Abstract
Natural petroleum gas occurring in the oil-bearing reservoirs is found toexist either as free gas associated with the oil and/or in solution in the oil. In some virgin fields practically no free gas is encountered under thereservoir conditions of pressure and temperature, although in other fields aconsiderable section of the "pay" horizon is occupied by free gas. Inother words, a given oil field mayor may not have a gas cap before theexploitation takes place. However, when the reservoir pressure is reduced to apoint below the critical saturation pressure by withdrawals of fluids from theproducing formation, the gas dissolved in the oil inevitably begins to come outof solution and becomes free gas. Under the controlled methods of productionand drilling, a portion of this gas is produced with the oil to the surface anda portion of it remains within the reservoir to accumulate at higher structurallevels toward which it migrates by virtue of its low specific gravity. The rateof growth of a gas cap in a given structure depends primarily upon the rate ofdecline in the reservoir pressure. If the rate of pressure decline is rapid, the gas cap likewise will grow rapidly. On the other hand, shrinkage in thegas-cap area was noted in some fields in which reservoir pressures wereincreasing. Determination of Gas-Oil Contact and Thickness of Gas Cap The gas-oil contact can be conveniently located in the process of drilling awell and by electrical logging. However, very little can be found in thepetroleum literature concerning methods of predetermining gas-oil contacts inproducing reservoirs. Therefore, the writer feels justified in describingbriefly a simple, workable, and inexpensive method of determining gas-oilcontacts and outlining the limits of gas-cap areas in a field where suchknowledge may be essential for efficient production and developmentoperations. The method involves the determination of the following data:Structuraldata, such as the depth or elevation of the top of "pay" horizon;reservoir pressures;tubing and casing pressures at the well head;static oil-pressure gradient;static gas-pressure gradient. T.P. 917
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