Primary production of heavy oil from unconsolidated sand reservoirs is a process that is not well understood. Laboratory depletion experiments of sand packs, which mimic the field situation at a laboratory scale, show that production behavior of heavy oils under solution gas drive depend in a complex way on diverse parameter such as: depletion rate1,2, fluid properties3, porous medium propertiesi4,5, etc.
This work focuses on understanding the effect of the clay fraction in the sand pack on the recovery process. Experimental results from depletion tests at constant total volumetric production rate are presented. The porous media consisted of synthetic sands with different proportions of clay, except for one of the experiments, where reservoir sand was used. The oil was extra heavy crude (8°API) from the Venezuelan Orinoco Belt.
Pressure, pressure drop over the core, and oil and gas production were measured as a function of time. It was found that critical gas saturation and recovery factor increase with increasing clay content, whereas the supersaturation or the deviation from thermodynamic equilibrium decreases. This is interpreted in terms of an increasing number of nucleation sites at increasing clay concentration. More activated nucleation sites lead to more and smaller bubbles that allow a lesser mobility for the gas phase. Gas then becomes significantly mobile at larger gas saturations. This hypothesis is supported by previously published data from micromodel experiments doped with clay particles6.
The fact that critical gas saturation and recovery factor depend on the composition of the reservoir sand may strongly affect the evaluation of reservoir production capacity, because large regional heterogeneities related to clay content are often present in oil-producing "sands".
Primary oil production, as observed in heavy oil fields in the Venezuelan Orinoco belt, typically differs from that predicted using reservoir simulation. The field shows higher produced oil rates, lower gas rates, and slower pressure decline. At least, three different mechanisms contribute to the primary production process in volumetric reservoirs: (i) compressibility of the liquid phase, (ii) solution gas drive, and (iii) rock compaction. Because the liquid compressibility is much lower than the gas compressibility, the main contributing mechanisms are solution gas drive and rock compaction. However, the contribution of solution gas drive to the recovery process is generally larger than that related to rock compaction, and very often large uncertainty exists on the existence of this last mechanism and its contribution to primary recovery. Therefore, solution gas drive is commonly considered to be the main mechanisms in this type of reservoirs, at least early in primary production process.
Solution gas drive depends in a complex way on many variables7, including porous medium properties. Characteristics of the porous medium play an important role in the nucleation of bubbles during depletion6. By consequence, they also influence phase distribution and morphology. It has been reported that recovery factor increases as the number of bubbles formed during depletion increases8. A possible explanation is that the probability of coalescence decreases when more and smaller bubbles are present and that the gas phase remains immobile up to larger gas saturations. Large values of critical gas saturation and low gas mobility, which support this explanation, have been observed in depletion tests9,10.
The fact that porous medium composition affects heavy oil solution gas drive behavior is the main motivation for this work. We carried out a set of depletion experiments in sand packs doped with clay in different proportions. In order to emulate closer field situation, we also carried out an experiment with reservoir sand.