Jarring device

1 . A jar for providing an impact to a tool string stuck in a wellbore, the jar comprising:

a housing and a mandrel configured to move relative to the housing, and

a release mechanism actuatable to fire the jar to cause the housing and mandrel to collide by the action of an applied tension to cause an impact to the tool string, and

a transducer configured to measure the axial position of the mandrel relative to the housing to allow for a real-time measurement of the jar status at the surface based on the measured axial position, and

wherein the transducer is a linear variable differential transformer comprising:

a primary coil and two secondary coils mounted to the housing, and

a core comprising a magnetic permeability material fixed to the mandrel and received within a bore of the coils to move axially with the mandrel therein, the core comprising a non-magnetic portion fixed to the mandrel and the magnetic permeable material mounted to the non-magnetic portion,

wherein the mandrel is hollow, and wherein the non-magnetic portion of the core is a hollow cylinder connected in-line with the mandrel, an inside of the mandrel and the core providing a conduit for electrical conductors to pass through the jar.

2 . A jar as claimed in claim 1 , wherein the release mechanism is configured to be actuated by axial displacement of the mandrel relative to the housing by a predetermined actuation distance, and

the transducer is configured to measure the axial position of the mandrel relative to the housing to allow for a real-time measurement at the surface of the jar status based on the measured axial position and the actuation distance.

3 . The jar as claimed in claim 1 , wherein the transducer is configured to continuously and incrementally measure the axial position of the mandrel relative to the housing.

4 . The jar as claimed in claim 1 , wherein the non-magnetic hollow cylinder forms a faraday shield to prevent electrical carrying conductors passing through the transducer from interfering with a signal from the transducer.

5 . The jar as claimed in claim 1 , wherein the jar comprises a compensation piston and an inside of the housing, mandrel and core is fluid filled, and the fluid is pressure compensated to an external ambient pressure by the compensation piston.

6 . A system for providing an impact to a tool string stuck in a wellbore, the system comprising:

a jar as claimed in claim 1 ,

a data acquisition system to provide communication between the jar and a surface wellsite, and

a surface user interface to communicate the real time measurement of the jar status at the surface based on the measured axial position.

7 . The system as claimed in claim 6 , wherein the jar release mechanism is configured to be actuated by axial displacement of the mandrel relative to the housing by a predetermined actuation distance, and

the system is configured to provide the real-time measurement of the jar status based on the measured axial position and the actuation distance.

8 . The system as claimed in claim 7 , wherein the system is configured to provide the real-time measurement of the jar status based on the measured axial position, a rate of axial travel of the housing relative to the mandrel and the actuation distance.

9 . The system as claimed in claim 7 , wherein the measurement of the jar status is or comprises one or more of:

a distance the mandrel has travelled or is yet to travel along the actuation distance,

a distance the mandrel has travelled or is yet to travel along the actuation distance as a percentage of the actuation distance, and/or

a time based on the distance the mandrel has yet to travel along the actuation distance and a rate of travel of the mandrel relative to the housing or known characteristics of the jar and an applied tension.

10 . The system as claimed in claim 6 , wherein the jar status comprises any one or more of jar fired, jar not-fired, jar cocked, jar not-cocked.

11 . The system as claimed in claim 9 , wherein the system is configured to provide the jar status as a continuously incrementing real time value and/or one or more real-time discrete visual or audible warning.

12 . The system as claimed in claim 11 , wherein the one or more warning is based on a distance and/or time threshold determined from i) when the mandrel has moved a portion of an actuation distance and/or ii) a time period based on a distance of travel of the mandrel relative to the housing, the actuation distance and a rate of travel of the housing relative to the mandrel.

13 . A method for firing a jar, the method comprising:

i) providing a system as claimed in claim 6 , and if the tool string becomes stuck in the wellbore:

ii) applying a pull tension to the jar,

iii) monitoring the real time measurement of the jar status to determine when the jar will fire, and continuing to apply the pull tension until the tool string is free or a hold threshold is reached, and

iv) if the tool string is not free and the hold threshold is reached, applying a firing tension to the jar, wherein the firing tension is less than the pull tension, and

v) firing the jar at the firing tension.

14 . The method as claimed in claim 13 , wherein if the tool string is not free after firing, then the method further comprises applying a pull tension to the jar without re-cocking the jar.

15 . The method as claimed in claim 13 , wherein if the tool string is not free after firing, the method further comprises:

vi) re-cocking the jar,

vii) determining an updated firing tension, and

viii) firing the jar at the updated firing tension.

16 . The method as claimed in claim 15 , wherein the updated firing tension is higher than a previous firing tension.

17 . The method as claimed in claim 15 , wherein if the tool string is not free after firing at the updated firing tension, the method further comprises refiring the jar at progressively higher firing tensions up to a maximum allowable firing tension or until the tool string is free.

18 . The method as claimed in claim 13 , wherein the hold threshold comprising any one or more of:

a distance the mandrel has travelled or is yet to travel along the actuation distance,

a distance the mandrel has travelled or is yet to travel along the actuation distance as a percentage of the actuation distance, and/or

a time based on the distance the mandrel has yet to travel along the actuation distance and a rate of travel of the mandrel relative to the housing or known characteristics of the jar and an applied tension.