Drill pipe pulled to lose hydrostatic pressure

You previously learn about hydrostatic pressure lose due to pulling out of hole . This post will use the same concept but we will determine how many feet of drill pipe pulled to lose certain amount of hydrostatic pressure in well bore.

The calculations below have 2 cases of pulling out of hole, pull dry and pull wet. They are different in calculation because amount of drilling fluid out of hole is different. Please follow and understand each case of calculation.

#1: How many feet of pipe pulled DRY to lose certain amount of hydrostatic pressure
Feet = (hydrostatic pressure loss in psi x (casing cap in bbl/ft – pipe displacement in bbl/ft)) ÷ (mud weight in ppg x 0.052 x pipe displacement in bbl/ft)

Example: Determine the FEET of dry drill pipe that must be pulled to lose the overbalance using the following data:

Hydrostatic pressure loss = 200 psi
Casing capacity = 0.0873 bbl/ft
Pipe displacement = 0.01876 bbl/ft
Mud weight = 12.0 ppg
Ft = 200 psi x (0.0873 – 0.01876) ÷ (12.0 ppg x 0.052 x 0.01876)
Ft = 1171 ft
You need to pull 1171 ft of dry pipe to lose 200 psi hydrostatic pressure.

#2: How many feet of pipe pulled WET to lose certain amount of hydrostatic pressure

Feet = hydrostatic pressure loss in psi x (casing capacity in bbl/ft – drill pipe capacity in bbl/ft – drill pipe displacement in bbl/ft) ÷ {mud wt in ppg x 0.052 x (pipe displacement in bbl/ft + (% of volume in drill pipe out of hole ÷ 100) x pipe capacity in bbl/ft)}

Example: Determine the feet of WET pipe that must be pulled to lose the overbalance using the following data:

% of volume in drill pipe out of hole = 100
Hydrostatic pressure loss = 200 psi
Casing capacity = 0.0873 bbl/ft
Drill pipe capacity = 0.01876 bbl/ft
Drill pipe displacement = 0.0055 bbl/ft
Mud weight = 12.0 ppg

Feet = 200 psi x (0.0873 – 0.01876 – 0.0055 bbl/ft) ÷ {12.0 ppg x 0.052 x (0.0055 + (100÷100) x 0.01876 bbl/ft)}
Feet = 832.9 ft
You need to pull 833 ft of wet pipe to lose 200 psi hydrostatic pressure.

Please find how many feet of drill pipe pulled to lose certain amount of hydrostatic pressure in well bore.

Ref book: Drilling Formula Book Formulas and Calculations for Drilling, Production and Workover, Second Edition

Formation Temperature

Formation temperature is one of the most critical parameters in drilling and workover operation and it varies by true vertical depth of wellbore.

The following formula shows relationship between formation temperature and true vertical depth of well.

Formation temperature = (ambient surface temperature ) + (temperature gradient x  Well TVD)

Where:

Formation temperature in F (Fahrenheit)

ambient surface temperature in F (Fahrenheit)

temperature gradient in F/ft (Fahrenheit / ft)

Well TVD in ft

Example: The temperature gradient in a specific area is 0.015 °F/ft of depth and the ambient surface temperature is 90 °F.

Determine the estimated formation temperature at a TVD of 12,000 ft:

Formation Temperature, °F = 90 °F + (0.015 °F/ft x 12,000 ft)

Formation Temperature, °F = 90 °F + 180 °F

Formation Temperature = 270 °F (estimated formation temperature)

Please find the Excel sheet used for estimating formation temperature.

Ref book: Drilling Formula Book Formulas and Calculations for Drilling, Production and Workover, Second Edition

Hydraulic Horse Power Calculation

Hydraulic Horse Power is a measure of the energy per unit of time that is being expended across the bit nozzles. It is commonly calculated with the equation HHP=P*Q/1714, where P stands for pressure in pounds per square in., Q stands for flow rate in gallons per minute, and 1714 is a conversion factor necessary to yield HHP in terms of horsepower. Bit manufacturers often recommend that fluid hydraulics energy across the bit nozzles be in a particular HHP range, for example 2.0 to 7.0 HHP, to ensure adequate bit tooth and bottom-of-hole cleaning (the minimum HHP) and to avoid premature erosion of the bit itself (the maximum HHP).
Ref: http://www.glossary.oilfield.slb.com

Hydraulic Horse Power (HPP) formula as follow:

HHP= (P x Q) ÷1714

where HHP = hydraulic horsepower
P = circulating pressure, psi
Q = circulating rate, gpm

Example : Determine Hydraulic Horse Power with these following data:

circulating pressure = 3500 psi
circulating rate = 800 gpm
HHP= (3500 x 800) ÷1714
HHP = 1633.6

 

Please find the Excel sheet for calculating Hydraulic Horse Power (HHP)

Ref book: Drilling Formula Book Formulas and Calculations for Drilling, Production and Workover, Second Edition

Hydrostatic Pressure (HP) Decrease When POOH

When pulling out of hole, volume of steel will be out of hole and mud volume will replace the steel volume.  If we don’t fill hole, hydrostatic pressure will decrease. There are 2 cases of pulling pipe which are pull dry and pull wet. Each condition is different in calculation concept because mud volume to displace pipe volume is different.

