Two heavy oil samples taken from different geographical areas, with high viscosities at reservoir pressure and temperature, but with similar macroscopic characteristics (API gravity, Rsi, SARA, etc), have been evaluated using a methodology developed at PDVSAIntevep to quantify the "foamy characteristics" of the system. The results show substantial differences in the production mechanisms of the two heavy oils studied, not only in terms of PVT behavior but also in terms of the production performance during core depletion tests.
The term "foamy oil" refers to some heavy oil reservoirs that exhibit better than expected primary production performance during cold production under solution gas drive mechanism1. This behavior has been reported by several authors in the literature2–3, and it has also been observed in some Venezuelan heavy oil reservoirs from the Orinoco Belt (FPO) 4–6. A lot of workon the understanding of this mechanism can be found in the literature7–10. PDVSA Intevep has carried out a large number of investigations to understand foamy oil flow trough porous media and pipes. The main motivation for this research is its strategic importance for Venezuela because of the large heavy oil reserves present in the FPO7.
The main goal of this paper is to show basic results of an extensive study on production performance of two heavy oils. The samples tested were extracted from different areas in Venezuela and all of the tests were conducted at reservoir pressure and temperature.
Heavy oil recovery using only a solution gas drive mechanism is often insufficient to obtain an economically attractive exploitation of a field. Therefore, thermal methods are usually required to produce such reservoirs. There exist field evidences of some heavy oil reservoirs in Venezuela and Canada that show an unusually good performance being produced by solution gas drive mechanism. Such reservoirs are called "foamy oil reservoirs".
A possible hypothesis with regard to this unusual behavior is that solution gas, liberated under a depressurization process, remains trapped because of the high viscosity of the oil2. The conceptual difference between foamy and non foamy oil is shown in Figure 1. Only after a "long time" a phase separation between the oil and gas phase would occur. Therefore, released solution gas could be entrapped in the reservoir in foamy oil systems.
Nevertheless, flow of foamy oil through porous media is currently not well understood. It is thought that its description involve mechanisms such as nucleation, growth and coalescence of gas bubbles, but their precise influences remains unclear.
A methodology developed at PDVSA-Intevep that consists of a combination between two types of PVT tests and porous media evaluations, allowed us to quantify foamy behavior at reservoir conditions. Bubble pressure, pseudo bubble pressure and gas-trapping factor can be calculated by comparing a conventional PVT test (with agitation) with the so-called "non-conventional" PVT test (without agitation) 8 Core depletion tests show the behavior of multiphase flow, critical gas saturation and production profiles which are very useful to understand foamy oil behavior in porous media and its relationship with the recovery factor.