In actual practice the 'bottom packer' method has not proved as successful as might be expected, because of various mechanical obstacles. Frequently the pack er does not completely fill the space be tween the tubing and casing, due to the wear on its outside edge while being in serted, thus leaving -an opening through which the cement works up inside the cas ing. The fact that the packer occupies such a large space also prevents the tubing from being rapidly withdrawn after the cement has been inserted, and this is a disadvantage since it is well to run the bailer as soon as possible and remove the cement remaining inside the casing before it has begun to set. The suction caused by withdrawing the packer tends to draw in cement from the outside of the casing, if the shoe is not landed squarely on bottom.
The methods now generally favored are various forms of the following typical example. Assume that the 8-in. casing has been to 2000 ft., where it is to be cemented, and a good landing place in the form of a hard shell or hard shale is the measure at the bottom of the hole. The 8-in, casing is suspended from 2 to 6 or 7 ft. above the bottom and 3-in. tubing is run in to within 2 or 3 ft. of the casing shoe. A packing head (see Fig. 130) is stripped over the top joint of tubing and screwed into the top casing-coupling, packing off the space between the casing and the tubing so that if the casing is filled with water, when the cement is pumped in through the tubing it will he prevented from rising inside the casing and must travel around the shoe and up on the outside. Fig. 131 shows the arrangement of the cement pump, mixing box, tanks, etc. The mixing box is 7 by 12 by 2 ft. and holds 8 tons of cement. The large tank, with a capacity of 100 barrels is used for water storage and the small tank as a receiving tank for the cement after it has been mixed in the mixing-box. A screen is placed over the top of the small tank to prevent lumps of cement or debris from entering the suction of the pump. The discharge-line, including a section of armored hose, connects the pump with the tubing.
When the tubing has been inserted, the packing head is screwed into the top casing-coupling and the tubing connected to the pump discharge, and fresh water pumped in again to make sure that there is a satisfactory circulation. The cement, which has previously been passed through a screen into the mixing box, is next mixed with water by opening the valve B, leaving the valve A still open slightly so as to maintain a circula tion in the well. Connected to the valve B is a section of hose
with a p-in. nozzle that is directed against the cement for mixing it. At the same time, six or eight men stir the cement with hoes and the batch, say 5 tons, becomes thoroughly mixed in from 10 to 15 minutes.
The mixed cement is then run into the small tank T, from which it is taken by the pump and forced down the tubing. This accomplished, the plug in the end of the tee at the top of the tubing is removed and a wood plug, from 1 to 3 ft. long, tapered at the lower end and with a canvas washer nailed to its top, is dropped into the tubing. The tubing-plug is replaced and water pumped in, forcing the wood plug and cement ahead of it down until the plug strikes the swage nipple, when a pressure on a gauge at the pump immediately goes up, indicating that the cement is all out of the tubing. The 8-in. casing is then landed on bottom, the tubing withdrawn and the bailer run in to remove the cement that remains inside the casing. At least seven days are allowed for the cement to set. The hole is then drilled about 10 ft. ahead of the shoe and bailed dry for the purpose of testing the cementing job. If at the end of a period of from 24 to 48 hours no water enters the hole drilling operations are continued.
When the well is bailed dry the greatest collapsing strain is placed on the casing, since no fluid remains inside the pipe to balance the pressure of that on the outside. The table on page 86 indicates the lengths of the different sizes and weights of casing that may be inserted, with an allowable factor of safety of 2; and while the limits set forth in this table are frequently exceeded, yet there is always the danger when doing so of sub jecting the pipe to greater collapsing strain than it can bear, especially if it has been weakened by wear or by corrosion and pitting due to the presence of salts in the waters.
Before the tubing has been run in, during the preliminary opera tion of securing a circulation, the fluid may come to the surface on the outside of the pipe even though it is not traveling around the shoe, if a leak exists in the casing. If such is the case it may be determined by continuing to pump and at the same time lowering the casing until the shoe strikes bottom. If the casing leaks, the circulation will continue ; but if no leak exists and the circulation has been entirely around the shoe, then when the latter is 'placed on bottom the fluid will be held and the pump pressure increased until it stops the pump.