This topic shows you how to calculate hydrostatic pressure loss for both cases of pulling pipe, pull dry and pull wet. Moreover, there is the Excel sheet for calculating pressure decrease due to pulling out of hole.

Case#1: When pulling DRY pipe

When pulling dry, we will consider volume of steel out of hole only.

Step 1: Determine Total Pipe Volume

Step 2: Determine Hydrostatic Pressure Decrease

Example: Determine the hydrostatic pressure decrease when pulling DRY pipe out of the hole:

Number of stands pulled = 10
Pipe displacement = 0.0055 bbl/ft
Average length per stand = 91 ft
Casing capacity = 0.0873 bbl/ft
Mud weight = 12.0 ppg

Step 1: Determine of pipe displacement in Barrels = 10 stands x 91 ft/std x 0.0055 bbl/ft displaced
Barrels displaced = 5.01 bbl
Step 2: Determine HP, psi decrease = 5.01 barrels x 0.052 x 12.0 ppg ÷ (0.0873 bbl/ft – 0.0055 bbl/ft)
Hydrostatic pressure decrease = 38.2 psi

Case#2: When pulling WET pipe

When pulling wet, we will consider volume of steel out of hole and volume of mud in drillpipe as well. Therefore, pulling wet will decrease hydrostatic more than pulling dry pipe.

Step 1: Barrels displaced = number of stands pulled per stand in ft
x average length x {pipe disp inbbl/ft + {(% volume in drill pipe out of hole ÷ 100) x pipe cap in bbl/ft)}

Step 2: Determine hydrostatic pressure in psi decrease = barrels displaced x 0.052 x mud weight, ppg ÷ ((casing capacity in bbl/ft) – (Pipe disp in bbl/ft + pipe cap in bbl/ft))

Example: Determine the hydrostatic pressure decrease when pulling WET pipe out of the
hole:

% of volume in drill pipe out of hole = 100
Number of stands pulled = 10
Pipe displacement = 0.0055 bbl/ft
Average length per stand = 91 ft
Pipe capacity = 0.01876 bbl/ft
Mud weight = 12.0 ppg
Casing capacity = 0.0873 bbl/ft

Step 1: Barrels displaced = 10 stands x 91 ft/std x {(.0055 bbl/ft + (100 ÷ 100) x 0.01876 bbl/ft)}
Barrels displaced = 22.08 bbl

Step 2: hydrostatic pressure in psi decrease = 22.0766 barrels x 0.052 x 12.0 ppg ÷ ((0.0873 bbl/ft) – (0.0055 bbl/ft + 0.01876 bbl/ft))
HP decrease, psi = 218.52 psi

Please find the Excel sheet for calculating pressure decrease due to pulling out of hole.

Ref book: Drilling Formula Book Formulas and Calculations for Drilling, Production and Workover, Second Edition

Buoyancy Factor Calculation

Buoyancy Factor is the factor that is used to compensate loss of weight due to immersion in drilling fluid.

Before explaining any further, I will explain you about the basic concept of Buoyancy. Buoyancy is the upward force that keeps things afloat. The net upward buoyancy force equals to the amount of the weight of fluid displaced by the body volume. This force will make objects lighter when it immerses in fluid. For example, we feel ourselves lighter when we are in swimming pool because this is the effect of buoyancy.

bouyancy-factor

In drilling operation, we need to know how much weight of string of drill pipe, completion string, etc in drilling fluid. Therefore,  Buoyancy Factor is value that we need to know and be able to calculate this value. Please follow the formulas below to calculate Buoyancy Factor in different mud weight units, ppg and lb/ft3.

Buoyancy Factor using mud weight in ppg

Buoyancy Factor (BF) = (65.5 – mud weight in ppg) ÷65.5

Note: 65.5 ppg is density of steel.

Example: Determine the buoyancy factor for a 13.0 ppg fluid:
BF = (65.5 – 13.0) ÷ 65.5
BF = 0.8015

Buoyancy Factor using mud weight in  lb/ft3

Buoyancy Factor (BF) = (489 – mud weight in lb/ft3) ÷489

Note: 489 lb/ft3 is density of steel.

Example: Determine the buoyancy factor for a 100 lb/ft3 fluid:
BF = (489 – 100) ÷489
BF = 0.7955

How to use the Buoyancy Factor

In order to figure out the actual weight of drilling string in mud, the air weight of drilling string times the buoyancy factor equal to actual weight in mud, called buoyed weight.

Example: Determine the string weight in 13.0 ppg mud. Air weight of string is 350 klb.

The buoyancy factor for a 13.0 ppg fluid:
BF = (65.5 – 13.0) ÷ 65.5
BF = 0.8015

Buoyed weight of drill string = String weight in the air x BF

The buoyed weight of drill string in 13.0 ppg mud = 350 x 0.8015 = 280.5 Klb.

 

Ref book: Drilling Formula Book Formulas and Calculations for Drilling, Production and Workover, Second Edition