The first attempted substitute for the formation shut-off, con sisted of clay and chopped rope which were put into the well to fill up and seal the space between the shoe and the wall. A now ob solete practice, which was in vogue before the use of cement, was to make a landing on a hard lens or "shell, " as it is usually termed, and then depend upon a bag of flaxseed to seal the space around the shoe. The hole was carefully trued up, cuttings removed, the seed bag lowered into the hole and the casing landed on it. The swelling of the seed from the absorption of water was depended upon to hold it back long enough to allow the mud to settle and the formation to close in sufficiently to make a permanent job.
The method of tamping, which has been in use for a number of years, now has but few followers. By this method a complicated expanding packer was attached to the oil string where it was desired to make a shut-off, and located below the next larger string of pipe. The packer was made of canvas and closely resembled an inverted umbrella. Sand and pulverized shale was then introduced between the casings by means of a stream of water, and the outer string constantly moved up and down to facilitate the downward movement of the material introduced, as well as to tamp it tightly around the shoe. As the space filled up, the tamping string was removed joint by joint. This operation took from three to six weeks to accomplish. Advocates of this prac tice claim in justification that the casing recovered repaid the labor involved. By this method a string of casing is saved, the last string serving the double purpose of an oil and water string. It is readily seen that this process could only be applied where forma tions stand up sufficiently to allow the removal of casing at will.
Hydraulic cement is now being generally used in California for excluding water from oil sands. The so-called "dump-bailer" method is the simplest in use. In this process the hole is first trued up and carefully cleared of cuttings if the shut-off is to be made at the bottom of the hole. If the hole has already been carried below the shut-off point it is necessary to put in a bridge or false bottom. The casing is then raised off the bottom and thirty or forty sacks of cement, mixed to the consistency of thin gruel, are lowered to the bottom by means of the dump-bailer. The casing is then filled with water to the top and a cap screwed on; then when the casing is lowered to the bottom, the cement is forced out behind it. Sufficient time is allowed for the cement to set, and if the displacements have been accurately figured, only a few feet will remain on the inside of the casing, which is easily drilled out. One of the advantages of this method is that the mud used in drilling can be left behind the casing. In experi enced hands this method has achieved a large percentage of successful shut-offs, even in deep territory. The amount of cement that can be used is necessarily limited, consequently this method is applicable only to cases where no large cavities are to be filled, such as those which often occur in redrilling, or are occasioned by other troubles in wells.
The pumping of cement into the space between the wall and casing is now the method in most general use. For this purpose complete cementing outfits are mounted on auto-trucks, and steam connections are made with the boiler at the well. Two pumps are generally used. The work is usually started with a pump capable of delivering a pressure of 300 to 400 lbs. and finished with one capable of delivering 700 to 800 lbs. The cement is mixed in a portable mixing trough. Neat cement is used, and as in the use of the dump-bailer method it is mixed with water to the consistency of thin gruel. The details of this method vary considerably. In all cases it is important to true up the bottom of the hole and remove the cuttings. The casing is raised a few feet from the bottom while the cement is being pumped, and then lowered to its final position after the cement is in place. Some operators force water upward outside the casing, prior to introducing the cement, until the clear water comes to the surface. Others leave as much circulating mud in the hole as possible, only pumping in sufficient clean water in advance of the cement to prevent the mud mixing with it, the idea being to allow this mud to settle around the casing and thus prevent the movement of water from one stratum to another.
An early method, now little used, was to pump cement through tubing, a packer being placed at the bottom of the casing to pre vent the return of the cement between the tubing and the casing. An improvement on this method, now extensively used, is to pump the cement through tubing which passes through a casing head fitted to the top of the water string and provided with a release valve. The tubing is lowered to within a few feet of the casing shoe, the exact distance being governed by the amount of cement it is desired to leave in the casing. Water is pumped in to obtain circulation outside the casing, then the casing-head release valve is opened and the casing is pumped full of water, which, when the release is closed, prevents the cement from coming up between the tubing and the casing, and forces it outside the cas ing. After the cement is in place, the casing is lowered and all connections closed until the cement has taken its initial set in order to prevent a possible return of the cement around the shoe. There are two methods of determining when the cement has been forced out from the tubing. The first is to calculate the capacity of the tubing and measure the required amount of water. The second is to reduce the diameter of the lower end of the tubing with a swedge nipple. After the cement has been applied, and before the displacing water has been pumped in, a wooden plug is inserted in the tubing. When the plug reaches the swedge nipple and the cement has been displaced the pump pressure suddenly rises and the circulation stops.