The lack of positive knowledge as to the positions of the top and bottom water strata gives rise to considerable doubt concern ing where the water should be sealed off in any new field during the early days of its development, and the principal damage by flooding, aside from that due to negligence during subsequent opera tions, may be traced to this uncertainty when the first few wells were being drilled. It is the general opinion of oil men, experi enced in excluding water, that after the precise relative positions of these measures have been ascertained little excuse remains for not protecting the oil-sand. A number of methods for accomplishing this, under the various drilling conditions, have been devised and few situa tions arise that cannot be met if handled properly.
The original method used for shutting-off the water, which is still successfully followed in the eastern and middle western states where the strata are hard and cave but little, is simply that of setting the casing on bottom and proceeding with a smaller size drill, thus leaving a shoulder upon which the casing may rest and effect a water-tight bond with the wall of the hole. To be of permanent value, however, it has been found that this one time universal method is far from satisfactory in many cases, and particularly unreliable in soft, loosely cemented measures that may hold the water back for a few months and then permit it to break in by gradually leaching through the interstices of the surround ing porous measures.
In some such cases the proportion of water that works its way down to the productive measures is slight, and gives little trouble if it can be pumped out with the oil. But such instances are not the general rule, and it has become apparent that more positive methods for excluding the water must be applied if the lives of the wells are to be protected. In the first attempts at improvement, bags of cereals were inserted at the bottom, before the pipe was landed, so that a portion of these would expand on the outside of the casing and seal off the water. This did prove very effective and the development of the use of cement followed as a natural consequence in the search for something that would hold back the water for all time. It has now been tried for several years, has come into increasing favor, and is generally recognized as by far the most satisfactory medium for permanently re taining the superficial water back of the casing.
The problem, then, is that of introducing from 2 to 8 or 10 tons of cement into the bottom of the well and placing it so that the major portion of it is situated on the outside of the casing at the bottom. The mechanical difficulties connected with accom
plishing this are considerable in some cases ; in others the actual work is simple and requires only care and experience. In all the processes to be described, the preliminary steps are the same and bear an important relation to the success of the work. The walls of the hole are under-reamed for from 75 to 100 ft. above bottom, in order that the column of cement may be as thick as possible, and the hole is washed by pumping in fresh water until all the mud, oil and gas have been removed. Both oil and gas tend to prevent the cement from setting properly and so interfere with the formation of a tight bond.
The simplest method of placing the cement is that known as `bailing' it in. The hole is first filled with water and the casing raised until the shoe is about 60 ft. off bottom. A 'stand' of three joints of casing is then unscrewed and placed to one side in the derrick. The cement, mixed to a thick grout, is next run into the hole in a specially-constructed bailer that dumps when it reaches bottom. When 1 or 2 tons (dry weight) of cement have been inserted in this way, the stand of casing is screwed back into the top of the string, filled with water, and a plug screwed into the top coupling. The casing is then lowered until the shoe strikes bottom, and since the pipe is full of water which is pre vented from escaping by the plug at the top, a large portion of the cement at the bottom is forced out into the formation and up between the casing and the wall of the hole. The casing is then driven, in order to seat the shoe into the bottom as far as possible. Some operators prefer, instead of lowering the cement in a bailer, to run it in in a series of long narrow bags tied to the end of the drilling-tools. When the bottom is reached, a few strokes of the drilling tools loosen the bags and break them so that the cement is free to flow when the casing is lowered.
In connection with these methods the Baker 'cement plug' (Fig. 128) is sometimes used instead of the plug that is screwed into the top of the casing before it is lowered. The plug is made of light cast iron and so constructed that it may be hung from the bailer with a piece of soft rope and lowered inside the casing. When placed below the casing-shoe and then raised with a slight tension, a set of slips catch on the shoe and the bottom opening of the casing is effectually closed. The casing is then lowered, the cement forced up on the outside, and the bailer loosened by a stronger pull that breaks the soft rope, leaving the plug in the hole. Being of cast iron, it is easily drilled up.