Drill Pipe Pressure Schedule Calculation for Wait and Weight Well Control Method

This example demonstrates how to determine drill pipe pressure schedule for wait and weight method.

 

Well Information

Current mud weight = 9.5 ppg

Pump output = 0.1 bbl/stroke

Well depth = 9000’MD/9000’TVD

Drill string capacity = 0.0178 bbl/ft

Surface line volume = 15 bbl.

Shut in casing pressure = 700 psi

Shut in drill pipe pressure = 500 psi

ICP = 1600 psi at 30 spm as kill rate

Please follow steps below to determine the drill pipe pressure schedule (step down chart).

Determine kill weight mud

KWM = OMW + [SIDPP ÷ (0.052 x TVD)]

Where;

KWM is kill weight mud in ppg. (Read more about it -> kill weight mud)

OMW is original mud weight in ppg.

SIDPP is shut in drill pipe pressure in psi.

TVD is true vertical depth of the well in ft.

 

KWM = 9.5+ [500 ÷ (0.052 x 9000)]

KMW = 10.6 ppg

 

Determine Slow Circulating Rate (SCR)

SCR = ICP – SIDPP

Where;

SCR is slow circulating rate in psi.

ICP is initial circulating pressure in psi.

SIDPP is shut in drill pipe pressure in psi.

SCR = 1600 – 500 = 1100 psi

 

Determine Final Circulating Pressure (FCP)

FCP = SCR x KWM ÷ OMW

Where;

FCP is final circulating pressure in psi.

SCR is slow circulating rate in psi.

KWM is kill weight mud in ppg.

OMW is original mud weight in ppg.

 

FCP = 1100 x 10.6 ÷ 9.5 = 1227 psi

 

Determine stroke from surface to bit

Drill string volume = Drill pipe capacity x TD ÷ Pump output

Where;

Drill sting volume is in strokes.

Drill pipe capacity is in bbl/ft. (Read more about it> capacity of pipe)

TD is well measured depth in ft.

Pump output in bbl/stroke. (Read more detail about it ->Mud Pump Output)

 

Drill string volume = 0.0178 x 9000 ÷ 0.1 = 1602 strokes

 

According to this example, you need 1602 stokes in order to bring kill mud to the bit and drill pipe pressure will change from 1600 psi (ICP) to 1227 psi (FCP) within 1602 strokes.

 

Hence pressure drop per stoke is (ICP – FCP) ÷ surface to bit

 

(1600 – 1227) ÷ 1602 = 0.2328 psi/stroke

 

This figure (0.2328 psi/stroke) is very small and difficult to make adjustment with equipment on the rig. Therefore, you need to know how much pressure drop per required strokes. For this example, I will determine pressure drop per 200 strokes.

Drill pipe pressure drop = 0.2328 x 200 = 47 psi

Then we need to create a table showing pressure schedule.

For the first line, you need 150 stokes to bring KWM to the rotary table then drill pipe pressure will be dropped approximately 47psi/200 strokes until it reach 1227 which is the final circulating pressure.

The step down table look like this.

Strokes Drill Pipe Pressure (psi) Remarks
150 1600 You need to pump 150 stokes in order tobring kill from mud pump to rotary table
350 1554 DP drops 47 psi/200 strokes.
550 1506 DP drops 47 psi/200 strokes.
750 1459 DP drops 47 psi/200 strokes.
950 1412 DP drops 47 psi/200 strokes.
1150 1365 DP drops 47 psi/200 strokes.
1350 1318 DP drops 47 psi/200 strokes.
1550 1271 DP drops 47 psi/200 strokes.
1750 1227 Final circulating pressure

Reference books: Drilling Formula BookFormulas and Calculations for Drilling, Production and Workover, Second Edition

 

 Well Control Books

Pump Output Calculation for Duplex Pump and Triplex Pump

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4 Responses to Drill Pipe Pressure Schedule Calculation for Wait and Weight Well Control Method

  1. Mostafa A. Sabry says:

    Please Explain how did you Know the Initial Circulating pressure (ICP) at a given SPM in the Start ??
    How is this calculated while on the Rigfloor ?

  2. Kirk says:

    How do i calculate shoe safety margin during well kill operation?

    Kirk

    • Krik,

      There is no formula for safety margin at shoe during kill operation. You have MASCP when you shut in. It will tell you that how much shut in casing pressure you can have before you break the shoe.

      Regards,
      Shyne

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