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: Formulas and Calculations for Drilling, Production and Workover, Second Edition