Method for Abandoning a Well The invention relates to the field of hydrocarbon exploration and more specifically to methods of efficiently abandoning wells when they have come to the end of their useful life. Background to the Invention In order to make the wellbore safe and to meet regulatory standards, it is necessary to set several plugs within the wellbore to resist any build-up of pressure that may occur in the future. In order to prepare the wellbore for the setting of such plugs it is necessary to remove the production tubing from the well. Tubing removal is a costly operation requiring the employment of expensive equipment, such as a drilling rig. The present invention seeks to provide a more cost-effective solution by leaving at least some of the tubing in the well, whilst providing confirmation that the well has been left in a safe state. In the course of constructing an oil or gas well, a hole is drilled to a pre-determined depth. The drilling string is then removed and a metal tubular or casing is run into the well. When the casing reaches the bottom of the well, cement is pumped down the casing and displaced up the annulus between the casing and the original wellbore. The function of the cement is to secure the casing in position and ensure that the annulus is sealed. This process of drilling, running casing and cementing is repeated with successively smaller drilled holes and casing sizes until the well reaches its target depth. At this point, a final tubular or tubing is run into the well. The tubing is secured at its top and at its bottom end, but it is not cemented in place so as to facilitate potential remedial operations, such as removal and replacement of the tubing in the event that it becomes damaged or corroded. A valve, known as a downhole safety valve, is positioned in the upper part of the tubing typically 500 ft below the wellhead. Should a safety problem occur, this valve can be closed to seal in pressure. Activation of the valve is accomplished by applying pressure from surface down control lines running alongside and clamped to, the tubing. During the well construction phase, after each cementing operation, confirmation of the quality of the cement sheath around the casing is desired. A typical way of providing such confirmation would be to carry out a cement bond log (CBL). The CBL will confirm whether the quality of the cement sheath is adequate. If it proves that the CBL shows that the cement quality is not adequate certain remedial operations may be possible. These processes are necessary so that when the construction phase is complete, the well operator has a record demonstrating that the successive annuli are secure. The well may now be put on production, with the hydrocarbons flowing up the tubing and gathered at surface. Over time, which may be several decades, the production of hydrocarbons reduces until the production rate is no longer economically viable, at which point the well has reached the end of its productive life. The well now needs to be plugged and abandoned. Although regulations vary between jurisdictions, a universal requirement is that the abandoned well should not leak hydrocarbons at any point in the future. In abandoning the well, the operator has the primary legal responsibility to demonstrate to the regulatory authorities that everything practical has been done to ensure a secure abandonment. Ordinarily, the CBL record would form part of the evidence that the operator would produce to support the case for a securely abandoned well, however in many cases the CBL record may not be available, or the quality of the CBL record may not be adequate, due to the limitations of the technology of the day or due to the acceptance criteria having become more stringent. It may even be the case that the operator has a good CBL, but decides that the risk of a leak is still too high given the uncertainty of the future regulatory framework and associated penalties. Unless the operator is both able and willing to convince the regulatory authorities that the cement bond is acceptable, they have to contemplate how to arrive at a position of acceptance. In many cases, it is decided that the best method is to gain access to the cemented casing by removing the final tubing from the well. In order to gain access to the casing for which a new CBL needs to be carried out, the operator needs to remove the final tubing from at least that point up to the surface of the well. This is because cement bond logs cannot be made through two strings of metal tubular. One method of doing this would be to pull the entire tubing string, alternatively the tubing can be cut just below the point of interest and the tubing above pulled from the well, leaving the lower portion in place. In either event, costly surface equipment such as a drilling rig capable of pulling tubing needs to be provided. A CBL can now be carried out. In the event that the CBL is shown to be good, the operator can set a cement plug inside the casing and move up the well to the next zone of interest. This may be repeated several times until the entire well is deemed secure. If however the CBL shows the cement to be of insufficient quality, the operator now has the choice either to mill away the casing and the old cement over a sufficiently long section and place a new cement plug or alternatively to perform a remedial cement job. Remedial cementing would involve perforating the casing, washing out as much of the old cement as possible and squeezing new cement through the perforations, known as a perforate, wash and squeeze job, whilst also leaving a cement plug within the casing. It can now be seen that the default option for the situation where existing data shows the cement to be of uncertain quality, or where there is no data available is to remove the tubing from at least that point up to the surface of the well. As has been explained, this is necessarily an expensive process. However, it can be seen that removal of the tubing is merely a facilitator in determining whether the cement bond is adequate, if the cement bond is shown to be inadequate then tubing removal has been justified, but if the cement bond is adequate then tubing removal has proved to be an unwarranted expense. Even in the situation where all the CBL's show that the cement quality is good, the operator still has to remove the upper part of the tubing. This is because an upper cement plug needs to be set just below the wellhead. Summary of the Invention It is an object of at least one aspect of the present invention to obviate or at least mitigate the foregoing disadvantages of prior art methods for abandoning a well. It is another object of an aspect of the present invention to provide a method of determining the status of a cement bond without removing the production tubing from the well. It is further object of at least one aspect of the present invention to provide a method to determine whether it is required to remove the tubing after the adequacy of the cement bond is known. It will be shown that in those cases where the cement bond is shown to be adequate there are significant savings to be made over the methods employed during current practices. It is another object of an aspect of the present invention to provide a reliable, quick and cost efficient method of abandoning a well. Further aims and objects of the invention will become apparent from reading the following description. According to a first aspect of the invention, there is provided a method of determining the condition of a cement bond of a casing in a wellbore comprising;

