DRILLING PRACTICES

1

JARS AND ACCELERATORS

• Types of Drilling Jars

• Forces required to fire Jars

• Accelerators

• Jar and Accelerator positioning

• Successful use of Jars

DRILLING PRACTICES

2

DRILLING JARS

• Two basic types of jar available

(1) Hydraulic: Impact is a combination of load and time.

(2) Mechanical: Impact is load dependant.

DRILLING PRACTICES

3

Successful Use of Jars

Reasons a jar may fail to fire:

• Incorrect weight applied, calculations incorrect.

• Pump open force exceeds compression force to fire jar down.

• Stuck above the jar.

• Jar mechanism failed.

• Jar not cocked.

• Drag does not allow sufficient force to be applied to fire jar (usually mechanical jars).

• Well path prevents compression being applied to down jar action.

• Jar has fired but unable to detect at surface.

• Right hand torque is trapped in torque set mechanical jars.

• Not waiting long enough for jar to fire.

DRILLING PRACTICES

4

Forces Required to Fire Jars

All jars have a firing force envelope for each direction they fire in:

The firing force envelope consists of two forces:

• One to cock the jar.

• One to fire the jar.

A dual acting jar has two force envelopes:

• One for down jarring.

• One for up jarring.

The jar envelope forces can be considered at the jar or at the surface.

The rig teams job is to estimate the surface firing force envelopes.

Forces applied to cock and fire a jar on the test bench are forces at the jar.

DRILLING PRACTICES

5

Jar Envelope

Down Jar

Envelope

Up Jar Envelope

0

200

400

600

800

X 1000 lbs

Weight Indicator

Jar cocked to fire

down

Jar fires down

Jar cocked to fire upJar fires up

DRILLING PRACTICES

6

Pump-Open ForceAssist firing the jar up

Assist cocking the jar after firing down Oppose firing

the jar down

Oppose cocking the jar after firing up

DRILLING PRACTICES

7

ACCELERATORS

The functions of a drilling accelerator:

• Compensate for lack of stretch in a short string.

• Compensate for slow contraction of the drill string due to drag.

• Act as a shock reflector to the shock wave travelling up the drill string from the jar blow.

• To intensify the jar blow.

DRILLING PRACTICES

8

JAR AND ACCELERATOR POSITIONING

Considerations:

• Likely places for sticking to occur.

• Most likely jarring direction required.

• Wellbore contact and differential sticking.

• Position of axial neutral point when drilling with max WOB.

• Depth of hole section.

• Drag in hole section.

• Minimum allowable measured weight for plastic buckling when not rotating.

DRILLING PRACTICES

9

JAR AND ACCELERATOR POSITIONING

Guidelines for vertical wells:

(1) Jar must be placed above buckling neutral point even when max WOB is applied.

(2) There should be at least two drill collars above the jars.

(3) Jars should have differential sticking prevention subs fitted if this is a risk.

(4) No stabilisers should be placed above the jars.

(5) Use accelerators in shallow hole section. Ensure cocking and firing jar is possible before running.

DRILLING PRACTICES

10

JAR AND ACCELERATOR POSITIONING

Guidelines for Deviated and Horizontal wells:

(1) Do not run jars buckled. Avoid placing jars in the pressure area neutral point

(2) If using two jars or two jars with an accelerator, be fully aware how to use the system.

(3) Jars should have differential sticking prevention subs fitted if this is a risk.

(4) Calculate measured weight reading at the jar for cocking and firing as drag may prevent seeing the jars open and close on the weight indicator.

(5) A common problem is getting sufficient force to a horizontally placed jar to fire down.