theory_of_well_control

7/30/2019 Theory_of_Well_Control

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Theory of Well Control

n Hydrostatic Pressure.

n Pore Pressure.

n Fracture Pressure.

n Definition of “U” tube.

n “U” tube in a wellbore.

n Wellbore static conditions.

n

Wellbore dynamic conditions.



Theory of Well ControlTheory of Well Control

In oil and gas well drilling, maintaining control of the well is one of themost important aspects of drilling operations. Maintaining a balancebetween the hydrostatic pressure in the well and the pressure in theformation around the well is crucial. For this purpose, an understanding of the different pressures that exist in and around a wellbore and affectdrilling operations, and how they relate to each other, is required to

perform operations safely.

At the end of this section you should be able to:

n Define hydrostatic, pore pressure and fracture pressures.

n Describe leak-off test and formation integrity test.n Describe the concept of “U” tube and demonstrate its application in

understanding the wellbore in static and dynamic situations.

n Calculate bottomhole pressure in a well in static and dynamic conditions.



Hydrostatic PressureHydrostatic Pressure

The hydrostatic pressure is the pressure exerted by a fluid column. This pressure is onlya function of the vertical depth and the density of the fluid, the size and shape of thecolumn do not affect this value.

HydrostaticHydrostatic Pressure (psi) = 0.052 x vertical height of fluid (ft) x fluid dPressure (psi) = 0.052 x vertical height of fluid (ft) x fluid d ensity (ppg)ensity (ppg)

The hydrostatic pressure at the bottom of the vessels in the figure below are all thesame:



Hydrostatic PressureHydrostatic Pressure

Example: calculate the hydrostatic pressure on the bottom of the following wells.

12 ppg

fluid10,000 ft

6,240 psi6,240 psi

15 ppg

fluid10,000 ft

7,800 psi7,800 psi

12 ppg

fluid9,000 ft

1,000 ftEmpty

5,616 psi5,616 psi

12 ppg

fluid9,000 ft

1,000 ft

Sea

Water

8.5 ppg

6,058 psi6,058 psi



Pore PressurePore Pressure

The pore pressure (formation pressure) is the pressure acting on the fluids contained inthe pore spaces of the rock. Pore pressure are classified as:

n Normal Formation Pressure: when the formation pore pressure is approximatelyequal to theoretical hydrostatic pressure for the given vertical depth.

The normal pressure gradient for most areas is generally between 0.433 and 0.465

psi/ft (equivalent to 8.33 – 8.90 ppg fluid density).

n Abnormal Formation Pressure: formation pressures greater than normal pressure.It is also known as surpressure, overpressure, and sometimes geopressure.

Abnormally high formation pressures are caused by the undercompaction of shales,claystone diagenesis, tectonic activity (e.g. faulting, uplift, salt diapirs) structural

features (e.g. an impermeable cap rock overlaying a gas reservoir).

n Subnormal Formation Pressure: pressure below the normal pressure. Among thecauses of subnormal formation pressure are: depleted reservoirs, temperaturereduction in an isolated fluid system, tectonic activity and osmosis.



Fracture PressureFracture Pressure

The fracture pressure is thepressure necessary to overcomethe formation pore pressure and thestrength of the rock matrix.

Formation fracture pressure isdependent on the type of formation(strength, permeability, etc.) mudproperties as well as on localgeology. Rock strength usually

increases with depth andoverburden load. Therefore, fracturepressure normally increases withdepth.



Formation Strength TestsFormation Strength Tests

The knowledge of the pressure at which the formation fracture will occur isessential when designing and drilling of a well, as the integrity of the well in awell control situation depends on the minimum formation strength.

The strength of the formation can be determined by the following tests:

n Leak-off Test (LOT):

Determines the pressure atwhich the formation begins totake fluid. It is used speciallyin exploratory wells to

determine the maximumpressure that the formation atthe test point can withstandbefore a loss of drilling fluidinto the formation occurs.



Formation Strength TestsFormation Strength Tests

n Formation Integrity Test(FIT):

The formation is tested to apredetermined pressure value

without breaking. The test isnot carried to the point whereformation failure occurs. Inmany cases, this value of formation strength is sufficientto ensure well integrity in awell control situation,

especially in developmentwells.



““ U” TubeU” Tube

The “U” tube is the combination of twocolumns of fluids that are communicatedon the bottom, although the fluids havedifferent density and height, thepressure at this point is the same.

AA BB

Pressure APressure A = Pressure B= Pressure B

The “U” Tube can be:n Static.

n Dynamic.

