tubing is placed to bottom with 10 ft. of perforations in the lower joints and 2 to column inserted inside the 4-in. "to a depth in the fluid equal to at least twice the distance from the level of the liquid to the surface." An air-head is placed at the surface and the air is forced down the 4-in. tubing outside the smaller tubing and returns inside the 2 or tubing, forcing out the dead oil and later carrying up the aerated fluid. This form of air-lift has also been successfully used in the United States. The Associated Oil Co. in California used an air-lift as shown in Fig. 162. An ordinary plunger pump is often used in conjunction with compressed air when the well is making water, the pump being placed at a point above the water level where the oil contains little water. The air lift raises the water with a small percentage of oil while the pump raises oil with a small percentage of water. Where water from one well is flooding the territory the air-lift is installed to protect the neighboring wells and the latter kept pumping, the reduced water level making it possible to obtain more oil. In the Kern River fields, it was found by continuous blowing of the key well that production in neighboring wells was materially increased. In many cases, how ever, where there is no water present, the air-lift has not met with such pronounced success, but this can be attributed largely to lack of sufficient oil in the well to furnish a continuous stream. When the latter condition obtains, plunger-pumping is usually the only alternative.
Perforations. The question of perforations to be used in the oil string 'is an important one. There is no rule governing the size or quantity -in--any particular oil field and in many cases only by re peated trial is a perforation found which gives a maximum produc tion. The gravity of the oil, the amount of sand the well makes, the quality of sand, that is, whether fine or coarse, the presence of shale or mud and the percentage, if any, of water, all have to be con sidered. In light gravity oils it often happens that the perforations become clogged with shale or mud. This prevents the oil from entering the pipe, thus reducing production. This condition sometimes may be remedied by repeated swabbing, by moving the casing to remove the shale from the perforations, by washing the oil or, in extreme cases, by withdrawing the oil-string from the sand until the shoe is just above the latter, the light oil working its way through the cavings and up into the casing. In washing, the oil is pumped cold or hot down the tubing for a period of a half-hour or more, a 3-in. tee having been previously attached to the bottom of the tubing to force the flow directly against the perforations. Some operators pull the standing-valve out of the barrel and simply pump the oil down the tubing without lowering the latter. The well will show an appreciable gain until the perforations again become clogged, when washing is again repeated. Some wells require washing every few days, while others will pump satisfactorily for several weeks.
A low gravity oil usually carries a large percentage of sand, and when first put to pumping often occasions considerable expense and trouble until the percentage of sand is reduced by reason of a cavity formed in the sand around the oil-string. If the sand is fine, with
a small percentage of water present, repeated sanding of the pump occurs and there is no perforation which will help this condition, continued bailing being the only means of removing the sand. In some of the Russian fields the wells cannot be pumped because of an excessive quantity of sand, and production is obtained only by steady bailing. Should the sand be coarse, however, different makes of screens or screen-pipe have been devised whereby the sand is excluded from the casing, allowing the oil to come freely through the interstices. In one form, the pipe is wound with a tapered wire over or h-in. round holes, the wire preventing large particles from entering the pipe, while in another, the holes are plugged with `buttons' having small slots, which answer about the same purpose as the wire. Wells in California producing from 20 to 40 barrels a day have been increased in production to 100 to 250 barrels -a day, while in the southern fields of the United States the use of this pipe is almost universal.
For ordinary producing wells in California, where the gravity of oil is light, /8 to round perforations are used, 72-in. being the common size. The holes are bored with a drill, each joint having three to six rows, from 4 to 12 in. apart. Many operators prefer perforating the casing with slotted holes, in the well after it has been landed (Fig. 201), the holes being 3A by 1%-in. for heavy oil and /8 by /8 for light oil with three or four rows to the joint. Should an oil-string become frozen while drilling into the oil-sand, it can al ways be perforated in the well.
Shooting Wells. Where the formation containing the oil is hard, such as the limestone and sandstone found in the fields of the eastern and central United States, a better production is often obtained by blasting the oil-bearing rock. A high explosive, such as nitro glycerine, is carefully poured into long cylindrical cans made for the purpose. The depth of the well to the oil-bearing strata is first ascertained and the charge lowered to the desired position. A firing-head is placed at the top of the upper can and a `go devil,' a piece of .cast iron with wings for a guide, is dropped upon the firing-head. After the blast, the hole is thoroughly cleaned out, leaving a cavity in the oil-formation where the oil may gather. The production in a well with hard formation is usually increased ap preciably by shooting, but care should be exercised in the quantity of explosive used, for an excessive charge may result in breaking the formation to such an extent as to ruin the well. The usual shot is from 10 to 300 quarts of nitro-glycerine, depending upon the forma tion. A shale or soft stratum may be so compacted by a blast that the oil cannot penetrate it. Shooting has been tried in the 'tight' oil-sands in California but with indifferent success.