In case where compressed air is used instead of gas, naturally air is easily procurable and compressed under most every con dition. The disadvantage lies in the fact that any gas remaining in the oil will be rendered valueless for gasoline or any other purposes if mixed with the compressed air, and the gravity of the oil as well, may be lowered.
The value of both methods is the greatest when used in a porous lenticular bed, and in such cases production that has fallen down to a barrel has been increased and in many cases a production of 3o or 35 barrels per day has been obtained. Another disadvantage of the process is the fact that it is impossible to control the action of the compressed gas or air in the wells, thus the adjoining property may reap as much benefit from it as the installer, and there are instances where the user of the system did not reap the benefit of his work, while another lease at con siderable distance has been greatly improved. Sometimes the current of air or gas will find a short cut or "break through" without going through the required sand; in such cases the air or gas is used without good results. The methods have proved of great success in porous lenticular sands, but without much success in limestone.
The gas or air is compressed and sent through the air holes into the sand and this process is kept up at all times without stopping. In some cases the looked-for results have been noticed in a few hours after the installation of the plant, while in others it showed good effects only after months of continued work, but it may be said that the advantages greatly exceed the dis advantages, and so far it is the most promising method devised. It is claimed that it has proved of value in 75 per cent of the cases employed. So far the system has been used mostly around the southeastern part of Ohio, having originated around Marietta, Ohio, and is known as the Smith & Dunn process, after the originators, who have installed many plants and put the process to practical test.
In case of increased recovery of gases, we have fewer methods at our disposal, inasmuch as, when a gas well so declines that it necessitates artificial methods of increasing the output of wells, such wells are really of no great value; however, the vacuum method, used similarly as employed for oil extraction, • has been tried, but without much success, and can best be used only in connection with a gasoline proposition. The use of vacuum has been in courts in various parts of the country and its legal standing is not clear even at the present day.
The only practical method is the storage of gas, which is really not an increased recovery method, but consists simply in the storing of gas in abandoned wells, through which it is stored in the exhausted sands, so that in case of heavy gas con sumption the storage wells may be utilized to assist the producing wells, so that the two used together may supply sufficient gas during a cold spell wherever needed. (Fig. 125.) This method has been discussed by the writer in Bulletin 145 of the American Institute of Mining and the other methods above described have been explained at length by J. 0. Lewis in Bulletin 148 of the Bureau of Mines.
In this connection the recovery of oil from shales may be mentioned. Great deposits of shales have been prospected with view of obtaining oil from them by a method of destructive distillation, in hopes that the source of oil supplies of the United States may be increased. Along these lines V. Ziegler says:* "These shales do not actually carry oil or gas as such. They contain organic materials such as partially altered plant remains which are broken up into oil and gas when subjected to heat." This is really a manufacturing process and devoid of the uncer tainties of the oil producer, and for the purpose of producing oil in that manner or in investing in such enterprises it should not be confused with the well-known oil and gas producing methods. (Fig. i26.)