9- free point and backoff -(houston)

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IPM Stuck Pipe Prevention

9- Free Point and Backoff

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Learning Objectives

• Have a better understanding FPIT

• Be able to interpret FIT results

• Understand operational procedure to perform a Back-off operation

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Concepts• Intensity of Stress

Is the stress per unit area due to a force of a F (pounds), producing

tension, compression or shear over an area of A, square inches.

S= F/A (lbs/in2)

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Concepts• Elastic Limit

Below the elastic limit, deformations are directly proportional to the forces producing them (Hooke’s law applies).

• Yield Point

Is the point above which permanent plastic deformation occurs with little or no increase in stress.

• Ultimate Stress

Is the maximum stress which can be produced before rupture occurs.

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Concepts

• Strain

Is the elongation e, per unit length of a bar or pipe. Strain may be tensile or compressive.

• Modulus of Elasticity E,

Is the factor of proportionality between stress and strain within the elastic limits of the material.

E =P/A

e/L(lbs/in2)

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Stretch in Drill String

• Hanging vertically from the block a

pipe is in tensile stress due to it’s

own weight.

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• When a pull is applied to a stuck pipe, above the weight of the pipe down to the stuck point, the pipe stretches an amount DLDLDLDL at surface proportionally to the additional pull DPDPDPDP (overpull)

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Torsion in Pipe StringsPipe in TorsionIf torque is applied at surface to a stuck pipe string, of a constant cross section, the

angular displacement or twist will vary linearly from a maximum at surface to zero at

the stuck point.

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• For estimating the length of free pipe, the stretch formula can be re-arranged as follows:

LLLLfreefreefreefree

DL.E.A

DP.12

Where: Lfree = length of free pipe ( feet )

DL = Stretch ( inches )

A = Cross sectional Area (inch 2 )

E = Modulus of elasticity ( lb / inch 2 )

DP = Drill string overpull ( lbs)

=

• Introducing:

A= ∏x(OD2 – ID2) and E =30x106 psi

Where: OD = Outside diameter of tubular to be stretched ( inches)

ID = Inside diameter of tubular to be stretched (inches)

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• Then:

LLLLfreefreefreefree(ft)(ft)(ft)(ft)DL (in) x [OD (in)2 – ID (in)2] x 1,963.5

F (1,000 lbs)=

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Tubular Free Point Calculation1. Note the following information:

a) Drill string data (diameters and weights)

b) Buoyancy Factor

c) Traveling Block weight (includes T. Block, Bails, Kelly or Top Drive)

d) Drill string weight before stuck

2. Calculate the weight of the string (drill string, tubing or casing) in mud.

3. Calculate the maximum overpull margin on the string (maximum allowable pull

– weight of string in mud)

4. Pull on the string until the weight indicator shows a pull of T1=string weight +

10% of the maximum safe overpull

5. Draw a mark at the rotary table / kelly bushing level

6. Increase the pull to string weight + 20% of maximum safe overpull.

7. Return to pulling string weight + 10% of the maximum overpull.

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Tubular Free Point Calculation

• 1st OPERATION

String

Weight Rotary

Table

Fig 1

T1=SW + T1=SW + T1=SW + T1=SW +

10% MOP10% MOP10% MOP10% MOP

1st Mark

Fig 2

Fig 3

Increase

SW+20% MOP

1st Mark

T1=SW+

10% MOP

Fig 4

Datum line A

2nd Mark

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Tubular Free Point Calculation

• 2nd OPERATION

T1=SW+

30% MOP

Fig 5

3rd Mark

Datum line A

T1=SW+

40% MOP

Fig 6

3rd Mark

Datum line A

T1=SW+

30% MOP

Fig 7

3rd Mark

Datum line A

4rd Mark

Fig 8

3rd Mark

Datum line A

4rd Mark

Datum line BDLDLDLDL

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Free Point Indicator ToolReading Stretch

• The FPIT measures the pipe elongation over the distance between

the two anchoring points.

• To determine if the pipe is stuck or free at a particular depth, the

driller first pulls the “normal weight” corresponding to the buoyant

weight of the entire string in the hole.

• The FPIT is then positioned, and an additional pull at surface is

applied to the pipe. The tool gives a strain measurement.

Reading Torque

• Measures the twist strain between the two anchoring points. The

anchors transmits the pipe deformation to a linear sensor.

• The ratio of this torque value computed by the program gives the

free percentage in torque

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Free Point Indicator ToolTaking Data

• Tie in CCL depths with the driller first by checking a prominent feature in BHA.

• Then record CCL log to 1000’ above estimated stuck point.

• Take readings in both stretch and torque every 90 ’ (every 3 connections) starting about 500’ above the suspected stuck point.

