FLOWING WELLS: SOME EXPERIMENTS AND OBSERVATIONS
Total Water Discharge of Oasis — Water-Pressures — Static
Head — Importance of Systematic Observations and Records of
Bores — Sensitiveness of Wells — Experiments showing Mutual
Interference of Wells — Bores Nos. 5 and 6 — Bores Nos. 4 and
42 — The Wells at El Dêr el Ghennîma — Decline in Discharge
of New Wells — Effect of Closing Bores — Rate of Flow of Water
in Sandstones — Danger of Over-Exploitation — Holding Capacity
and Porosity — Experiments on Porosity of Nubian Sandstone —
Large Quantities of Water held in Storage Beds — Economically
only partially available at Surface.
At the present day there are about 230 native-owned wells in the
oasis, yielding a total discharge of some 295 qirats. We have seen
that the qirat has not a fixed value, but represents a discharge
varying from 22 to 38 or more gallons a minute. Applying these
values as far as possible to the old wells, and adding the known
discharge of the new, we shall not be very far from the truth if we
estimate the total discharge of the oasis wells at 8,000 gallons
a minute, or 11,500,000 gallons (53,000 cubic metres) a day. The
largest well in Kharga Oasis is Ain Estakherab at Gennâh, with a
discharge of between 700 and 800 gallons per minute. The average
yield of those which might be described as the best wells probably
does not exceed 150 to 200 gallons, while there are a great many
which only discharge 20 or 30 gallons per minute.
In some parts of the world the discharges of artesian wells are
measured by thousands, not hundreds, of gallons. In Queensland and
other parts of Australia, for instance, there are numerous bores
throwing over 1,000 gallons a minute; in these regions the depth
of individual bores in many cases amounts to thousands of feet,
the pressure frequently rising as high as 100 pounds to the square
inch. It must, moreover, be borne in mind that the artesian basins
of Australia have only been exploited during a comparatively short
period, and that from the nature of the country the wells are,
as a rule, at considerable distances apart.
The exploitation of the artesian basin of the Libyan Desert has been
in progress for hundreds and thousands of years, and it is probable,
therefore, that in the oases-depressions the general average pressure
has been very much reduced. At the present time the water-pressures
seldom exceed a very few pounds per square inch, so that the static
head, or absolute height to which the water will rise, is seldom more
than a few metres above the actual surface of the ground. The outlets
of the native wells are, unfortunately, of such a nature that it is
not possible to make even approximate determinations of pressure;
but after taking into account the influence of ground-level on the
volume of flow, we still find differences which point to there being
considerable variation in the absolute static head in different
parts of the oasis—that is to say, the water will rise higher in
some places than in others. This is probably partly to be accounted
for by variations in the level of the underlying strata and by the
presence of faults—for instance, the static head to the west of
the central line of disturbance is certainly considerably higher
than to the east.
In the absence of accurate data, the static head in a district can
be approximately gauged from the ground-level and flow of any one of
the higher and more isolated wells. The maximum static head judged
in this way appears to be 88 metres above sea-level in the Kharga
district on the west side of the fault; but in the Headquarters
area, to the east of the fault, it can be definitely proved to be
very much less.
In order to determine the extent of the local variations throughout
the district where the recent boring operations have extended,
I carried out a number of experiments. For this purpose four bores
in different parts of the area were selected, of similar diameters,
and of approximately equal depths into the water-sandstone. By means
of a flange the casing of each bore was carried vertically upwards,
the ordinary outlet being kept shut until the pressure ceased to
rise, as indicated by a pressure-gauge. It was then found that in
Bore No. 36 the water had risen to 61·6 metres, in Bore No. 38
to 59·55, in Bore No. 42 to 64·26, and in Bore No. 44 to 62·0
metres above sea-level. The average static head in this district
may therefore be taken as 61·85 metres. The positions of the four
bores in question, with regard to Headquarters, are as follows:
No. 36, 7·7 kilometres N.; No. 38, 2·6 kilometres N.W.; No. 42,
1·6 kilometres S.E.; and No. 44, 3·9 kilometres W.N.W.
To the north of Meheriq the static head appears to rise, as flowing
wells are found up to 76 metres above sea-level (Ain Mohammed
Delaib). The water-level at Ain el Ghazâl is even higher (84·56),
but this well does not actually run.
In the development of an artesian basin this question is of the
utmost importance; only by a knowledge of the static head and the
ground-level can we, with any likelihood of success, estimate
beforehand the discharge to be looked for in any particular
district. If, for instance, we sink a bore in a locality whose
surface is above the static head, the result, so far as a flowing
well is concerned, can only be failure, however great the amount
of water existing in the underlying strata. If a sufficient number
of observations are available, it is possible to construct charts
showing the isopotential lines, or lines of equal pressure,
and these may be of great value when boring is contemplated in
intermediate districts.
It is seldom that accurate records of bores are preserved in the
initial stages of the development of new artesian basins, and to
this neglect is due much of the doubt which frequently arises at
a later period as to the extent and permanence of the underground
water-supplies.