Fundamentals:

n The sum of the pressures in one column is exactly equal to thesum of the pressures in the other column.

n In static “U” Tube the sum of the pressures above a given point isexactly equal to the sum of the pressures below that point.



The Well as “ U” TubeThe Well as “ U” Tube

The well can be consider as “U” tube: thetwo column of fluids are the drill pipe andthe annulus connected in the bottom, asshown in the figure.

The pressure exerted on the bottom of thewell for the column of fluid in the drill pipeis the same than pressure exerted by thecolumn of fluid in the annular.

BHP = Bottom Hole Pressure

BHP = Hydrostatic Pressure Drillstring = Hydrostatic Pressure Annulus



Wellbore Static ConditionsWellbore Static Conditions – – “ U” Tube Static“ U” Tube Static

In a well control situation, the wellbore is under external pressure, due to the formationpressure applied on the fluid columns when the hydrostatic pressure of the fluid is lessthan formation pressure and the well is closed.

In a shut-in well condition, the bottomhole pressure is the sum of the hydrostatic

pressure of the drilling fluid column inside the drill string and the pressure indicated bythe surface pressure gauge connected to the drill string. The reading of the pressuregauge is called Shut-In Drill Pipe Pressure (SIDPP).

BHP = Hydrostatic Pressure Drill String + SIDPP

Also, the bottom hole pressure is the sum of the hydrostatic pressure of the fluid in the

annulus and the pressure indicated by the casing gauge. The reading if the pressuregauge is called Shut-In Casing Pressure (SICP).

BHP = Hydrostatic Pressure Annulus + SICP



Wellbore Static ConditionsWellbore Static Conditions – – “ U” Tube Static“ U” Tube Static

Example: After a gas kick, a well is shut-in and therecorded pressures are: SIDPP = 500 psi and SICP =700 psi.

• Well Depth = 10,000 ft.

• MW= 10 ppg.

• Gas Densityy = 0.1 psi/ft (1.923 ppg).• Influx height = 476 ft.

Calculate the BHP on the Drill String side and on theannulus side.

SIDPP = 500 psi

SICP =

700 psi

P1 = P2

476 ft

M W = 10 ppg

BHPDP = (0.052*10 ppg*10,000 ft) + 500 psi = 5,700 psi5,700 psi

BHP An = (0.052*10 ppg*(10,000 ft – 476 ft)) + (0.052*1.923 ppg*(476 ft)) + 700 psi = 5,700 psi5,700 psi

Both pressures are equal because the weBoth pressures are equal because the wellll is an “U” Tubeis an “U” Tube



Wellbore Dynamic ConditionsWellbore Dynamic Conditions

“ U” Tube Dynamic“ U” Tube Dynamic+PDP +PDHT

In dynamic conditions, pressurelosses in the system must beconsidered.

The pressure losses contributors are:

n Surface pressure loss.

n DP/BHA pressure loss.

n Bit pressure loss.

n Annular pressure loss (ECD).

n Back pressure from choke.



Wellbore Dynamic ConditionsWellbore Dynamic Conditions – – “ U” Tube Dynamic“ U” Tube Dynamic

The bottom hole pressure is equal to the

hydrostatic column of fluid in the Drill String

(HPDP), plus the pump pressure (PP), minus

the pressure losses inside the drill string and

bit (PLDP).BHPBHP == HPHPDPDP + PP+ PP – – PLPLDPDP

The concept of “U” Tube applied to dynamic conditions, is used when calculating the bottom holepressure, either through the Drill Pipe or Annulus side.

Similarly, the bottom hole pressure is equal tothe hydrostatic pressure of the fluid in theannulus (HP AN), plus the pressure losses in theannular space (ECD).

BHPBHP == HPHPANAN + ECD+ ECD



Wellbore Dynamic ConditionsWellbore Dynamic Conditions – – “ U” Tube Dynamic“ U” Tube Dynamic

Example: a well is circulated with no influx. Thecirculation pressures are: CDPP = 2,000 psi and CCP =500 psi (holding back pressure).

• Well Depth = 10,000 ft.

• MW= 10 ppg.

• DP pressure losses = 1,300 psi

• Annular pressure losses = 200 psi

Calculate the BHP on the Drill String side and on theannulus side.

BHPDP = (0.052*10 ppg*10,000 ft) + 2,000 psi – 1,300 psi = 5,900 psi5,900 psi

BHP An = (0.052*10 ppg*10,000 ft) + 200 psi + 500 psi = 5,900 psi5,900 psi

Both pressures are equal because the weBoth pressures are equal because the wellll is a dynamic “ U” Tubeis a dynamic “ U” Tube

P1 ≥ P2

CDPP =

2,000 psi

CCP =

500 psi

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