Conventional analysis has two main drawbacks: (1) difficulty of finding a given flow regime and (2) absence of parameter verification. Both slope and intercept of such linear tendency are used to characterize the reservoir. Conventional analysis-the oldest pressure transient test interpretation technique-is based upon understanding the flow behavior in a given reservoir geometry, so the pressure versus time function is plotted in such way that a linear trend can be obtained. The use of the oil flow equations and interpretation techniques is carefully extended for gas flow so that reservoir permeability, skin factor, and reservoir area can be easily estimated from a gas pressure or gas rate test by using conventional analysis and characteristic points found on the pressure derivative plot ( TDS technique). This chapter will be devoted to provide both fundamental of gas flow in porous media as well as interpretation of pressure and rate data in gas reservoirs. On the other hand, buildup pressure tests require linearization of both pseudotime and pseudopressure. For instance, when wellbore storage conditions are insignificant, drawdown tests are best analyzed using the pseudopressure function. Then, another artificial function referred as pseudotime is included to further understand the transient behavior of gas flow in porous media. The gas system’s total compressibility highly depends on gas compressibility which for ideal gases changes inversely with the pressure. Pseudopressure is a function that integrates pressure, density, and compressibility factor. Depending on the values of reservoir pressure, viscosity, and gas compressibility factor, the gas flow behavior can be treated as a function of either pressure to the second power or linear pressure with a region which does not correspond to any of these and it is better represented by a synthetic function call pseudopressure. Then, the gas flow equation is normally linearized to allow the liquid diffusivity solution to satisfy the gas behavior when analyzing transient test data of gas reservoirs. It is important, however, to try to provide the same mathematical treatment to oil and gas hydrocarbons, so interpretation methodologies can easily be applied in a more practical way. In general, gas viscosity is about a 100 times lower than the least viscous crude oil. This is not the case of the Tiab’s Direct Synthesis technique (TDS) which is indifferently applied to either drawdown or buildup tests and is based on features and intersection points found of the pressure and pressure derivative log-log plot.Ĭontrary to liquids, a gas is highly compressible and much less viscous. Its disadvantages are the accuracy in determining of the starting and ending of a given flow regime and the lack of verification. The conventional straight-line method has been customarily used for well test interpretation. Buildup pressure tests, for example, require linearization of both pseudotime and pseudopressure. Besides, since the viscosity-compressibility product is highly sensitive in gas flow then, pseudotime best captures the gas thermodynamics. When wellbore storage conditions are insignificant, drawdown tests are best analyzed using the pseudopressure function. Depending upon the viscosity-compressibility product, three treatments are considered for the linearization: square of pressure squared, pseudopressure, or linear pressure. The gas flow equation is normally linearized to allow the liquid diffusivity solution to satisfy gas flow behavior. This assumption does not apply for gas flow case in which the gas compressibility factor is also included for a better mathematical representation. Modeling liquid flow for well test interpretation considers constant values of both density and compressibility within the range of dealt pressures.
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