[Illustration: BORE NO. 5.]
[Illustration: BORE NO. 14.]
In some countries, however, the water-resources have long been the
subject of exhaustive examination, notably in the United States,
where a most valuable and instructive series of water-supply reports
has been issued by the Geological Survey. The chief difficulties
arise from the fact that bores are generally the property of private
individuals, who are seldom both able and willing to supply accurate
information. Discharges, for instance, are usually given in the
roundest of figures, and without regard to the conditions under
which they were taken. In Australia more than one geologist has
called attention to the matter, and quite recently Mr. G. H. Knibbs,
F.R.A.S., of the University of Sydney, in a valuable and suggestive
paper on the hydraulic aspect of the artesian problem, refers to the
want of comprehensive and deliberate investigation in the past, and
admits the inadequacy of the available data for the determination
even of the one question only—_i.e._, the extent to which
exploitation can be pushed without fear of exhausting the supply.
When drilling was first commenced in the Headquarters area, the
bores were placed at an average distance apart of 500 metres;
circumstances, however, led to there being a considerable
variation in the depths of the wells, with the result that those
of shallow depth and those situated on comparatively high ground
were adversely affected by the deeper and more favourably placed
bores. The sensitiveness of any one well to the influence of its
neighbours is, I believe, far greater than is generally supposed,
and appears to be especially dependent on the amount of difference
between the depths, discharges, and surface-levels of the bores. In
investigating this subject I made a number of experiments with the
object of determining the mutual influence of wells, and perhaps
some reference to these may not be without interest and value.
The first experiment to which I shall refer was made on wells
situated comparatively close together. Bore No. 5 is 570 metres
W.S.W. of Bore No. 6, the outlet of the former being at 57·38,
that of the latter at 59·18, a difference of 1·8 metres. No. 5
has an internal diameter of 5⅝ inches, is 197 metres deep and 95
metres into the water-sandstone; No. 6 has a diameter of 8 inches,
is 146 metres deep, and 61 metres into the sandstone. The two
wells had been flowing continuously for a considerable period,
and during the experiment neighbouring wells were kept shut down,
so that there is no reason to suppose that the observations were
affected by other bores.
Bore No. 5, discharging 114 gallons a minute, was shut down at
7 p.m. on June 12, 1907, and reopened at 7 a.m. on June 13. The
hourly observations, as given in the following table, show the
effects produced on Bore No. 6.
EXPERIMENT TO SHOW MUTUAL INTERFERENCE OF BORES.
+-------------------------------++-------------------------------+
| Bore No. 5 closed at 7 p.m., || Bore No. 5 opened at 7 a.m., |
| June 12, 1907. || June 13, 1907. |
+-----+---------+-----+---------++-----+---------+-----+---------+
|Time.|Discharge|Time.|Discharge||Time.|Discharge|Time.|Discharge|
| | of Bore | | of Bore || | of Bore | | of Bore |
| | No. 6. | | No. 6. || | No. 6. | | No. 6. |
+-----+---------+-----+---------++-----+---------+-----+---------+
|P.M. | Gallons |A.M. | Gallons ||A.M. | Gallons |P.M. | Gallons |
| | per | | per || | per | | per |
| | Minute. | | Minute. || | Minute. | | Minute. |
| | | | || | | | |
| 7.0 | 61·2 | 1.0 | 76·6 || 7.0 | 83·7 | 1.0 | 69·0 |
| | | | || | | | |
| 8.15| 65·6 | 2.30| 77·4 || 8.0 | 78·4 | 2.0 | 67·7 |
| | | | || | | | |
| 9.0 | 68·4 | 3.0 | 79·2 || 9.0 | 75·0 | 3.0 | 66·8 |
| | | | || | | | |
|10.0 | 69·6 | 4.0 | 79·7 ||10.0 | 73·0 | 4.0 | 66·2 |
| | | | || | | | |
|11.0 | 73·2 | 5.0 | 82·1 ||11.0 | 70·8 | 5.0 | 66·6 |
| | | | || | | | |
|12.0 | 74·7 | 6.0 | 83·1 ||12.0 | 69·6 | 6.0 | 65·3 |
| | | | || | | | |
| | | | || | | 7.0 | 64·0 |
+-----+---------+-----+---------++-----+---------+-----+---------+
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 flowed 13·2 when lower well was open (flow 50·3 g.p.m.).
„ „ 20·5 „ „ had been closed 10 minutes.
„ „ 23·7 „ „ „ „ 20 „
„ „ 26·3 „ „ „ „ 30 „
„ „ 39·4 „ „ „ „ 24 hours.
„ „ 35·6 „ „ had been open 30 seconds.
„ „ 32·5 „ „ „ „ 2 minutes.
„ „ 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.