perforating a production tubing in the wellbore at a zone of interest;

displacing a settable composition through the perforations into the annulus between the casing and tubing to secure the production tubing;

cutting the tubing; and

assessing the status of the cement bond of the casing. The method may comprise running a perforating tool through the tubing to a

predetermined and/or desired depth. The method may comprise perforating the tubing using explosive charges or a perforating tool. Preferably the settable composition supports the tubing and secures the tubing in position. The method may comprise securing the tubing rigidly in position in the wellbore.

Preferably, the method comprises securing the tubing rigidly by allowing the settable composition to set hard in the annulus between the casing and tubing. The method may comprise securing the tubing temporarily in position before the cutting and/or milling operation is started. The method may comprise providing a tubing cutter to cut a slot through a wall of the production tubing. The method may comprise deploying a milling tool to mill away the tubing. The method may comprise milling away the securing settable composition in the annulus between the casing and the tubing. The method may comprise milling in an upward or downward direction. The method may comprise milling away the tubing up to the top of the securing settable composition. Preferably the method comprises assessing the quality of the cement bond at a zone of interest by deploying a cement bond logging tool through the tubing to the zone of interest. The method may comprise pulling the tubing out of the casing if the cement bond is shown to be of poor quality. The method may comprise deploying a cement plug if the cement bond is shown to be of adequate and/or good quality. The method may further comprise assessing the quality of the cement of a second zone by running a tubing cutter tool in the tubing and cutting the tubing at the upper end of the second zone. The method may comprise cutting the tubing and dropping the cut tubing further downhole. By dropping the tubing further downhole the second zone is exposed and the quality of the cement of a second zone may be assessed. The method may comprise assessing the quality of the cement bond at multiple zones. The method may comprise running a tubing cutter tool in the tubing and cutting and/or milling the tubing at a second and/or further zone to expose the cement bond to allow assessment of the quality of the cement bond. The method may comprise positioning the cutting tool higher up the well in the event a cement bond is not identified in the second and/or further zone. The method may comprise making further cuts to the tubing and exposing cement bonds until a zone with good quality cement is identified. It will be appreciated that wells vary in complexity and there may be either more or less zones of interest than described above, however it will also be appreciated that the sequences of operation described heretofore can be applied as many times as are necessary and are not limited to two zones of interest. The settable composition may be selected from the group comprising cement, resin and/or gel. Preferably, the settable composition is cement. The method may comprise deploying a cementing tool to displace a pre-determined amount of cement through the perforations into the annulus between the casing and the tubing. The current invention provides a method for determining the status of a cement bond without removing the tubing from the well. As has been seen, removal of the tubing is an expensive process which in certain circumstances may have proved to be unnecessary. The tubing is perforated at the zone of interest and cement, resin and/or gel is displaced through the perforations into the annulus between the casing and tubing. The cement resin and or gel is then allowed to set. The tubing is then cut and milled away, along with the previously placed cement, resin and/ or gel. Under normal circumstances it would not be possible to mill tubing because it is not supported securely in the lateral direction. Any attempt at milling would fail due to vibration and the milling tools would be quickly damaged. According to the current invention however there is an intermediate step of providing cement, resin and/ or gel to the tubing over the zone of interest. The cement, resin