EXPERIMENT TO SHOW MUTUAL INTERFERENCE OF BORES.

Bore No. 5 closed at 7 p.m., June12, 1907.Bore No. 5 opened at 7 a.m., June 13, 1907.
Time.Discharge of Bore No. 6.Time.Discharge of Bore No. 6.Time.Discharge of Bore No. 6.Time.Discharge of Bore No. 6.
P.M.Gallons per Minute.A.M.Gallons per Minute.A.M.Gallons per Minute.P.M.Gallons per Minute.
7.061·21.076·67.083·71.069·0
8.1565·62.3077·48.078·42.067·7
9.068·43.079·29.075·03.066·8
10.069·64.079·710.073·04.066·2
11.073·25.082·111.070·85.066·6
12.074·76.083·112.069·66.065·3
7.064·0

From these figures it will be seen that the shutting down of a flowing or the opening of a closed well may produce a most marked effect on a neighbouring well within the short space of sixty minutes, even when the intervening distance is over 500 metres. In the above instance the rate of increase was most rapid at first, there being a gain of 7 gallons per minute, equivalent to about 12 per cent., in the first two hours. The total increase in the twelve hours amounted to 22½ gallons, or about 37 per cent. On reopening No. 5 it is seen that the discharge of No. 6 at once commenced to fall, the loss being nearly 9 gallons in the first two hours; afterwards the rate of decrease gradually diminished, until at 7 p.m., when the observations were discontinued, the flow had fallen to within 3 gallons of its normal.

A second series of observations was made between two bores considerably farther apart, No. 4 being 835 metres N.N.W. of No. 42. The difference of level in this case was found to be 1·18 metres, the outlet of No. 4 being 60·74, and that of No. 42, 59·56 metres. Bore No. 4 has an internal diameter of 4¼ inches, is 141 metres deep, and draws from 19 metres of sandstone; Bore No. 42 is 6 inches in diameter, 218 metres deep, and 69 metres into the water-sandstone. Previous to the experiment, No. 4 was flowing 36·75, and No. 42 about 68·5 gallons per minute. Precautions were taken against other wells influencing the results, the nearest bores having been opened twenty-four hours previously and being kept in the same condition throughout the experiment. Bore No. 42 was closed down at 9 a.m. on March 4, 1908, periodical observations being then made of the discharge of No. 4 during the next thirty-six hours.

Briefly stated, the result of this experiment was as follows: The discharge of No. 4 had not perceptibly increased at the end of the first half-hour, but had done so after one hour. It continued to increase at a very slow rate, the net gain after thirty-six hours being only 3 gallons, or between 8 and 9 per cent. In this case the mutual interference is very much less than that between Nos. 5 and 6, doubtless largely owing to the greater distance apart, and to the lesser difference between the outlet-levels of the wells. In all probability there are many other conditions which combine with the above in determining the amount of interference, such as the positions of the wells with regard to the main lines of underground flow, the relative depths of the bores, and the thicknesses of sandstone from which they draw their supplies.

The most marked example of interference with which I have met was in the case of two ancient wells at El Dêr el Ghennîma, situated only 88 metres apart, on the crest of an anticlinal fold running north and south. These wells had been sanded-up for centuries, but were recently taken in hand and cleaned out. The difference of level in the outlets is 2·07 metres, the higher well being 34½ metres in depth, the lower 41 metres. The opening or closing of the lower well produces an almost instantaneous effect on the higher, the difference in flow of the latter within thirty seconds amounting to as much as 11 per cent.

A great many observations were made, but the following are sufficient to show the rates of decrease and increase:

Gallons per Minute.
Upper well flowed13·2when lower wellwas open (flow 50·3 g.p.m.).
„ „20·5„ „had been closed10minutes.
„ „23·7„ „„ „20„
„ „26·3„ „„ „30„
„ „39·4„ „„ „24hours.
„ „35·6„ „had been open30seconds.
„ „32·5„ „„ „2minutes.
„ „26·7„ „„ „7„
„ „24·9„ „„ „9„
„ „23·4„ „„ „16½„
„ „19·7„ „„ „45„

The closing down of the lower well is thus seen to have influenced the discharge of the upper to the extent of 100 per cent, in the short space of thirty minutes, while the flow was trebled in twenty-four hours. On opening the lower well the discharge of the upper fell to within 50 per cent. of its normal within forty-five minutes.

As already mentioned, most bores show a marked decline in discharge for some time after completion, and except in special cases it seems doubtful if large bores can be expected to maintain their original flows for long periods of years. During the early part of its existence a well draws its supplies from fully saturated beds, the water being forced into it from every side, not only through the pores of the sandstone, but through any fissures the bore may have struck. The flow of water through a compact sandstone is, however, extremely slow, and it is probable that as time goes on every bore becomes more and more dependent on fissures for the maintenance of its supply. This supersaturation of the water-bearing beds, if we may be permitted to use the term, is well illustrated by the closing of a bore for a few days. The water at once commences to accumulate around it, and when the bore is reopened the discharge will generally be found to have increased to a very great extent. As an example of this I may mention Bore No. 14, which, on April 19, 1907, was flowing at the rate of 225 gallons per minute. The well was then closed down for five days; on reopening the discharge was found to be 370 gallons per minute, an increase of 145 gallons, or about 65 per cent., the pressure during the same time having risen from below 9 to nearly 16 pounds per square inch. The discharge took about twelve hours, or one-tenth of the time, to fall to its normal. On another occasion the same well had its output increased from 217 to 339 gallons by being closed for twenty-four hours, a gain amounting to 55 per cent.