Basic Concept - Well Control

In Drilling Operation, pressure is everything. The entire concept of well control revolves around one key principle: "Maintaining Sufficient hydrostatic pressure to prevent formation fluids entering the wellbore"

understanding pressure in well control involves several key components, including hydrostatic pressure (HP), surface pressure (SP), and friction Pressure (FP). These pressures are fundamental in evaluating bottom hole pressure (BHP) and maintaining well stability during drilling operations. 

to evaluate and predict these pressure behaviors utilize the basic principles such as Boyle's Law and the U-tube principle to maintain BHP constant. 

HP = Pressured caused by the weight of a column of fluid = constant x Mud Weight x TVD 

HP (psi) = 0.052 x MW (ppg) x TVD (ft) 

The HP per foot depth of a fluid is called its pressure gradient, or more commonly, Gradient (G)

G (psi/ft) = 0.052 x MW (ppg) 

Pressure in Well

In a static well condition, pressure at any depth is basically generated by the fluid column above it (hydrostatic pressure) plus any additional pressure applied from the surface (if may exist) 

Simply but, the deeper the point inside the wellbore, the higher the pressure will be because of the fluid weight above it.

Pressure between any two points is equal to the hydrostatic pressure between the points. 

Bottom Hole Pressure (BHP)

Bottom Hole Pressure (BHP) can be simplified as the combination of Surface Pressure (SP) and Hydrostatic Pressure (HP).

BHP = SP + HP

In drilling operations, the hydrostatic pressure generated by the mud column acts as the primary well control barrier.

As long as the mud hydrostatic pressure is sufficient to balance the formation pressure, the well remains under control.

• If BHP > Formation Pressure (FP), Then overbalance 
• if BHP = Formation Pressure (FP), Then at balance 
• if BHP < Formation Pressure (FP), Then underbalance

The U-Tube Principle

The U-tube concept is a simple analogy of a well with tubing inside it. Since both sides are connected, at the End of Tubing (EOT), the BHP on both sides must be equal.

Example: there is a gas inside the annulus 2 ppg and brine inside casing 10 ppg 

SPtbg + HPtbg = BHPtbg 

                             BHPann = SPann + HPann

1664 psi + 416 psi = 2080 psi 

                                  2080 psi = 0 psi + 2080 psi

1664 psi tubing pressure required to maintain BHP 

Friction Pressure (FrP)

the principle of friction pressure is the pressure that opposes flow 

during circulation, the friction pressure will increase the BHP. Several factor might affect the amount of friction inside the wellbore, including :

• Tool-to-casing clearance
• Packer swabbing effect
• Changes in brine weight
• Changes in fluid viscosity
• Well geometry
• Forward circulation vs reverse circulation
• Flow rate

In general, higher friction losses will result in higher circulating BHP.

Friction pressure becomes relatively small at low pump rates and is usually neglected in most well control applications. at the well control procedure, the reason we use SPR (slow pump rate) because the friction facture value is neglected. 

Effect of FrP on BHP (ECD) While Forward Circulating

Equivalent Circulating Density (ECD) is simply the total pressure acting at the bottom hole while the fluid is circulating. It combines the mud hydrostatic pressure with the additional friction pressure generated during circulation.

• Static Condition 
• BHP = HP or HP + SP
• Dynamic Condition (Forward Circulation)
• BHP = SP + HP + FrPacting on bottom hole
• Standpipe pressure = FrPsurf + FrPds + FrPbit + FrPann
• BHP will increase by FrPann 
• BHP = HP + FrPann 
• BHP also expressed in ECD will be

ECD = MW + (FrPann/(0.052 x TVD))

 

              

Effect of FrP on BHP (ECD) While Reverse Circulating

  ECD is the expression of BHP in equivalent MW

• Static Conditions: 
• BHP = HP 
• Dynamic Conditions (Reverse Circulation): 
• Standpipe pressure = FrPann FrPbit + FrPds + 
• BHP will increase by FrPbit + FrPds 
• BHP = HP + FrPbit + FrPds 
• BHP expressed in ECD will be:

ECD = MW + ((FrPbit + FrPds)/(0.052 x TVD))

During reverse circulation, FrPtubing, FrPbit, FrPsurf line will add extra pressure at the bottom hole.
As a result, the Bottom Hole Pressure (BHP) becomes higher during circulation.

Gas Behavior - Boyle's Law

the principle of Boyle's Law 

Pressure1 x Volume1 = Pressure2 x Volume2

If the gas expands to 25 gallons, what will the pressure be after the expansion?

Pressure2 = Pressure1 x Volume1/Volume2 

Pressure2 =  200 psi

Gas Behavior in Shut-in Well

the data given as picture above, please calculate Surface Pressure (SP) in PSI 

1. Since the gas volume remains constant, the gas pressure will also remain unchanged.
• P1 x V1 = P2 x V2 =
• 5500 psi x 10 bbls = 5500 psi x 10 bbls
2. The gas moved upward by 5000 ft TVD. Calculate the BHP below the gas column. The BHP will increase by the vertical fluid height displaced by the gas.
• BHP =  SP + HP
• BHP = 5500 psi + (0.052 x 5000 ft x 9.6 ppg)
• 7996 psi or 8000 psi 
3. The upper section can be considered as a separate wellbore.
• BHP = SP + HP
• 5500 psi = SP + (0.052 x 9.6 x 5020)
• SP = 5500 - 2505 
• SP = 2995 psi or 3000 psi 

Gas Behavior With Constant SP

• Attempting to control the BHP by maintaining constant surface pressure will reduce the BHP, as the hydrostatic pressure decreases
• If the surface pressure is continuously kept constant, the gas volume in the wellbore will continue to increase.

 

the well data given

1. the gas will be bled 10 bbls, how much the decrease of HP?
• TVD = Volume Bleed/Annulus Capacity 
• TVD = 10 bbls/(0.0476 bbls/ft) 
• TVD = 210 ft 
• The Decrease of HP = 0.052 x 210 x 9.6
• The Decrease of HP = 105 psi

Gas Behavior With Constant BHP Control 

Well control using BHP control means maintaining a constant BHP while the surface pressure (SP) and hydrostatic pressure (HP) change.

HP decreases as fluid is bled from the well. SP increases to compensate for the reduction in HP

1. Assumse 6 bbl of mud creates 80 psi of hp 
2. if bleed 6 bbls resulting decreasing 80 psi of hp 
3. to maintain HP keep constant while HP decreases due to bleed of 6 bbls mud.