q921 de2 lec3 v1

Drilling Engineering 2 Course (1st Ed.)

mailto:[email protected]

http://www.about.me/AlamiNia

mailto:[email protected]

http://www.about.me/AlamiNia

1. General Notes

2. Pore Pressure Prediction

3. Abnormal vs. normal Pressure

4. Fracture Gradient determination

1. Mud Weight Planning

2. drilling hydraulics: A. the hydrostatic pressure

mud weight selection

Selecting the correct mud weight for drilling the individual sections comprises a key factor to realize a in-gauge hole and avoid various borehole problems.Too low mud weight may result in

collapse and fill problems (well cleaning),

while too high mud weight may result in mud losses or pipe sticking

Fall 13 H. AlamiNia Drilling Engineering 2 Course: 5

constant mud weight program

Practice has also shown that excessive variations in mud weight may lead to borehole failure (fatigue type effect), thus a more constant mud weight program should be aimed.

Along with a more constant mud weight program, the equivalent circulation density (ECD)

as well as the surge and swab pressures shall be kept within limits.


Benefits of higher mud weight

Washouts of the borehole are sometimes caused by jet actions of the bit nozzles but also sometimes by to low mud weight causing a breakdown of the borehole wall. A higher mud weight will therefore

balance the rock stresses better and tend to keep the borehole more in-gauge.

A decease in hole diameter is often due to swelling (clay swelling) requiring wiper trips or backreaming. This necessity is sometimes reduced by

higher mud weights.


differential stuck

An increased mud weight increases the danger of becoming differential stuck at permeable formations. Therefore mud weight shall not be chosen to be to high.However, what is sometimes believed to be

a differentially stuck drillstring is sometimes due to a borehole collapse which packs the hole around the bottom-hole assembly.

A lower mud weight also causes breakouts of shale layers leaving sand formations in-gauge, (next fig).This can increase the danger of

getting differential stuck at the exposed sand stringers.


Partial collapse in mixed lithology


lost circulation

Thus when considering the danger of differential sticking, it is recommended to keep the mud weight

below a certain value but it shall not be as low as possible.

The same is true for lost circulation problems. As long as the mud weight is kept below a critical value,

lost circulation will not occur.


Mud weight vs. penetration rate

It is often argued that to have a as high as possible rate of penetration, the mud weight shall be kept as close as possible

to the formation pressure gradient plus a safety margin of around 100 [psi].

Although it is true that a small reduction in mud weight increases the penetration rate, but this increases has to be weighted against the possible

induction of hole problems and additional lost time.

A higher mud weight requires the use of more mud additives which makes the well more expensive, but it was found that

these extra costs are usually neglectable.


drilling for a kick

When drilling within areas where the subsurface pressure regimes are not well known, it is often argued that a lower mud weight

easies the detection of abnormal pressures.

In some locations, a practice called “drilling for a kick” was applied to detect overpressured formations. For this, a relatively low mud weight was applied

until a kick was detected (pressure gradient at this depth was equal to the used mud weight) and handling the kick, the mud weight was increased.

Therefore and since a higher mud weight also suppresses high gas readings, the mud weight of exploration wells are often

designed to be lighter than the ones for development wells.


median line concept

Based on all these considerations, the “median line concept” is recommended generally for mud weight planning.Thereto, the mid-point between

the fracture pressure and the pore pressure defines the borehole pressure that is equal to the ideal in-situ stress.

Maintaining the mud pressure close to this level causes least disturbances on the borehole wall.

This principle is sketched in next two slides


Effects of varying the borehole pressure


average horizontal in-situ stress

Mentioned principle is mathematically found with following equation:

𝜎𝑎 =𝑃𝑤𝑓+𝑃𝑜

2 𝜎𝑎 [psi] average

horizontal in-situ stress,

Pwf [psi] fracture stress,

Po [psi] pore pressure

Pressure gradients for a wellFall 13 H. AlamiNia Drilling Engineering 2 Course: 15

Changing mud weight

Experience had shown that new drilling fluid exacerbates fracturing/lost circulation

and

leakoff tests applying used drilling muds give higher leak-off values than when carried out with new ones.

Therefore it is a good practice that, when the mud weight has to be changed

after setting casing, drilling is usually started with a lower mud weight.

After drilling about 100 [m] below the casing shoe, the mud weight is then gradually increased

to the desired value.


Changing mud weight (Cont.)

Furthermore it should be noticed that within an open-hole section, the mud weight shall only be increased and

not decreased since tight hole may result.

