The Action of Oil WellsThe Illustrated London News, vol. 45, no. 1292, p. 618.
December 17, 1864
The Action of Oil Wells has been examined by Professor E. W. Evans, of Marietta College, Ohio, U.S., and the results are given in Dr. Silliman's American Journal of Science. We can only select a few points. The principal supplies of petroleum are not diffused between the planes of stratification, but are collected in cavities more or less sunken in the strata; whence the oil is less liable to be carried away by running water. It is common to find large quantities in places where there are marks of disturbance and displacement of the rocks. The cavities have been probably caused sometimes by uplifts and sometimes by erosion and the dissolving action of water; but, whatever may be their origin, they are not usually of very great horizontal extent. On Oil Creek, in Pennsylvania, the greatest quantities of oil are found in the horizontal stratum of sandstone. It would seem that this rock is very porous, and perforated, like a honeycomb, with numerous cells and fissures containing petroleum. The history of many of the wells is as follows:--When oil is entered the gas begins to raise it up over the top of the boring, increasing gradually in force until it projects it into the air often to a height of 40 ft. or 50 ft.; then alternately diminishing and increasing in force at regular intervals, but without any cessation in the flow for a long time. These variations in the force of the gas (the breathings of the earth as they are called) are to be explained on the same principle as before, by supposing that as the tension of the gas is relaxed by the removal of oil, the gas and oil from other cavities around rush in through the pores and slight fissures till a certain maximum tension is reached and the influx ceases; then by the expansion of gas already in the chamber the oil continues to come up, but with a diminishing flow, until a relative vacuum is again created; after which the influx is renewed and gradually increases as at the beginning. These regular alternations vary in different wells from two or three times a day to as many times an hour, the intervals, however, gradually increasing in length as the supply of oil is diminished--unless, as sometimes happens, new communications are forced, and the well, deriving new supplies, starts off again with a new period. It often happens that the same well has two periods--one of variation in the flow, and another of cessation, consequent on the escape of gas. A more uniform flow may be secured by making the orifice at the mouth of the tube smaller. This is often desirable in order to prevent the escape of gas by the exhaustion of the oil in the cavity down to the bottom of the boring. Sometimes such a quantity will thus rush out, before the oil raised up by the water closes the passage again, as not only to render the pump necessary after that to raise oil, but also to diminish materially the influx of oil from other cavities by reducing the pressure of the gas in them. Oil wells commonly vary in depth from 100 ft. to 800 ft. The deepest are as apt to raise oil to the surface as the shallowest. This indicates a greater compression of the gas at the greater depth--owing, doubtless, to its connection with higher columns of water. The activity of some wells is increased by rains; others, with less gas, are rendered unproductive till the water can be reduced.