• Once the stuck point determined, reduce the measurement intervals to 30 ‘ and take a few readings on either side of stuck point.

Interpretation

• The free point is selected as the deepest point at which the pipe can be backed-off and recovered.

• To cover all possible manners in which the pipe can be stuck requires both

torque and stretch readings.

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Free Point Indicator Tool

Interpretation

Four “typical cases” of stuck pipe are given on the following slides.

One may not be able to determine which cases exactly fit, however,

if the torque and stretch reading are > 80% of free pipe (in Torque and

Stretch), the pipe will most likely backoff.

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Free Point Indicator Tool InterpretationInterpretation

1. Straight hole, straight pipe, stuck in drill collars

• Both stretch and torque reading show the stuck point in the same DC

• Where there is a little wall friction, a sharp drop off in both readings observed below the stuck point

• The pipe is considered recoverable when torque and stretch indicate 80-85% of free readings.

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Free Point Indicator ToolInterpretation

2. Differential or packed stuck pipe

• In this case,transmission of pull and torque becomes more difficult

• The readings decreases below collar B, pipe should be backed-off either at collar A or collar B

• Pipe should be backed-off at the collar nearest 80 to 85% of the reading in free pipe

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3. Straight hole, extremely crooked pipe

• This case illustrates the inability to obtain normal torque in a badly bent string

• Normal stretch and torque for this string are read above collar B

• Below collar B, stretch reading is normal but torque reading is decreasing with increasing depth

• Normal torque reading can often be obtained below collar B by applying an overpull on the pipe

• The Backoff should be performed where the torque readings is still high

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4. Crooked Deviated Wells

• In deviated wells it is normally possible to transmit torque deeper than stretch

• The torque reading is often a function of the pull on pipe. Generally, best torque transmission is obtained at relatively low values of pull

• Some judgment must be exercised in what can be backed-off. While a Backoff may at times be made with less than 25 % stretch reading, it is not recommended to attempt a backoff without a torque reading of 50 % free pipe

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FPIT Example

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Backoff ProcedureBasic requirements to assure successful Backoff

1. Tighten up the entire pipe string with torque 30% higher than the value that will be used for backing off, and “work this torque down”.

2. Count how many turns come back.

3. When the same amount of right hand turns come back, the pipe is ready to back off.

4. Have the connection to be unscrewed in slightly in tension

• The FPIT tool can be an excellent indicator of torque / tension being transmitted to the connection to be backed off – Use It !

5. Apply sufficient left hand torque to the connection to be unscrewed

• First, apply LH torque and gradually work down hole to the connection that has been chosen to attempt to backoff. Apply ½ to 1 LH turn per 1000’of drill pipe.

• Apply as much left-hand torque that can be safely transmitted to the joint without risking a mechanical backoff somewhere else in the drill string.

• Place the buoyed string weight at the neutral point of the connection, plus a slight overpull ( 2,000 to 10,000 lb ).

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Backoff Procedure

Backoff Torque recommendations

Steel Drill Pipe

Pipe in unknown condition requires discretion as low grade, badly

worn or fatigued pipe may not accept the average values given in

the table above

½ - ¾0 – 4,000

¾ - 1 Over 9,000

½ - 14,000 – 9,000

Rounds per 1,000’Backoff depth (ft)

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Backoff Procedure5. Accurately position and fire a string shot of adequate strength at the Backoff point.

• The size of the shot must be strong enough to Backoff tool joint without splitting the pipe body. This can be calculated by using WL tables for the diameter of the pipe.

Applying sufficient left torque to the string and properly working it down to the Backoff point prior

to firing the explosive charge is often the most critical and difficult part of the operation.

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Backoff ProcedureDetermination of the proper Backoff weight

Theoretically the connection to be backed off should be in a neutral tension

condition. However, experience shows that is better to have the joint slightly in

tension rather than in compression.

Three different approaches may be used to determine the exact pull at surface:

1. Calculate buoyant weight of pipe down to the back off point. Pull this amount plus 10 % of buoyant weight above stuck point.

2. Obtain tension, when moving up before sticking from the drillers log. Subtract the weight of the fish to be left in the hole and add 10% of buoyant weight above stuck point

3. Use FPIT measurements to determine what kind of over pull puts the string in tension where the pipe is going to be backed off and use this value plus 5 Klbs overpull.

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Backoff ProcedureDetermination of the proper Backoff weight

Corrections

• Note whether or not the weight of the block was included when the weight

indicator was zeroed

• Check if the weight of kelly in the string is included in the readings

• The mud pumps should be turned while taking the load readings.

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Backoff Preparation

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Excessive Primacord

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9- free point and backoff -(houston)

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