and/ or gel holds the tubing securely in place whilst the tubing milling operation is carried out. A CBL logging tool can then be run through the tubing to the milled-away section and a log taken of the cemented section outside the casing. The advantage gained by performing these operations is that they can all be done by using a low cost surface package, for example a boat rather than a rig. The status of the cement bond can thus be established before the tubing has been removed. Where the cement proves to be of adequate quality, the tubing need not be removed. Where the cement proves to be of inadequate quality, the operator now has the choice of performing a perforation, wash and squeeze job to improve the cement behind the casing, or the well is suspended pending availability of an appropriate rig and then the tubing can justifiably be removed. In the case where the cement is of adequate quality, the next operation would be to move up the well to the next zone of interest (if any), however at this point there is no need to perforate, cement and mill away the tubing. The tubing is simply cut at the top of the zone of interest. The tubing is then free to fall under gravity into the space created by the previous tubing milling operation and the next CBL can be taken and the process repeated. Thus it can be seen that tubing need not be removed from the well unless and until a poor quality cement is detected and even then, a perforation, wash and squeeze job may still obviate the need to remove tubing. If all the cement bonds are good then no tubing is removed at all, obviating the need for an expensive rig on location entirely. If only some of the cement bonds are of good quality, then less tubing may need to be removed and this amount has been justified by obtaining the CBL information before taking the decision to deploy a rig. According to a second aspect of the invention, there is provided a method for abandoning a well comprising: determining the condition of a cement bond of a casing in a wellbore according to the first aspect of the invention and

pulling the production tubing out of the wellbore and/or deploying a cement plug. Preferably the method comprises assessing the quality of the cement bond at a zone of interest by deploying a cement bond logging tool through the tubing to the zone of interest. The method may comprise pulling the tubing out of the casing if the cement bond is shown to be of poor quality. The method may comprise deploying a cement plug if the cement bond is shown to be of adequate and/or good quality. The method may comprise assessing the quality of the cement of a second zone by running a tubing cutter tool in the tubing and cutting the tubing at the upper end of the second zone. The method may comprise cutting the tubing and dropping the cut tubing further downhole. By dropping the tubing further downhole the second zone is exposed and the quality of the cement of a second zone may be assessed. The method may comprise assessing the quality of the cement bond at multiple zones. The method may comprise running a tubing cutter tool in the tubing and cutting and/or milling the tubing at a second or further zone to expose the cement bond to allow assessment of the quality of the cement bond. The method may comprise positioning the cutting tool higher up the well in the event a good quality cement bond is not identified in the second and/or further zone. The method may comprise making further cuts to the tubing and exposing cement bonds until a zone with good quality cement is identified. Embodiments of the second aspect of the invention may include one or more features of the first aspect of the invention or its embodiments, or vice versa. According to a third aspect of the invention, there is provided a method of milling a production tubing section in a wellbore casing comprising

perforating the tubing section;

passing settable composition through the perforations into the annulus between the casing and tubing to secure the tubing; and milling the tubing section and the settable composition securing the tubing section. The method may comprise perforating the tubing using explosive charges or a perforating tool. The method may comprise running a perforating tool through the tubing to a

predetermined and/or desired depth. Preferably the settable composition supports the tubing and secures it in position.

