The majority of operators now pump the cement directly into the casing and then force it out behind the casing by filling it with water. Some operators figure the contents of the casing and stop the pumps when the calculated amount of water has been pumped in. In some cases the water is measured with a meter, while in others it is measured in tanks, the latter method being considered less risky and more accurate.
Another method consists in using two wooden plugs about three feet long, which are made to fit tightly against the casing by means of rubber belting. These plugs are introduced into the casing by the use of a special arrangement of gates and fittings. The first plug is started down the casing ahead of the cement to prevent the water and cement mixing. The usual practice is to raise the casing about 18 in. SD that the upper portion of the first plug remains in the casing. Sometimes a wooden "spreader," 2 in. by 4 in. and 6 ft. to 15 ft. long, is inserted ahead of the second plug. This "spreader" serves to retain within this casing cement which has become excessively diluted, and which would otherwise rest at the critical position immediately around the shoe of the casing. The plugs and "spreader" are afterwards easily drilled out. The second plug is introduced after the cement is pumped in and the casing lowered, so that the plugs and "spreader" will not pass out of the casing but come to rest, and thus increase the pressure of the pumps when all the cement has left the casing. In using this method it is customary to measure the water pumped in behind the second plug as a check on the progress of the operation.
The amount of cement used varies from 5 to 30 tons, according to local conditions. On an ordinary job, without any com plications resulting from caving, 10 to 15 tons are used. Most operators use larger quantities than are needed for the simple purpose of plugging off water behind the shoes, because it serves to reinforce the casing by filling the space between the casing and the wall, and also prevents corrosion from waters carrying mineral salts. It is not uncommon to find in redrilling aban doned wells, where large amounts of cement had been used, that the cement has followed up the casing as much as 1000 ft. In localities where formations showed a decided tendency to cause the casing to collapse after the completion of wells, as much as 30 tons of cement have been used with the hope of holding back the walls and preventing this collapse.
The shutting off of "bottom water" is a different problem, and usually a much more difficult one to overcome than is presented in shutting off "top water." Oil sands have been drilled through where the operator has often had to contend with gas and loose sand, as well as sidetracked or slivered casing. To remedy these conditions the hole must first be cleaned out and, if any casing was left in the well when the oil string was pulled, it must be shattered with dynamite and plugged above the water sand. Some operators have succeeded in meeting these difficulties by inserting rope, brick, or similar materials, and tamping them down thoroughly with the tools. On top of such a foundation
is placed a mixture of dry cement and metal lathe cuttings in tin cartridges and tamped down. These cartridges are then broken up and the cement mixed with water in the hole by the action of the tools. Other operators have been successful by simply placing enough neat cement in the well with a dump bailer to fill it up to the bottom of the lowest oil sands.
Probably the best method, and the one that has met with success under the most difficult conditions, consists in pumping the cement through tubing, which enables the operator to exert sufficient pressure to force it into cavities and channels that would not otherwise be filled. The outfit used is similar to that described above for cementing a water string with tubing and casing-head. The space between the oil and water string is closed at the top of the water string, thus preventing the cement from coming up between these two casings. The packing head closing the space between the two casings is so constructed as to allow the movement of the oil string through it, if that is desired. It frequently happens by the use of this method that the oil sands will absorb considerable water, and continue to do so until the cement comes up and shuts them off to some extent. Thus the pump pressure is increased and forces the cement into the formation below. The oil sands thereafter become some what deadened, and it may be some time before the former production is obtained from these sands; however, it is improb able that the cement forms a solid wall around the casing in the oil sands that would shut out all the oil, for the reason that there is sufficient gas and movement of the cement to keep it from setting. Some operators advocate first mudding up the oil sands to keep the cement out and prevent circulation, but this procedure undoubtedly would prevent the return of the oil to a greater extent than the cement would.
The most difficult problem operators have to deal with is the case of a water sand lying between oil sands, where it is desired to produce oil from the lower oil sand and give the upper oil sand the proper protection from water. In many cases such protec tion would require two additional strings of casing. The first string being cemented above the water sand, the second being cemented below the water sand, and the third serving as the water string, the diameter of the hole would be so reduced as to make it impractical. Instances are known to exist where the intermediate water was excluded from the upper oil sand with one string of casing. In those instances the casing was cemented below the water sand, enough cement being used to reach above it and bind the casing firmly to the overlying shale. Then the casing opposite the upper oil sands was perforated and they were proved to be free from water.
For further detailed specifications of methods of excluding water from oil wells the reader is referred to the descriptions by F. B. Tough.i