Gas Saturation Pressure of Crude Under Reservoir Conditions as a Factor in the Efficient Operation of Oilfields

THIS contribution deals with general principles and their application to oilfield operating problems. The observations were made on limestone fields in which reservoir-productivity varies locally within wide limits. Data from sand-fields and other limestone fields would be especially welcomed. Some points may be clearer after reference to the sketches in Platei II, wherein also terms liable to misinterpretation are defined. The main feature of this paper is that it deals with the conditions of gas solution-equilibrium in oil reservoirs, and emphasises the necessity of determining these conditions on individual fields by examination of crude samples collected from the reservoir under pressure. Other published papers have reported results of quantitative investigations into the dissolved gas-content of crude at vary, ing pressures and temperatures and the resultant changes in the physical. properties of the crude. Nothing, however, as far as the author is aware, has been published on actual conditions in oil reservoirs.as reservoirs. as determined by bottom-hole samples. It is generally assumed in oilfield literature that crude at any point in a reservoir is saturated with gas to its actual reservoir pressure at that point. Theoretically this should not be so. For true solution-equilibrium the saturation pressure of all crude in a reservoir at any time should be the gas pressure at gas-oil level. A survey of "saturation pressures" in Persia has proved that this is only the case in highly productive zones where fracturing and fissuring are favourable to convection. In other areas the crude was super-saturated, showing that diffusion alone is a negligible factor, in human time, in the maintenance of Solutionequilibrium. Saturation pressures in most old-producing fields should, therefore, vary substantially. Wide variations may also be expected in some virgin fields where processes of convection and diffusion have not re-established solution-equilibrium disturbed by seepages. Other points are the unreliability of dissolved gas-content/ pressure curves based upon regâssing dead crude and, to a lesser extent, of curves based upon samples collected under pressure at well-heads. The chief application of these conclusions is in proration for gas conservation. The ideal method is to restrict production of individual wells so that bottom-hole pressure-drop on production does not exceed a "critical" value, which is the observed difference between the reservoir pressure and the "saturation pressure." In comparing producing with dissolved gas-oil ratios allowance must be made for the method and temperature of separation.