These methods, by which the cement is placed at the bottom of the hole and then worked out to its final position on the outside of the casing, have been largely replaced by processes in which the cement is pumped down, either through the casing or through an auxiliary smaller string of tubing lowered inside the casing for that purpose. With methods of this class, a necessary pre liminary step is the securing of a 'circulation,' i.e., the space between the casing and the wall of the hole must be sufficiently cleared of caved materials so that there is a free passage for fluid pumped. down inside the casing to come to the surface on the outside, thus insuring that when cement is pumped to the bottom it will pass readily around the casing-shoe and up on the outside of the pipe, if prevented from rising inside the pipe in cases when tubing is used. When endeavoring to secure a circulation it frequently becomes necessary to pull tip the casing 100 ft. or more from bottom and resort to a pump pressure of several hundred pounds before the fluid will break through to the surface on the outside. The pipe is then gradually lowered, and worked up and down, until the fluid circulates readily when the shoe is only a few feet off bottom.
The type of pump ordinarily used is the 10 by 5 by 12-in. duplex mud-pump, used in the oil fields for pumping mud in wells being drilled by the rotary or circulator methods. It is connected to the top of the casing by a section of 272 or 3-in. pressure armored hose.
In the Perkins method, which is of particular value in very deep wells or those in which the water-string of casing tends to `freeze' unless moved at frequent intervals, the cement is pumped directly inside the casing to the bottom. It is also known as the disc, or packer method from the fact that the cement is inserted between two moving packers that have an outside diameter almost as great as the inside diameter of the casing. After a circulation has been obtained, the casing is suspended so that the shoe is 2 or 3 ft. from the bottom, and enough fresh water is pumped in to clean the bottom thoroughly. The two packers have been prepared, one about 3 ft. in length, and the other of such length that when its lower end reaches the bottom of the hole, the tipper end still remains in the casing. They are made of either wood or cast iron, with ends consisting of heavy canvas or rubber washers of just the proper size to pass down inside the casing. The casing is filled with water, the shorter packer inserted in it and against this is pumped the cement, mixed to a grout just thin enough to be pumped readily. When the contents of the cement box is all pumped in, the longer packer is placed in the casing ' above the column of cement, and water is next pumped in, pushing the combination of lower packer, cement and upper packer down inside the pipe. When the lower packer has passed the casing shoe it falls to the bottom of the hole, permitting the cement to pass around the shoe and up on the outside of the casing, as it is pushed from the inside of the pipe ahead of the upper packer. When the latter has
reached bottom it cannot leave the casing entirely, because of its length ; it therefore stops the further flow of water and retards the pump, thus indicating that the cement is out of the pipe.
The casing is then landed on bottom, the cement has been placed on the outside of the pipe at the bottom of the hole and the packers, like the cement plug, are easily drilled through after the cement has set. The main objections to this method are the danger of the packers sticking while going down inside the casing, and the fact that the lower packer may fall to the bottom in such a way as to prevent the casing shoe from being landed squarely on bottom, getting underneath the shoe in such a position as to result in the cement bond breaking when the packer is drilled.
It is possible to follow the general lines of the above method, and dispense with the traveling packers by haying previously measured into a tank, connected to the pump, the exact amount of water necessary to fill the bore of the casing from the surface to the bottom. The cement is pushed ahead of the water and is known to have passed out of the casing when the tank is drained.
In other methods a string of tubing is used as a conductor for carrying the cement to the bottom of the hole, whence it is made to pass to the outside of the casing. Probably the earliest of these was the 'bottom packer' method. In this there is attached to the lower end of the tubing a packer similar to the type described in connection with pumping-wells (Fig. 140), or a more simple one made from strips of belting confined between two metal plates (Fig. 129). The duty of the packer is to close off the space between the exterior of the tubing and the interior of the casing, leaving no room for the cement, when it is pumped down inside the tubing, except to pass around the casing shoe and up on the outside of the casing.
In all tubing methods it is necessary that the precise in stant at which pumping should cease be known, lest the cement he forced up a considerable distance on the outside of the casing. Pro vision for this may be made by previously measuring into a tank the necessary amount of water to fill the tubing, as described in the Perkins method, or by at taching a swage-nipple or bushing to the lower end of the tubing. A wood plug, with a rubber or canvas washer nailed to the top, is inserted in the tubing between the cement and the water used for forcing it down, and when the plug reaches the re stricted opening at the bottom of the tub ing the pump-pressure goes up and it is known that the cement is all out of the tubing. .