Pressure-volume-temperature (PVT) analysis is key to understanding how hydrocarbon composition, viscosity, density and compressibility change with pressure and temperature. Knowing how a specific hydrocarbon will react during production is essential to maximizing recovery in the shortest possible time and at the lowest possible cost.
With this in mind, BPS offers our clients one of the largest and most comprehensive collections of PVT laboratory analysis of Brazilian basins on the market. Among other essential data, the Brazil Geodatabase currently includes flash expansion, differential expansion, oil formation volume factor, gas formation volume factor and solubility ratio results from thousands of samples across a wide variety of Brazilian basins.
Taking things a step further, the integration of all these PVT analysis results with pressure and reservoir data provides an interpretation summary through which gravity segregation and compositional grading, among other important features, can be identified and explained.
Absence of Gravity Segregation
The PVT analysis of fluid samples collected in the pre-salt wells of the Santos Basin can provide important insights to understand the absence of gravity segregation in the very thick interconnected zones of the carbonate reservoirs.
In this article, we reinforced that the prevailing tectonic-sedimentary environment in the pre-salt Santos Basin created the proper conditions for the development of a naturally fractured system, together with the reactivation of normal faults, which in turn provided hydraulic interconnectivity between the deeper source rocks and the overlaid carbonate units.
We also emphasized that the hydrocarbon composition and properties are virtually the same across all gross-pay intervals of the Barra Velha and Itapema formations.
The absence of gravity segregation can be explained by the fluid circulation generated by natural thermal convection across the entirety of the fractured and interconnected carbonate units, where the oil homogenization process has been going on since hydrocarbons charged the reservoirs.
Very similar oil and reservoir fluid compositions have been found in samples from the top of the microbialite carbonate reservoir to the base of the coquinas reservoir, as shown in Figure 1 for well 3-BRSA-1064-RJS of the Buzios field. Notice that the fluid properties such as API gravity, Gas-Oil-Ratio (GOR) and bubble point pressure are very close to each other.
The same behavior has been observed for the hydraulic continuity in the reservoir zones crossed by well 1-BRSA-1146-RJS in the Atapu field. In fact, this phenomenon has actually been observed in many pre-salt fields.
In contrast, hydrocarbon fluids in the pre-salt accumulations of the Santos Basin with very high CO2 content in the very thick reservoirs have shown abnormal behavior known as compositional grading, where two distinct phases are formed, with gas and condensate in the upper section and heavy oil in lower section, both with supercritical dissolved CO2.
This was observed in well 3-BRSA-967A-RJS of the Jupiter block. Relevant physical properties obtained from the PVT analysis of samples collected from microbialite carbonate reservoirs thicker than 150 meters are shown in Table 1, along with compositional analysis in Figure 2, highlighting the presence of gas and condensate in the upper section and heavy oil in the lower section.
It is also worth mentioning that the exploration areas in the southeastern area of the Santos Basin, including the Peroba and Pau Brasil blocks, are favorable for the occurrence of higher CO2 content that can be accompanied with the consequent compositional grading phenomenon.