Compressibility and density are important fluid properties that are used in dynamic reserve calculations such as the material balance equation and reservoir simulation. Compressibility is normally obtained from pressure-volume-temperature (PVT) measurements performed in the laboratory. However, for samples obtained at reservoir pressures exceeding 15,000 psi, extrapolation techniques are sometimes used, introducing uncertainty in the calculated results. A new approach has been developed to obtain compressibility from downhole fluid analysis measurements up to 25,000 psi. A formation testing tool pumps formation fluid from the reservoir. Pressure and density of the pumped fluid are measured in the flowline of the tool. A change in pressure may be induced by a change in pump rate and/or by closing valves in the tool. These dynamic pressure and density data are used to calculate compressibility.
When the density sensor is placed between the formation interface module and the pump, density is measured at and below formation pressure. In this scenario, no extrapolation is required to derive compressibility in situ at reservoir conditions. It is possible to place the density sensor downstream of the pump. Density is then measured at and above mud pressure. The obtained pressure-density cross plot can be used, not only to derive the fluid compressibility, but also to extrapolate the density and compressibility to reservoir pressure or, if desired, to saturation pressure. The measurement of pressure and density, the compressibility calculation, and the density extrapolation are all performed in real time during data acquisition with the tool in the well.
This method has been successfully applied in Gulf of Mexico and other deep-water wells for various fluid types. The presented data examples cover high pressure (>20,000 psi) environments. The calculated compressibility and measured or extrapolated density values are validated by laboratory measurements for the lower pressure examples. Additionally, a best practice has been developed for various formation testing tool configurations to maximize the quality of the obtained compressibility data.