Preferably, the method comprises rigidly securing the tubing by allowing the settable composition to set hard in the annulus between the casing and tubing. The method may comprise securing the tubing temporarily in position before the milling operation is started. The method may comprise providing a tubing cutter to cut a slot through the wall of the tubing. The method may comprise deploying a milling tool to mill away the tubing. The method may comprise milling away the settable composition in the annulus between the casing and tubing. The method may comprise milling in an upward or downward direction in the wellbore. The method may comprise milling away the tubing up to the top of the securing cement. The settable composition may be selected from the group comprising cement, resin and/or gel. Preferably, the settable composition is cement. The method may comprise deploying a cementing tool to displace a pre-determined amount of cement through the perforations into the annulus between the casing and the tubing. Embodiments of the third aspect of the invention may include one or more features of the first or second aspect of the invention or their embodiments, or vice versa. According to a fourth aspect of the invention, there is provided a method of determining the condition of a cement bond of a casing in a wellbore comprising;

cutting the production tubing at a zone of interest;

dropping at least a section of the production tubing downhole; and

assessing the status of the cement bond of the casing. The method may comprise running a cutting tool through the tubing to a predetermined and/or desired depth. The method may comprise cutting a slot through a wall of the production tubing. The method may comprise deploying a milling tool to mill away the tubing. The method may comprise milling in an upward or downward direction. Preferably the method comprises assessing the quality of the cement bond at a zone of interest by deploying a cement bond logging tool through the production tubing to the zone of interest. The method may comprise pulling the tubing out of the casing if the cement bond is shown to be of poor quality. The method may comprise deploying a cement plug if the cement bond is shown to be of adequate and/or good quality. The method may further comprise assessing the quality of the cement of a second zone by running a tubing cutter tool in the tubing and cutting the tubing at the upper end of the second zone. The method may comprise cutting the tubing and dropping the cut tubing further downhole. By dropping the tubing further downhole the second zone is exposed and the quality of the cement of a second zone may be assessed. The method may comprise assessing the quality of the cement bond at multiple zones. The method may comprise running a tubing cutter tool in the tubing and cutting and/or milling the tubing at a second or further zone to expose the cement bond to allow assessment of the quality of the cement bond. The method may comprise positioning the cutting tool higher up the well in the event a cement bond is not identified in the second and/or further zone. The method may comprise making further cuts to the tubing and exposing cement bonds until a zone with good quality cement is identified. The method may comprise cutting and/or milling the production tubing at a position in the wellbore lower than the zone of interest prior to cutting the production tubing at the zone of interest to provide and/or create a space for the protection tubing to fall when the production tubing is cut at a zone of interest. The term "lower" in this context means a position further from the surface than the zone of interest. Embodiments of the fourth aspect of the invention may include one or more features of the first to third aspects of the invention or their embodiments, or vice versa. According to a fifth aspect of the invention, there is provided a method of allowing a section of tubing in a well to be milled, wherein the tubing is temporarily secured rigidly in position before the milling operation is started. The method may comprise securing the tubing by perforating the tubing and cementing through the perforations. The method may comprise securing the tubing by perforating the tubing and displacing resin and/or gel through the perforations. The method may comprise subsequently assessing the quality of cement outside the casing surrounding the tubing. The method may comprise cutting the tubing a distance above the milled section. The method may comprise relocating the section of tubing below the cut further down the well. Embodiments of the fifth aspect of the invention may include one or more features of the first to fourth aspects of the invention or their embodiments, or vice versa. Brief description of the drawings There will now be described, by way of example only, various embodiments of the invention with reference to the following drawings (like reference numerals referring to like features) in which: Figure 1 shows a sectional diagram of a typical well with two strings of casing and tubing installed. Figure 2 shows a sectional diagram of the well of Figure 1 demonstrating the current practice of removing the tubing before performing a CBL to assess the