From 2 to 8 tons (dry weight) of cement is the amount cus tomarily used, although greater quantities are inserted when unusually large cavities are to be filled. Preferences for different brands are found in different districts but there appears to be little advantage in any one make, provided the cement contains • enough gypsum to retard the set so that the time of initial set is long enough to cover the period of mixing, pumping and land ing the pipe. Ordinarily, when everything is running smoothly, this occupies about a half hour. Since what is desired is a tight bond, rather than strength, no sand is mixed with the cement.
It sometimes happens, particularly with the early wells drilled in a new field, that after the productive sands have been drilled and the well is carried still deeper the so-called 'bottom' water is encountered, in water-bearing strata situated below the oil-sands (Fig. 132). The exclusion of such water is liable to be more difficult than that of the top water because of the presence of gas and oil in the hole, especially when the lower water occurs only a few feet below the oil-sand. Particular care must be exercised, under such conditions, in gauging the amount of cement injected, so that its level does not rise to the oil-sand and inter fere with the production from the latter. If a streak of hard ground is between the two measures it may be possible to drive pieces of stone and brick, with a few sacks of cement, into this space and form a plug that will prevent the water from rising.
If a distance of 2 ft. or more intervene between the oil-sand
and the water-bearing strata (Fig. 133) a 'bridge' may be formed in the hole above the water measure by driving down tightly bricks, stones, etc. These tend to hold back the water temporarily and provide a landing place for a body of cement; which is pumped in through a string of tubing, run in until it is a few feet above the bridge. A similar bridge is also used when after the oil-sand has been penetrated and the well is finished, it is found that the water has broken in around the casing-shoe of the water-string. In such a case it is necessary, if the water string can be loosened, to pull it a short distance up the hole and build a bridge a few feet below its old landing place, thus providing an artificial bottom for the hole while cementing the water-string by some of the methods described. In this way the bridge prevents the entrance of the cement into the productive measure.
In other instances, however, it is found to be impossible to loosen or move the entire water-string and either the next smaller size pipe must be inserted and cemented where the bridge is formed, or else the original string is cut off at a point where it can be moved and the hole re-drilled from this point off at the side of the original hole. Should the latter alternative be fol lowed, the bottom of the old water-string should be filled with cement above the bridge prior to cutting it so that there will be no subsequent infiltration of water to the oil-sand through this old hole.