Drilling can now be carried on through the mud-fluid, and if desired casing can be set to case off the gas sand.
There are, of course, cases where "gas blow-outs" are so serious that the casing may be blown from the hole and, as in the case at White Point, just opposite Corpus Christi, Texas, craters large enough to hold sky scrapers may be formed. Under such condi tions mudding might be successful, but such large craters demand a great supply of mud and water and equipment sufficient to fill them.
There have been a few cases where the gas pressure has been so high that it was impossible to drill after lubricating. At Sand Draw, Wyoming, the Producers and Refiners, in their drilling, encountered a pressure of 1550 lb. per cubic inch at a depth of 2300 ft. The company lubricated the well, but when it was opened and drilling with a rotary was commenced the mud-fluid was blown from the hole and no headway in drilling could be made. As yet no method of drilling through the gas has been successful.
Increased Oil Recovery.—One of the greatest single problems facing the oil industry is the problem of increasing the oil recovery. From all the data available average recovery on oil properties will not exceed 20 per cent of the oil in the sand. It has been proven by experimental tests on sands that from 40 to 50 per cent of the oil always remains in the oil sand, but the other 50 per cent should all be recovered. One and one-half times the amount at present recovered is left in the ground. Such condi tions do not speak well for our methods of handling producing wells. That it is possible to obtain more oil than has been done so far was demonstrated at Bradford, Pennsylvania, and at Marietta, Ohio. At the former place a water-flooding system is employed, and at Marietta the Smith-Dunn compressed air system is used. The success of those ,systems should encourage further study along the lines of increased recovery.
Smith-Dunn Process.—The Smith-Dunn or Marietta process consists in forcing compressed air into the oil sands to take the place of the gas that was drawn from the sands. The air is introduced in central wells (see Fig. 85a) and forces the oil to the surrounding wells. The air well must, of course, be sealed tight so that the air cannot escape (see Fig. 86). Production has been increased several hundred per cent due to this use of compressed air.
This method can be used to advantage in areas where there are thick shale bodies which overlie the oil horizon and form practi cally impervious covers which will not allow the air to escape.
The low dip of 10 to 20 ft. per mile and the ideal conditions at Marietta, Ohio, do not exist in all places, but experiments with this method may prove successful in other fields.
Bradford Flooding system em ployed at Bradford, Pennsylvania, has given excellent results for that field. At Bradford the main oil producing sand is 40 to 50 ft. thick, is coarse-grained and very uniform in size of grain.
central well is selected from a group of four or five wells. This well is filled with water to its full depth, which is 1100 ft. The hydrostatic head is 477.4 lb. (1100 X 434). The water spreads away from the hole in all directions and drives the oil outward from the water hole. The rate of travel is slow, 150 ft. per year, or less. Where the producing wells are 500 ft. apart a ring of new wells may be drilled inside to catch the oil. Old wells have gained from 100 to 1000 per cent by this method. Wells making bbl. have, when flooded, made 2 to 5 bbl. Wells making 2 bbl. have increased to 10 bbl.
The same principle of flooding may be applied in other cases. The uniform, coarse-textured sand, the very low dip of the sand, 10 ft. per mile, give ideal conditions for the application of such a method. Modifications may apply in other places.