cement bond. Figure 2a shows a sectional diagram of well of Figure 2 after cement plugs have been set according to current practice. Figures 3a to 3f show sectional diagrams of a well demonstrating the typical sequence of operations to assess the condition of a cement bond at zone 2 according to the current invention; Figures 4 to 6 shows sectional diagrams of a well demonstrating the typical sequence of operations to assess the condition of a cement bond at zone 1 according to a second embodiment of the invention; and Figure 7a and 7b shows sectional diagrams of a well demonstrating the typical sequence of operations to assess the condition of a cement bond at zone 3 according to a further embodiment of the invention. Detailed description of preferred embodiments Figure 1 shows a typical well with two strings of casing and tubing installed. The initial section of wellbore 1 was drilled to a certain depth, after which casing 2 was run into the well. Cement 3 was set over a portion of the outside of the casing 2, sealing the annulus between the casing 2 and the wellbore 1. The next section of wellbore 4 was then drilled to the target depth of the well. A next section of casing 5 was run into the well, suspended inside the first casing 2 with a hanger 5a and likewise cemented 6 to seal the annuls between the second casing 5 and the wellbore 4. Production tubing 7 was then run into the wellbore and suspended at its upper end with a hanger 8 and sealed and anchored at its lower end by a packer 9. As has been described, when it comes time to abandon the well the cement sheaths at zone 1 and zone 2 need to be assessed for quality. Figure 2 shows a typical operations demonstrating current practice. In order to expose zone 2 for assessment, the tubing 7 and tubing hanger 8 have been removed by deploying a drilling rig. A cement bond logging tool 10 is now deployed on drill pipe, coiled tubing or wireline 11. Only when the CBL has been assessed will it be known whether the operation to remove the tubing was worthwhile. Figure 2a shows a typical well after plugs have been set according to current practice. A lower cement plug 13 has been set inside casing 5 just above the packer 9. This plug 13 in combination with the previously set cement 6 provides a barrier at the lower part of the well. An upper cement plug 14 has been set inside casing 2 just above hanger 5a. This plug in combination with the previously set cement 3 provides a barrier at an intermediate position in the well. Figures 3a-f show a typical sequence of operations according to the current invention and in particular show zone 2 in detail. According to the present invention the first operation is to perforate the tubing. In fig 3a, a perforating tool (not shown) is run through the tubing 7 to a first desired depth and explosive charges produce holes 20a in the tubing 7. The perforating tool (not shown) is moved to a second desired depth and explosive charges produce holes 20b in the tubing 7. In fig 3b, a downhole tool (not shown) deploys a pre-determined amount of a settable composition 21 , through the lower set of perforations 20b into the annulus between the casing 5 and the tubing 7. In this exemplary embodiment, cement is used as a settable composition 21. When the pre-determined amount of cement has been deployed through the lower set of perforations 20b the level of cement has reached the upper set of perforations 20a in the tubing 7. The downhole tool may have sensors to detect cement coming back into the tubing through the upper set of perforations 20a. The cement 21 is then allowed to set hard, thus securing the tubing 7 rigidly in preparation for the next operations. In Fig 3c, a tubing cutter (not shown) is deployed, cutting a slot 31 through the wall of the tubing 7, and fig 3d shows a tubing mill 35 deployed through, and milling away the tubing 7 up to the top of the previously placed cement 21. The length of tubing 7 milled away is pre-planned and is labelled 'A' and might typically be 200ft. The tubing mill 35 is removed from the well. In fig 3e, a cement bond logging tool 36 is deployed through the tubing 7 to assess the quality of the cement 6 of zone 2. If the cement is shown to be of poor quality, then the well is suspended pending deployment of a rig to pull the tubing as per Fig 2. However, if the cement 6 is shown to be of adequate quality the next operation, as shown in fig 3f, is to run a cementing tool (not shown) and deploy a cement plug 41 at the lower end of the milled section TV. Typically, the cement plug 41 might be 100ft thick. Importantly, this leaves a gap 'B' of 100ft for example, between the lower end of the tubing 7a and the top of the cement plug 41. Figure 4 shows the state of the well after the operations of figs 3a-f. The lower part of the well (zone 2) has been secured and a gap 'B' has been left between the lower end of the tubing 7a and the top of the cement plug 41. The next operation is to assess the quality of the cement over zone 1. However due to the gap 'B' left below the lower end of the tubing 7a it is not necessary to repeat the operation of figs 3a-f. In order to expose the cement 3 of zone 1 for assessment of the cement quality, a tubing cutter (not shown) is