As the petroleum industry moves to operations in more harsh environments, many of the tools commonly used for engineering estimates must be used in calculations for these extreme conditions. Among the conditions encountered by the industry range from highly undersaturated, high pressure oil reservoirs in the deep water Gulf of Mexico to the cool temperatures found along the ocean bottom. Deep, hot, high pressure gas reservoirs attract interest as well as carbon dioxide injection projects associated with carbon sequestration. All of these conditions challenge methods routinely used to accurately estimate PVT properties. Accurate estimates of oil and gas PVT properties are required to properly quantify resources and their potential. This talk will provide on overview of natural gas compressibility and oil viscosity properties as determined with correlations.
Natural gases are derived from two basic sources – associated gas which is liberated from oil and gas condensates where hydrocarbon liquid, if present, is vaporized in the gas phase. Either type of gas may contain nonhydrocarbon impurities such as hydrogen sulfide, carbon dioxide and nitrogen. Failure to properly distinguish between the two types of gases and address the nonhydrocarbon content can result in calculation errors in excess of those allowable for technical work. Mathematical representations of compressibility factor charts commonly used by the engineering community and methods used by the geophysics community are investigated. Generally these methods are robust and have been found suitable for ranges beyond those originally recommended.
The calculation of pressure drop resulting from the flow of oil through porous media or pipes requires the evaluation of viscosity. This is the single most important transport property necessary to accurately calculate pressure drop. The basis for oil viscosity calculations using a traditional black oil approach is the determination of dead or gas free oil viscosity. Most of the correlations do not properly account for the actual physical behavior of viscosity with temperature. Dead oil viscosity is modified to account for the effect of dissolved gas up to the bubble point pressure. In the undersaturated region, the pressure effect on viscosity results in a significant additional increase in this property. Correlations for undersaturated viscosity are often limited by the level of pressure differential.