AMOUNT OF CEMENT NECESSARY The amount of cement required to fill certain spaces in an oil well should be approximately known when a job is commenced and Table II, where the space is exactly stated, can be used for such a purpose. In actual practice the exact cavity may not be known. The table is based on the fact that a sack of cement, weighing about 100 pounds, will occupy about 1.1 cubic feet after being mixed with water and allowed to set.
Capacity of Casing and Tubing.—The amount of fluid con tained in casing or tubing must be determined before certain cementing operations. Tables III and IV present the infor mation in convenient form.
either allowed to exhaust itself by flowing into the air or if possible was cased off and allowed to waste gradually through under ground channels.' Diminishing fuel supplies called forth public disapproval of such waste and also brought about conditions which made the collection of gas commercially profitable. The methods of preventing waste of gas are therefore worthy of careful considera tion by many oil producers.
In a general way prevention of gas waste involves much the same procedure as is followed in preventing damage to oil wells by infiltrating water. In the latter case the aim is to identify water bearing formations and prevent their contents from enter ing the well, while in the case of gas the aim is to identify the sources and prevent it either from entering the well or in any other way escaping.
Two mechanical methods have been developed in order to save natural gas. The first and most expensive method involves landing and securely seating several strings of casing which will separate the various strata carrying water and gas. Preceding pages furnish details to be considered in such procedure. The second method depends upon the introduction of mud-laden fluid or slime behind a single string of casing, so that it will enter and seal all porous formations.
The use of mud-laden fluid has become quite general since its introduction by the U. S. Bureau of Mines in Oklahoma in The popularity of the method has doubtless been in large measure due to the fact that it obviates many difficulties and makes it easier to drill through a heavy flow of gas and pro ceed with the search for oil. Careful investigation has not yet brought forth all the evidence that might be desired to prove that the conservation feature is entirely effective. Some in stances have occurred where both the overlying gas and the underlying oil have been taken from a field developed by the mud laden fluid process of drilling. In many other instances, how ever, the final reports deal only with the oil which has been recovered and fail to show definitely whether the gas was saved.
However, the basic theory involved is so entirely reasonable that the burden of proof rests largely if not entirely upon oppo nents of the method. One instance of absolute proof of its effectiveness, as applied to water, is noted on page 99.
The principle involved may be thus briefly stated: The mud laden fluid is a mixture of water and any clay-like material which will remain suspended for a considerable time. Sand or other small rock particles must not be a part of the mixture because they will quickly settle down and tend to leave only clear water. The consistency of the fluid varies, with the conditions of its use, between specific gravities of 1.05 and 1.15, and sometimes even 1.5. The mud-laden fluid enters the porous rock forma tions, gradually deposits its load of clay and seals the pores. This action is similar to that of a filter.
Muddy water is always used as the circulating medium with rotary tools. Mud-laden fluid, conforming to the foregoing specifications, is readily obtained by merely allowing the sand from the drill cuttings to settle before re-introducing the fluid into the well. Mud-laden fluid can also be used with cable tools by means of a special casing-head provided with pipe line connections and stuffing box. Mud-laden fluid can be run into an open well, and even in the face of a heavy gas flow it can be introduced into the well by means of a trap consisting of pipe and valves. The description of tools and operations by Lewis and McMurray is complete enough to guide all work, and need not be here repeated.
Exact specifications for mudding a well are difficult to formu late, and further experience may alter such rules as are here tentatively presented.
1. The mud must be entirely free from sand or grit.
2. Constant circulation of mud must be maintained during operations until the casing is ready to land. Just before the casing is landed the exit of mud from the well must be stopped and pumping continued at a pressure of from 200 to 500 lbs. per square inch, until no more mud can be pumped into the well.
3. The casing must be landed in a thick bed of sticky clay. Before landing the casing a hole 2 in. smaller in diameter than the shoe must be drilled at least 3 ft. below the larger hole.
4. A casing shoe not less than 3 ft. long must be used. The outside diameter at the point of the shoe must not be larger than at any point above.
5. Proof should be obtained that the casing is securely and tightly landed. The well should be drilled about 5 ft. below the shoe, bailed to a specified depth and allowed to stand at least 24 hours.