run and the tubing 7 is cut at the upper end of zone 1. Upon cutting the tubing 7, the lower part is able to fall under gravity until it lands on top of the cement plug 41. Figure 5 shows the lower part of the tubing 7b with its lower end 7a located on the cement plug 41. There is now a gap of length 'B' between the upper end 7c of the lower part of the tubing 7b and the lower end 7d of the upper part of the tubing 7e. This gap Έ 'has now exposed zone 1 for assessment of the quality of the cement 3. In a similar manner as previously described, a cement bond logging tool now assesses the cement quality and if poor, the well is suspended until a rig is available to pull the tubing 7e from the well. If the cement quality is good, then, again as previously described, a cementing tool is run to place a cement plug 42 in the lower part of gap 'Β'. In the case where there are only two zones of interest, operations concerning the tubing 7b and 7e are complete and the final state of the well is shown in figure 6. The term "upper part" in this context means that this part is closer to the surface than the "lower part". In general, relative terms such as "upper" and "lower" are used to indicate directions and locations as the apply to the drawings. If the cement quality at zone 1 is poor, an alternative to pulling the tubing 7e from the well is to move the cutting tool upward in the wellbore to a depth closer to the surface and a make a further cut in the tubing as shown in Figure 7a. Figure 7a shows that when the further cut in the tubing is made the cut section of tubing 7e drops down the well onto the previously cut tubing section 7b and exposes a new section of casing for evaluation of the cement bond. The tubing 7e with its lower end 7d is located on the upper end 7c of the lower part of the tubing 7b. There is now a gap of length 'C between the tubing end 7f of the tubing 7e and the lower end 7g of the upper part of the tubing 7h. This gap 'C" has now exposed zone 3 for assessment of the quality of the cement 3. As previously described, a cement bond logging tool now assesses the cement quality and if the cement quality is good, a cementing tool is run to place a cement plug 42 in the lower part of gap 'C as shown in Figure 7b. If the cement quality is poor at zone 3, the cut and drop operation is repeated by moving the cutting tool upward in the wellbore to depths closer to the surface and a further cuts in the tubing are made until a zone with good quality cement is identified and a cement plug may be placed. By applying this cut and drop operation it is not required to provide costly surface equipment such as a drilling rig in order to pull the tubing and perform remedial operations. It will be appreciated that wells vary in complexity and there may be either more or less zones of interest than described above, however it will also be appreciated that the sequences of operation described heretofore can be applied as many times as are necessary and are not limited to two zones of interest. Throughout the specification, unless the context demands otherwise, the terms 'comprise' or 'include', or variations such as 'comprises' or 'comprising', 'includes' or 'including' will be understood to imply the inclusion of a stated integer or group of integers, but not the exclusion of any other integer or group of integers. Furthermore, relative terms such as "upper", "lower" and the like are used herein to indicate directions and locations as they apply to the appended drawings and will not be construed as limiting the invention and features thereof to particular arrangements or orientations. The foregoing description of the invention has been presented for the purposes of illustration and description and is not intended to be exhaustive or to limit the invention to the precise form disclosed. The described embodiments were chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilise the invention in various embodiments and with various modifications as are suited to the particular use contemplated. Therefore, further modifications or improvements may be incorporated without departing from the scope of the invention as defined by the appended claims. The invention provides a method of reducing the cost of plugging a well during the course of the abandonment process; consisting of utilizing a low cost surface package, such as a boat, to perforate, cement and mill away a section of tubing to gain access to and perform a cement bond log (CBL) of the casing outside the tubing. Should the CBL prove the cement to be of adequate quality the tubing need not be removed from the well. In the event that the cement is of inadequate quality and depending on the particular circumstances the operator may choose either to perform remedial operations or the tubing may justifiably be removed. The invention provides a method of determining the condition of a cement bond of a casing in a wellbore. The method comprises perforating a tubing in the wellbore at a zone of interest and displacing a settable composition through the perforations into the annulus between the casing and tubing to secure the tubing. The method also comprises cutting the tubing and assessing the status of the cement bond of the casing.