32. Techniques such as those described in WO2020/256563, which is incorporated herein by reference, can be used to identify the AFCD 32. The perforations 28 can be made by perforating guns, punches and the like tools which are known to those in the industry.

Starting at the upper end 30 of the perforations 28, a fluid 34 is introduced from a wash tool 38 into the annulus 14 to clear debris 22. This is illustrated in FIG. 1(c). A first length 36 of the perforations is washed so that fluid 34 flows through the perforations 28 and up the annulus 14 dislodging debris 22 and circulating it out of the well 12. The first length 36 is shorter than the perforation length 28. The wash tool 38 can be lowered in a continuous motion or discretely, stage by stage along the first length. Optionally, the wash tool 38 can be moved up the well 12. It will be realised that the wash tool 38 may include swab cups at one or both ends to direct fluid 34 into the annulus 14 and/or include jets to direct fluid 34 into the annulus 14 and/or be combined with a packer which seals between the string 40 and the casing 10a above the wash tool 38.

Once a portion of debris 22 is cleared from the first length 36, a spear 42 is engaged to grip the casing 10a and the cut section of casing 10a is pulled. This is shown in FIG. 1(d). If the cut section of casing 10a is free, then the cut section of casing 10a is recovered. It is noted that the entire cut section of casing 10a is removed to the CCTD 24, leaving only the remaining casing 10b, as shown in FIG. 1(e). It will be appreciated that this method will take a significantly shorter period of time than the prior art in which the entire perforation length 18 requires to be cleaned.

In the event that the cut section of casing 10a remains stuck and cannot be pulled, a second length of the perforations 28 can be washed using the wash tool 38. The second length is shorter than the perforation length 26 and extends the length of cleared debris 22 in the annulus. The spear 42 is engaged again and the cut section of casing 10a pulled. If the casing 10a is free it is recovered to surface. These steps illustrated in FIGS. 1(c) and 1(d) can be repeated for increasing lengths of washed perforations 28 until the cut section of casing 10a is free and can be pulled from the well 12. The first and second lengths may be sequentially located along the perforation length. Alternatively, the first and second lengths overlap by at least a portion. Preferably, the first, second and subsequent lengths are sequentially located along the perforation length. In this way, the perforation length may entirely be washed. Alternatively, one or more of the first, second and subsequent lengths overlap. This provides a more intense wash over the same length of perforations.

By pulling the section of casing periodically during washing, the section of casing can be pulled from the well as soon as it is free. This saves significant time in reducing the amount of washing required.

Referring to FIG. 2, there is illustrated a bottom hole assembly (BHA) 46 which can be used in the method, according to an embodiment of the present invention. BHA 46 includes the wash tool 38, the spear 42 and a downhole power tool (DHPT) 48. As illustrated, the DHPT 48 has an anchor mechanism 47 configured to grip the outer casing 20 above the inner casing 10 and is anchored thereto. The DHPT 48 includes pistons 49 which can act on a lower mandrel 50 and by application of pressure, pull high forces on the stuck casing 10. This procedure is known and is described in GB2473527B to Ardyne Holdings Ltd, which is incorporated herein by reference. In this way, the DHPT 48 is able to apply higher forces than the rig alone is able to apply. In the method of the present invention, the step of pulling the cut section of casing 10a, can include pulling using the DHPT 48. Accordingly, a reduced amount of washing will be required to remove the cut section of casing 10a.

By incorporating a DHPT 48 in the BHA 46, the system has advantages over using just a spear in that the DHPT 48 will always be able to pull the casing in less time than the spear. With the DHPT 48 in the same string then one can periodically apply DHPT 48 pulling loads at intervals during the washing. As soon as the release load 52 falls below the max DHPT load 54 the casing 10a would come free. Referring to FIG. 3 there is illustrated a graph of washing time 56 against force 58, being the force required to pull the casing. The maximum loads 54, 60 for the DHPT and the spear are indicated, respectively. The release load 52 is a falling line with increased washing time, which crosses the maximum load 54 for the DHPT in a shorter washing time 62, than for the spear washing time 64. This illustrates the benefit in reduced time in utilising the DHPT 48.

Reference is now given to FIG. 4 of the drawings, which illustrates a BHA 146, according to an embodiment of the present invention. Like parts to those of the BHA 46 of FIG. 2, have been given the same reference numeral with the addition of 100 to aid clarity. BHA 146 is mounted on a drill pipe string 140 with a through bore and includes: a DHPT 148 to engage outer casing 120; collars 66 to extend the mandrel 150 into casing 10; a spear 142; a wash tool 138 which incorporates a packer; an optional casing cutter 68; an optional logging tool 72; an optional vibration tool 74; and a perforating tool 70, being perforating guns.

In use, as described with reference to FIGS. 1(a) to 1(e), the BHA 146 is run into the well 112 and the perforating guns 70 are positioned at an appropriate depth adjacent the casing section 10 packed with debris 122. The depth may be the FCD 132 determined from use of a logging tool 72 as is known in the art. The casing 110 is perforated, after which the perforating guns 70 may be dropped into the wellbore as per known procedure. Alternatively other perforating tools such as hydromechanical punch tools may be used which are not required to be dropped from the BHA 146. The packer is then set adjacent perforations towards the upper end of the packed casing section. Fluid is then circulated down the drill string 140 and out of the wash tool 138. The fluid flows through the perforations 128 and up the annulus 14 dislodging debris 122 and circulating it out of the wellbore. The BHA 146 is then lowered, the packer sequentially exposes new perforations, thereby circulating out debris from successively lower portions of the casing. This is continued until a desired portion has been washed. The BHA 146 may be lowered in a continuous motion or discretely, stage by stage.