An increase of mud weight in steps of 0.05 [g/cm3] is good practice and in convenience of the mud engineer.


aspects of optimum hydraulics

To realize a safe, efficient and cost-effective drilling project, drilling hydraulics,

also known as rig hydraulics, play an important role.

The different aspects that make up optimum rig hydraulics are:1. Hydraulic energy impact on the bit,2. Friction pressure losses through the surface

equipment, drillstring, annuli and drill bit,3. Efficient hole cleaning,4. Nozzle selection and,5. Produced pump pressure.


improper drilling hydraulics consequencesSome of the drilling problems that are

due to improperly designed drilling hydraulics are failure of sufficient hole cleaning

leaving cuttings in the hole and lead to stuck pipe,

lost circulation causing kicks and slow penetration rates.

To understand the various dependencies of efficient drilling hydraulics, following concepts should be knownthe hydrostatic pressure inside the wellbore, types of fluid flow, criteria of fluid flow and commonly used fluid types for different drilling operations


Hydrostatic Pressure Inside the WellboreHydrostatic definition:

relating to or denoting the equilibrium of liquids and the pressure exerted by liquid at rest.

For oil well applications, the fluid may be mud, foam, mist, air or natural gas.

For a complex fluid column consisting of multiple fluids, the hydrostatic pressure is given in field units by:

𝜌𝑚𝑖 [ppg] mud weight of the ith fluid column


barometric formula

When gas is present in the well, the hydrostatic pressure developed by the gas column is calculated with:

z [1] ... real gas deviation factorpo [psi] ... surface pressureD [ft] ... total depth (TVD)Tf [F] ... bottom hole temperature of the formationThe molecular weight M of the gas is found as:

• where:

o 𝜌𝑔[ppg] ... density of the gas

o T [F] ... average gas density


equivalent mud weight

For practical purposes, the hydrostatics due to a complex fluid column are converted to an equivalent single-fluid hydrostatic pressure. To do this,

all individual hydrostatic pressures are summed up for a specific depth pd and then converted to an equivalent mud weight 𝜌𝑒 [ppg] that would cause the same hydrostatic pressure.

Therefore the equivalent mud weight has to be always referenced to a specific depth.


average mud weight

As the mud is used to transport the cuttings from the bottom of the hole to the surface and penetrated formations often contain a certain amount of formation gas, the mud column at the annulus is usually mixed with solids and gas.This alters the weight of the mud at the annulus.The new average mud weight m of a mixture containing

mud and solids can be calculated as:

mi [lbm] mass of component iVi [gal] volume of component i𝜌𝑖 [ppg] density of component ifi [1] volume fraction of component i


Notes about mud weight

only solids that are suspended within the mud do alter the mud weight.Settled particles do not affect the hydrostatic pressure.

If gas is present in the mud column as well, the density of the gas component is a function of the depth and will decrease with decreasing pressure.In this way, the density of mud containing gas is

decreasing with decreasing depth.


over-balanced drilling techniques (OBD)well control and the safety of drilling operations are

strongly depended on the maintenance of proper hydrostatic pressure. This pressure is needed to counterbalance the formation pressure.

In case the hydrostatic pressure in the borehole is higher than the formation pressure, the situation is called “over-balanced”. This prevents kicks (fluid flow from the formation into the borehole)

and causes at permeable formations an intrusion of some mud (water component) into the formation. The intrusion is stopped by the built up of mud cake

that seals off permeable formations. On the other hand, the hydrostatic pressure inside the borehole

must not be higher than the fracture pressure of the formations penetrated since this would fracture the formation artificially, cause loss of circulation and lead to well control problems.


underbalanced drilling techniques (UBD)To obtain maximum penetration rates

the hydrostatic pressure should be kept as close as practical to the formation pressuresince a higher differential pressure

(hydrostatic pressure - formation pressure) leads to worst cutting removal from the bottom of the well.

Due to this circumstance, UBD techniques have been developed that use air, foam or mist as drilling fluids. Here the formation pressure is higher than the hydrostatic

pressure caused by the mud and thus the well is constantly kicking.

With UBD techniques much higher penetration rates are possible but well control can be a problem.

Therefore UBD is prohibited by some governments and/or in some areas.


1. Dipl.-Ing. Wolfgang F. Prassl. “Drilling Engineering.” Master of Petroleum Engineering. Curtin University of Technology, 2001. Chapter 3 and 4