PRODUCTION methods originally aimed at obtaining the maximum immediate supply of oil from the ground without regard to the' ultimate efficiency of recovery. Unrestricted development, competitive drilling and offsetting encouraged the employment of various methods to obtain this objective. The cessation of natural flow was followed by swabbing, bailing, pumping or air-lift. Merits of various systems were compared according to the volume of the fluid handled. Present-day economic conditions, coupled with greater knowledge of the technical laws governing production, have caused the introduction of unit operation and legislation, which have allowed production methods to be considered from a different angle. Production engineers are now concerned with the following factors in the selection of operating methods (a) The maximum ultimate recovery from the field. Recovery with minimum loss of gas energy and slowest pressure decline. Wells are now considered under groups, instead of individually as heretofore. This will give greater flowing life to fields, and hence cheaper lifting costs as a result. (b) Quality of the Crude. The maintenance of high gasoline content, coupled with the freedom from water contamination, calls for careful production control. (c) The Economic Factor. Systems must be analysed to determine comparative lifting costs. (d) Choice of any system will partly depend on the simplicity of control, avoidance of shut-down time and local power available. (e) Topography of the country in which operations are taking place will have some bearing on the ultimate selection of equipment. Usually no 'particular method of production can be regarded superior to another in all the above-mentioned points. In fields with unit control, the maximum ultimate recovery is the chief object and the choice of any particular system will principally have this point in view. Initial expense and skill required for operation must be the secondary consideration. _ It is quite possible that the method chosen may have a lower overall mechanical efficiency than some other system, but in view of the fact that the ultimate recovery would be greater, it will be given first consideration. Recovery of oil from the formation may be divided into two stages (1) Movement of the fluid into the borehole. (2) Lifting of the fluid out of the hole. During the natural flowing life of the well the gas energy in the formation has to perform both these, functions. To maintain natural flow, the rate of production in most cases must be sufficient to supply enough gas to lift the oil to the surface (where, however, hydraulic control is the governing factor, gas energy is not the sole lifting medium).Much study has been devoted to the design of flow strings to reduce the amount of gas energy required for lifting. The back pressure exerte

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