The rate of flow of water through an underground sandstone depends
upon a number of conditions, the most important being the size of
the pores or spaces between the component grains, the porosity
or water-holding capacity of the sandstone, the temperature of
the water, and the pressure acting on it. The yield of a well
will depend, of course, not only on all these factors, but also
on the diameter of the bore, its depth into the water-stratum,
the size and number of fissures passed through, and, last and most
important of all, on the absolute height of its outlet. Large pores,
high average porosity, and high temperatures make for strong flows,
though in the absence of pressure greater than that due to a column
of water equal in height to the distance between the water-stratum
and the outlet of the well, they are in themselves of no avail in
the production of an artesian flow. Moreover, although some of the
above conditions may be known beforehand, the resistance to flow of
the strata immediately surrounding a bore can never be more than
approximately conjectured, as the size and mode of arrangement of
the individual grains of any sedimentary rock must always vary,
both horizontally and vertically, to a very great extent, and on
these factors depends in very large measure the capacity of the
strata to transmit water.
Data are as yet far too insufficient to warrant an attempt to
calculate the supply which can safely be drawn from a given area
without unduly reducing the pressure, lowering the average static
head, and endangering the continuance of the artesian supply. In
some parts of the oasis there are bores many hundreds of years
old still pouring forth their hundreds of gallons a minute; such
wells are probably situated in exceptionally favourable localities,
and are very possibly fed to a great extent by fissures. At the
same time it must not be forgotten that there are throughout the
oasis scores of wells which have ceased to run, either through
local exhaustion of the water-bearing strata, or through failure to
keep the bore-channels open; possibly through a combination of both
circumstances. In some cases time seems to have remedied matters,
as it is not uncommon to meet with instances where new bores, sunk
in the immediate neighbourhood of long extinct wells, have produced
strong discharges of considerable volume.
As I have already stated, the rate of flow of water is largely
influenced by both the size of the pores and the porosity of the
rock, the capacity to transmit water being very much greater for
large than for small pores, and for high than for low porosity. It
must, however, be pointed out that large pores and high porosity
do not necessarily go together, and that small pores in a rock
frequently accompany high holding capacity. For instance, a
fine-grained sample of Nubian Sandstone will absorb from 25 to 28 per
cent. of water, a medium-grained sample 20 per cent., while a very
coarse sample may take up as little as 15 per cent. The pores and
transmitting capacity of the coarse-grained variety will, however,
be very much greater than in the case of either of the others.
In order to arrive at some sort of idea as to the holding capacity
of the artesian-water strata of the oasis I made an examination of
between sixty and seventy samples of sand brought up from varying
depths from a few selected bores in the oasis. As, however, owing
to the methods of drilling employed, only powdered samples were
available, it was necessary in the first instance to ascertain the
relative porosity of sandstone in its ordinary state and broken up
into the form of sand. For this purpose I collected eight specimens
of the Surface-water Sandstone from various points in Northern
Kharga, and subjected them to absorption tests, both in the whole
and in the powdered states. In six out of the eight examples the
absorption was more when powdered than when whole, the average for
the eight rock samples being 22·44 per cent.; for the same when
powdered, 23·55 per cent. There is, moreover, no reason to suspect
that in ordinary lithological characters the Surface-water Sandstone
differs in any important respect from the Artesian-water Sandstone,
so that if we estimate the porosity of the latter from powdered
samples, we shall obtain a figure only about 5 per cent. too high.
The average porosity of sixty-four samples, collected from Bores
14, 16, 18, 31, 39, and 44, at various depths in the water-bearing
strata, was found to be 19·45 per cent. If we confine our attention
to a single bore, and examine a representative sample from every
stratum of the water-bearing sandstone, and, in deducing the
porosity, take into account the thickness of each individual
bed, we shall obtain a still more reliable figure. This was
done in the case of Bore No. 18, which passed through 122 metres
of water-bearing strata, thirty-one samples being collected and
subjected to examination. The absorptions obtained varied from 15·3
to 25·5 per cent., the average porosity of the whole column being
calculated as 19·6 per cent. I believe this figure may be accepted
without misgiving as a satisfactory working value for the porosity
of powdered Nubian Sandstone, that of the solid rock being taken
as 5 per cent. lower, or, say, 18·5 per cent.
The Artesian-water Sandstone has been proved to reach a thickness
of 122 metres, and probably its total thickness is considerably
more. Assuming, however, a vertical extent of only 122 metres,
the water-bearing beds under 1 square kilometre would, if fully
saturated, hold 4,965,000,000 gallons, which is the equivalent of
the water which would be discharged in ninety-four years by a well
flowing at the rate of 100 gallons a minute. When we consider that
the area of the floor of the depression is some thousands of square
kilometres, and that the water-sandstones, except in the immediate
neighbourhood of the existing wells, are probably fully saturated, we
realize the vast amount of water which is stored under the depression
alone, irrespective of the still greater quantities which underlie
the surrounding plateaux. The movement of the water through the
sandstones, except along fissures and through particularly porous
beds, is, however, very slow, so that the amount of water which
can economically be made available at the surface is more or less
limited, self-flowing wells being only obtainable over that portion
of the area lying below the general static head.