PERMEABILITY OF SAND ROCK.
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.