By FREDERICK H. NEWELL, M.E.
Among oil producers, there has been much discussion as to whether the sand rock in which petroleum occurs is of necessity fissured or is still in its original unbroken condition.
The earliest and most natural theory, which for years was indisputed, and is still given by some textbooks, was, that oil wells reached a cavity filled with petroleum.
Within the past few years, however, the opinion has been gaining ground that the oil is stored in the sandrock itself in the minute spaces between the small grains of sand, not entirely filled by cementing material, and that crevices holding and conducting oil are rare, all fissures as a rule being confined to the upper fresh-water bearing rocks of the well. Mr. Carll, in III. Pennsylvania Second Geological Survey, has discussed this subject very fully, and has made estimates of the quantity of oil that the sand rock can hold and deliver into a well; also, T. Sterry Hunt, in his Chemical and Geological Essays, has made deductions as to the petroleum contained in the Niagara limestone that outcrops about Chicago.
While the experiments and conclusions of these geologists go to prove that these rocks are capable of holding the oil, there are on record no facts as to the phenomena of its flow, other than by capillarity, through the rock. To obtain some data of the flow of liquids under pressure through certain oil-bearing stones, series of tests on small pieces were made. These tests were carried on during this spring, and many results quite unlooked for were obtained. When crude oil, kerosene, or water (river or distilled) was forced through the specimens, the pressure being constant, the rate of flow was variable. At first, the amount flowing through was large, then fell off rapidly, and when the flow had diminished to about one-quarter of its original rate, the decrease was very slight, but still continued as long as measurements were made, in some cases for three weeks.
When using crude oil, this result was not surprising, for, as the oil men say, crude oil "paraffines up" a rock, that is, clogs the minute pores by depositing solid paraffine (?); but this so-called paraffining took place, not only with crude oil, but with refined oil, and even with distilled water.
The only explanation as yet is, that liquids flowing under pressure through rock on which they exert little or no dissolving effect, instead of washing out fine particles, tend to dislodge any minute grains of the stone that may not be firmly held by cement, and these block up extremely fine and crooked pores in which the fluid is passing.
Several tests indicated that this blocking up was largely near the surface into which the fluid was passing. When this surface was ground off, even 1/50 of an inch, the flow increased immediately nearly to the original rate.
Reversing the flow also had the effect of increasing the rate, even above that of any time previous.
With the moderate pressures used - from 2" to 80" of mercury - the results show that the rate of flow, other things being equal, is directly proportional to the pressure.
The porosity of rock is not always a criterion of its permeability; a very fine grained marble, containing about 0.6 per cent. cell space, transmitted water and oil more freely than a shale that would hold 4 per cent. of its bulk of water.
If the above conclusions hold on a large scale as on the small, they may aid in explaining the diminished flow of oil wells. Not only will the flow lessen from reduced gas pressure, but the passages in the rock become less able to allow the oil to flow through.
The increase in flow following the explosion of large shots in a sand rock may be due not only to fissuring of the rock, but to temporary reversal of the pressure, the force of the explosive tending to drive the oil back for an instant.
The large shots now used (up to 200 quarts, or say 660 pounds of nitroglycerine) must exert some influence of this kind, especially when held down by 500± feet of liquid tamping. In the course of these tests, it was noticed that fresh water has a more energetic disintegrating action on the shales and clay than on salt water.
This may furnish a reason for the fact, noticed by the oil men, that fresh water has a much more injurious effect than salt in clogging a well. No oil-bearing sand rock is free from laminae of shale, and when fresh water gets down into the sand, the water must, as the experiments show, rapidly break up the shale, setting free fine particles, which soon are driven along into the minute interstices of the sand rock, plastering it up and injuring the well. - Engineering and Mining Journal.