Well abandonment and slot recovery
A method of recovering casing in a wellbore using a vibratory casing recovery bottom hole assembly. In a vibratory casing recovery bottom hole assembly having a casing spear, a flow modifier and one or more further elements including a shock sub. The flow modifier produces cyclic variations in fluid flow through the assembly at a first frequency and at least one of the elements is configured to have a natural or resonant frequency when vibrated to be near or at the first frequency. By tuning elements of the bottom hole assembly to be close or at the frequency of the output of the flow modifier, the dynamic amplification factor of the system is maximised and longer lengths of casing can be recovered. A method of recovering casing using the vibratory casing recovery bottom hole assembly is also described. Further embodiments include a casing cutter and a hydraulic jack.
1. A method of casing recovery in a wellbore, comprising the steps;
(a) running a string into the wellbore, the string including a vibratory casing recovery bottom hole assembly, the vibratory casing recovery bottom hole assembly comprising:
a casing spear having a gripping mechanism to anchor the spear to casing to be recovered;
a flow modifier for producing cyclic variations at a first frequency in the flow of fluid through the string, and
one or more elements, the one or more elements including an extension and retraction means for location in the string and adapted to axially extend or contract in response to variations in the flow of fluid through the string; and
the extension and retraction means being arranged between the casing spear and the flow modifier within the casing to be recovered;
(b) anchoring the casing spear to the casing to be recovered by use of the gripping mechanism;
(c) pumping fluid from surface through the string to produce cyclic variations at the first frequency in the flow of fluid through the string to induce vibration in and axially extend and contract the extension and retraction means;
(d) pulling the string and the vibratory casing recovery bottom hole assembly to recover the casing to be removed;
characterised in that:
at least one element is configured to have a resonant frequency in the assembly when vibrated wherein:
0.6<first frequency/resonant frequency<1.2.
2. The method of casing recovery in a wellbore according to claim 1 wherein at least one element is configured to have a resonant frequency in the assembly when vibrated wherein:
0.9<first frequency/resonant frequency<1.1.
3. The method of casing recovery in a wellbore according to claim 1 wherein the extension and retraction means comprises a housing forming part of the string and containing a fluid actuated member and an oppositely acting biasing arrangement, fluid force tending to actuate the member in one direction and the biasing arrangement acting in the opposite direction.
4. The method of casing recovery in a wellbore according to claim 3 wherein the fluid actuated member is a piston and the biasing arrangement a spring.
5. The method of casing recovery in a wellbore according to claim 1 wherein the at least one element comprises the string and all tools suspended from the flow modifier wherein a length of the bottom hole assembly between the extraction and retraction means and an end of the string inside the casing to be recovered is selected to provide the resonant frequency.
6. The method of casing recovery in a wellbore according to claim 1 wherein the at least one element includes one or more pipe members arranged between the extension and retraction means and the casing spear wherein a length between the gripping mechanism and the extension and retraction means is selected to provide the resonant frequency.
7. The method of casing recovery in a wellbore according to claim 1 wherein the first frequency is between 15 Hz and 20 Hz.
8. The method according to claim 1 wherein the method includes providing a downhole pulling tool on the string above the vibratory casing recovery bottom hole assembly and using the downhole pulling tool to pull the vibratory casing recovery bottom hole assembly and casing to be recovered before pulling the string to recover the casing.
9. The method according to claim 1 wherein the method includes the additional steps of providing a casing cutter in the vibratory casing recovery bottom hole assembly and cutting casing to provide a cut section of casing to be removed on the same trip as recovering the casing.