EXPERIMENTAL STUDIES OF SUBSURFACE RELATIONSHIPS IN OIL AND GAS FIELDS.1

During the writer's studies of subsurface problems in various oil and gas fields of the United States he has been impressed with the need for broad, systematic, and painstaking studies of subsurface relationships. New criteria for the interpretation of subsurface conditions must be established and new methods of work developed and applied in the search for oil and gas as well as in their recovery. The protection of our known fields against the inroads of water, and the development of more efficient methods of recovery are imperative, but both lines of endeavor depend primarily upon an understanding of subsurface principles and relationships. The drilling and operation of wells furnish valuable subsurface data, together with much costly experience, but the facts and relationships revealed through studies of materials and data collected in the field are not adequate because they are incomplete.

1 Published by permission of the Director of the Bureau of Mines.

The United States Bureau of Mines is endeavoring to meet this situation through systematic and scientific field and laboratory experimentation. While investigating various phases of the oil-field water problem for the Bureau of Mines the writer has pursued experimental studies in conjunction with his field. work. This has led to the development of a simple apparatus for the laboratory study of subsurface relationships in oil and gas fields. A series of preliminary experiments, with the apparatus demonstrated its value in studying some of the fundamental principles and relationships governing the occurrence and recovery of oil and gas. As there is a growing demand for this type of work, the apparatus, together with the methods of study that have been developed and some of the results of the work, will be of interest to other investigators.

APPARATUS.

Construction. The apparatus consists essentially of a rectangular steel tank with removable top and front plates, and with openings and cocks arranged as shown in Fig. 51 and the accompanying plates. Three of these tanks, two small and one large,

are now in use by the Bureau of Mines. All pipe connections for the small tanks have 3/8 inch (0.95 cm.) internal diameters, and those for the large tank 1⁄2 inch (1.27 cm.) internal diameters. Standard brass service cocks are used on both sizes of tanks. All interior corners are slightly rounded through the bending of the steel members during construction, or through filling in corners with aquarium cement. The internal dimen

sions of the two small tanks are 92 by 48 by 9 cm., or 36.22 by 18.90 by 3.54 inches; and of the large tank, 183.5 by 124 by 13.3 cm., of 72.24 by 48.82 by 5.24 inches. The fluid capacities of the two sizes of tanks are 39.7 liters (10.49 U. S. gallons) and 302.6 liters (79.94 U. S. gallons), respectively.

The steel bodies of the two sizes of tanks are built up of 16inch plates riveted and welded to 4-inch and 6-inch channels respectively, the channels being bent so as to form the bottoms and ends of the tanks when standing vertically as shown in Plates XII-XV. The surfaces of the steel members exposed on the insides of the tanks are enameled to prevent corrosion and to provide surfaces that approximate glass and quartz in their coefficients of friction with oil and water.2 The legs of the channels, to which the plates are riveted, project outward as shown in the photographs. The backs of the small tanks are single plates, but the back plate of the large tank is equipped with two armholes and two removable plates to close them. Around the tops of the tanks there are planed flanges 16 inches thick, to which the top or cover plates may be bolted. The cover plates, which are 16 inches thick, fit snugly upon the flanges, the contacts being made tight by rubber gaskets. The cover plates are equipped with threaded openings and packing boxes which are closed unless in use. These openings serve principally to receive miniature wells. (See Fig. 51.)

The fronts of the tanks are removable glass plates that fit inside the steel frames and slide into place through the openings at the tops of the tanks when the cover plates are removed. The dimensions of the glass plates are 1⁄2 by 16 by 36 inches for the small size, and 34 by 48 by 72 inches for the larger size. The tanks are constructed so that thicker plate glass may be used if desired. These plates are made tight to the steel frames by aquarium cement, and are braced from the outside by buckstays when necessary.

Uses. This type of apparatus can be readily transported to field laboratories where it is desired to make repeated use of large 2 Experiments with different enamels are in progress.

volumes of the gases, oils, and waters there found. When a tank is in use, such materials as sand, clay, gas, oil and water can be placed in the tanks and arranged to similate varied natural conditions. Loose sands can be compacted both by sedimentation and by tamping, and can be lithified through the use of different chemical reagents that react so as to cause cementation thus approximating the rock formations in nature.3 Fluids can be introduced and made to pass through at will. Jet-like currents can be induced or prevented through regulating the velocities of flow and through the use of baffles which break up such currents. Pipes representing wells can be inserted and the fluids extracted through them. Moderate pressures can also be exerted, but for high-pressure work an auxiliary apparatus, to be described later, is being developed. While in use, the apparatus can be tilted to any desired angle. The total surface of glass in contact with the fluids is practically negligible in comparison with the total sandfluid contacts. In other words, the conditions of experimentation can be made to approach more nearly to deep-seated rock conditions than can the conditions of experimentation in glass tubes or in open vessels. The plate glass fronts of the tanks afford every facility for making observations and photographic records, and the dimensions of the areas of glass exposed in the frames, together with graduations on the edges of the frames around the glass, afford size scales for the pictures. Furthermore, if the glass plates become badly scratched or broken, they can be easily replaced without injury to the apparatus. These features, together with the wide range of experiments that can be performed in the apparatus, are decidedly advantageous.

Three reservoir tanks for gases, oils, and solutions, together with an air pressure line connecting the reservoir tanks to an air receiver, form a part of the equipment. Pipe connections, gas meters, water meters, pressure regulators, thermometers, and gages, were arranged differently for the various experiments. In using an apparatus of this kind it is imperative that the

3 This process involves the formation of interstitial silicates and carbonates. It is used for the manufacture of artificial sand stone.

limitations of the experimental values be recognized. The difficulties of reproducing subsurface conditions in the laboratory necessitate the development of experimental methods to establish broad principles and relationships rather than concrete facts. In this connection, it is recognized that laboratory data have indicative and relative rather than absolute values as criteria for the interpretation of subsurface conditions and relationships; but in conjunction with field studies, the laboratory experiments broaden our conception and strengthen the foundation of facts upon which valid conclusions must be based. Field investigations, together with the laboratory study of materials collected in the field, are therefore supplemented by experimentation in which comparisons are made between the behavior of different gases, oils, and aqueous solutions in different kinds of sands under different sets of conditions. The number of variables in each experiment is restricted so that the relative values of the factors and relationships under scrutiny, may, under the conditions of experimentation, be very definitely determined. In this respect, laboratory experimentation has a decided advantage over subsurface experiments in the field. Both types of work should be pursued; one or the other taking precedence according to the requirements of the problem. The reduction of the number of variables and the substitution of known for unknown values in formulating conclusions from both field and laboratory studies are just as important as the corresponding steps in solving an equation or a set of equations.

EXPERIMENTS.

Photographic Records. To illustrate the use of the apparatus, photographs and brief descriptions of a few of the Bureau of Mines experiments in the laboratory are submitted for consideration. More complete descriptions of these experiments, together with notes showing the quantitative results, will be pre

4 See U. S. Geol. Survey Bull. 693. The evaporation and concentration of waters associated with petroleum and natural gas, by R. Van A. Mills and Roger C. Wells.