The objective of abandonment is to prevent the migration of all formation fluids, but especially hydrocarbons, between formations and to the surface. There are two types of conduit which must be closed off - the open hole plus casing and the annuli.

Key requirements

In the open hole all intervals which contain hydrocarbons should be covered by a cement plug extending some 50 m above and 50 m below the limits of the hydrocarbons.
Water-bearing porous and permeable formations from different geological units or under different pressure gradients should be separated by a plug, across one or the other or anywhere between them, in order to prevent cross flow.
There is no need to set a plug at TD unless it is required by the above criteria, but when considering that requirement it should be taken into account that any unlogged pocket may contain a porous and permeable interval.
A plug should always be placed across the shoe of the previous casing - at least 50m above and below. If there has been production testing the perforations should be covered by a plug as for hydrocarbon bearing intervals in open hole.

Plug setting and testing

Whether or not a plug should be tagged and weight tested depends on where it is, how critical the placement is and whether any losses may have occurred during placement. If the plug is placed on bottom with a relatively light mud in the hole and no losses there is no need to feel for it. In many cases it is quicker just to fill the whole of the open hole with cement than to set several smaller plugs and spend time waiting on cement and feeling for it. This is also a convenient method of disposing of surplus cement. In cases where the mud density and the cement density are similar the plugs should be felt for; and since high mud weights imply high pressures a pressure test of the plug in the casing shoe should also be done, to above the leak-off pressure. In critical cases where a plug separates a lower high pressure zone from a shallower, open, lower pressure zone, an inflow test should be done.

A tubing stinger, preferably made of plastic or aluminium, should be used for the abandonment plugs. This will disturb the cement less than DP as it is pulled out, and if the stinger is cemented in, either accidentally or deliberately, it can be parted by pulling. With a 700-1,000 m stinger, several 500 m plugs can easily be set one after the other (washing off between plugs but not waiting to feel for them). Again, this could only be done if there are no losses.

Poor cement

A pressure seal in the annulus can be guaranteed, and it is still very common that pressure appears on an annulus from a hydrocarbon bearing interval across which casing has been cemented. That is direct evidence of an open conduit to surface, which cannot be allowed to remain. If the cementation has been designed so that top cement is above the shoe of the previous casing it will be almost impossible to cure that leak by perforating and squeezing - and perforation will also open yet more channels for leakage into/from the formation behind the previous casing. The only remedy is to cut and recover the inner casing above top cement, and plug the outer casing above the cut. If the top of cement was designed to go into the shoe then in practice it may be much higher, and the higher the plug has to be set the longer any hydrocarbon column can be and the higher the differential pressure it has to withstand. In the worst case it can happen that a cement "tongue" is holding the casing and a series of cuts are made without being able to recover the casing.

The best method of dealing with this problem is not to cement into the previous shoe. A good quality ring of cement around the shoe of the casing being cemented is sufficient for what is required while drilling. However if there are hydrocarbon bearing formations open to the annulus the cement column should extend above them in line with the above guidelines. During subsequent drilling operations onshore or on a jack-up the annulus should be monitored for pressure and bled off/mud lubricated into it if necessary. At abandonment the casing strings should be cut in turn at the level of the previous shoe and a plug set across the cut - 50 m below to 50 m above. To prevent the plug falling a bridge plug should be set in the inside casing before cutting it. If gas flow in the annulus had been a significant problem then the cement plug can be dressed off in the outside casing and a bridge plug set, plus possibly a further cement plug.

Surface Plug

The final stage onshore is to set a ca. 50 m plug in the top of the conductor. If desired a plate can be welded over the flange of the casing head housing. It is not worthwhile removing the latter, unless it is above ground level. A post can be welded to the housing extending to above ground level and carrying a plate with the name of the well.

From a jack-up a plug should be set in the conductor from ca. 50 m below sea bed, and dressed off 3 - 4 m below sea-bed. The conductor and marine conductor can then be cut mechanically (possibly in one run) and recovered (in one run again as they will probably be cemented together). Explosive cutting cannot be used from a jack-up.

With a sub-sea well head there is no access to the annulus for checking the pressure, and therefore it is necessary to take adequate precautions before cutting each casing string. What is considered adequate will depend on the circumstances, but it may involve perforating just below the well-head and at the level of the previous shoe and circulating through a packer before cutting fully and recovering the casing. For the final cut through the conductor and foundation pile an explosive cutter can be considered as well as a mechanical cutter.

Note that with a sub-sea wellhead the abandonment procedure will be substantially simpler and quicker if the hanger lock-down rings were omitted when hanging off the casing. This will have no adverse effect on an expendable exploration well.