure measurement is taken.

Drillers will often watch their ECD to ensure that the hole is being cleaned effectively and that cuttings are not building up against the BHA downhole – a situation that could lead to stuck pipe.

Monitoring the ECD also allows timely adjustments to the mud density to maintain pressure control of the borehole.

Wellbore pressure control is critical for safe and smooth drilling operations. If the mud pressure is too high the formation may fracture resulting in loss of borehole fluid and subsequent drilling problems. If the mud pressure is too low then the fluid in the formation may flow into the well and begin migrating to surface resulting in a “kick”. Depending on the mechanical properties of the rock, too high or too low a mud weight may result in a variety of borehole failure modes, most of which are detrimental to efficient drilling.

Well pressure control design involves determining the upper and lower pressure limits within which the well can be drilled safely. These limits are usually expressed as fluid densities. The upper limit is called the fracture gradient and defines the fluid density above which the formation will fracture resulting in mud loss. The lower gradient is often the pore pressure gradient, below which formation fluid will flow into the borehole. During drilling, the ECD must be kept within the safe drilling window. An example of a pressure window is shown in Figure 4-38.

Casing points

1000 ft

Figure 4-38: A typical pressure window for a deepwater well.

The overburden pressure (purple) is the fracture gradient and defines the upper limit of the pressure window. The predrill seismic pore-pressure estimate (black) defined the pressure window lower limit. The closeness of the two curves indicates that there is little margin for error in mud weight. The resistivity–derived pore pressure is shown in red. The actual mud-weight profile plotted as annular pressure-derived ECD is shown in blue. Overall the drilling plan succeeded in staying within the narrow pressure window. However, at two depths where mud weight dropped below the lower pressure limit, the well took kicks.