Slimming down wells can have a substantial impact on the cost and size of wellheads, Xmas trees, and BOP stacks. As a rough guide the weight, size, and hence costs of these components is proportional to the square of the final hole size for a given well. When using the downsized conventional concept the use of liners can reduce the size of surface equipment even further, as can the use of monobore completions.
On land operations, the cellar can be shallower or even eliminated. Cellars act as a collection point for gas, oil and rain water. Apart from the fire hazard, it is also an H2S trap. A deep cellar can add to the difficulties of doing a surface kill on a blowing well. Alternatively a rig with a lower substructure can be used, giving gains in cost, time and safety. The diameter of the BOPs and wellheads will be reduced enabling a less bulky substructure to be used. In some extreme cases all or part of the weight of the rig and hook load can be transferred to the conductor giving potential savings in location costs.
For swamp or shallow water drilling a land rig mast and floor could be mounted on a driven conductor complete with fixed landing plate. The drilling operation could then be supported by a tender. This could be barge(s) or a self elevating work over platform supported by barges.
Deep land wells, including HP/HT, and wells drilled from floating rigs will generally use standard wellheads and BOPs. It may be possible to downsize wells drilled from jack ups and hence their wellheads, although economies in drive pipe or stove pipe size may not be possible due to compressive and structural loads.
Drilling smaller wells on floaters will enable subsea BOP stacks, wellheads, and risers to be reduced in size. Riser tension and variable deck load requirements will be reduced, thus enabling floating rigs to work for longer between re-supply. The high cost of wells and their tie-in systems is a major economic barrier to the wider use of subsea completions and floating production systems.
Land wellheads
Typically the wells are designed with a 9 5/8" (or 10 3/4") surface casing, 7" (or 7 5/8") intermediate casing, and 5" (or 5 1/2") and 3 1/2" liners, with a 3 1/2" monobore completion. However, a variety of other configurations are possible. Compact wellheads give safety, height and time advantages over conventional stack ups. The slightly larger design, in brackets, allow the installation of gas lift valves in the 3 1/2" tubing x 5 1/2" liner annulus.
Blow out preventers for land/swamp/jackup rigs
A wide range of small blowout preventers are available. These were typically developed for snubbing and workover service. Microdrill were even able to use wireline rams as drilling BOPs. Several smaller BOPs can more readily be combined into one housing while still retaining a light enough assembly for safe handling. One such development is the Annuram [739]. This approach enables substructure heights to be minimised. To minimise height still further orientating successive bonnets at right angles has been proposed, although this may require a wider substructure. Normally the shale shaker pan is about 3-4 metres above ground level and this determines the minimum height for the bell nipple. Once the total stackup height of small BOP stacks, and hence bell nipple height, has been reduced below a critical height the large knock on benefits of reducing substructure height can no longer be realised.
For heli rigs separate bodies for each BOP may have to be used to minimise lifting weight. To minimise weight and height tie rods can be used to supply the preload on the interconnecting faces.
Shaffer, who have been purchased by Varco, have proposed a system where rams can be changed or replaced from inside the stack by means of a running tool run through the rotary table. This avoids breaking the bonnet seals when changing rams. Also operations conducted from the floor tend to be faster and safer than those conducted in the BOP area. This is being considered by Sedco Forex for their slim hole "Simplex" rig. A similar tool is available from Quality Oilfield Products, of Houston.
Control systems, choke manifolds, etc. are at present all conventional. Stackups for BOPs should take account of the flow out meter for the Kick Detection System. This should be arranged for ease of cleaning.