This paper presents the results of laboratory tests, performed in a mobility control foam design for Western Venezuelan Reservoirs. In this operational area, two Water Alternate Gas (WAG) injection projects are currently under way using natural gas, but they are expected to change to nitrogen.
The tests presented in this paper were performed mainly with nitrogen, although a final displacement test was designed using fluids that are currently under use in the Lagocinco VLE WAG injection project. The final goal was to achieve a mobility control foam, for gas profile modification in WAG injector wells.
The experimental work was divided in several steps:preliminary screening tests for adequate surfactant selection;mobility reduction tests at average Western Venezuelan conditions;adsorption tests; anda final displacement test in a composite reservoir core using fluids and conditions for the Lagocinco VLE Field.
The selected surfactant for the foam was an alpha oleofinsulfonate (AOS) combined with a fluorinated surfactant for added stability to oil. Surfactant-gas co-injection was used in all core displacement tests as injection scheme. The foam achieved a Mobility Reduction Factor (MRF) of 100 at average Western Venezuelan conditions, while at Lagocinco VLE conditions the MRF values were as high as 800.
Introduction
PDVSA's Western oilfields were the first discovered in the country and are probably the most highly exploited reservoirs in Venezuela. For more than 4 decades, they have been under water and gas injection with recovery factors that sometimes fall below the expected values of 29 and 44% of the original oil in place, respectively.1,2 As these oil resources have approached maturity, it has become clear for PDVSA that improving recovery will only be possible through the incorporation of new and proven technologies, designed to meet specific regional needs.
Worldwide, oil recovery obtained by injecting fluids, not initially present in the reservoir, only represent 3.5 % of the world's oil production. Approximately, 82 % of this oil is obtained by steam or gas injection.3 However, oil recovery efficiency by these gas processes has not been as high as expected, mainly because not all the oil is contacted by the injected fluids.4 Gravity override and channeling are responsible for the low sweep efficiency of any gas-injection project. Additionally, early gas breakthrough in producer wells increases operational costs.
WAG processes have been commonly applied in gas floods to reduce mobility, add stability to the gas displacement front and reduce override effects caused by the domination of gravity forces over viscosity forces in highly heterogeneous reservoirs.5 However, even in these cases, early gas breakthrough and gravity unstable displacements have been evidenced as potential or mayor problems in the field.6,7
Foams, were first evaluated as a gas-blocking agent, in a porous media, by Fried8 in 1961. Since then, foams have been used in Improved Oil Recovery (IOR) processes to mitigate gravity override and channeling by reducing gas mobility. The use of foams in WAG injection projects has been documented by Hanssen et al.7 as a process that could allow a considerable improvement in the efficiency of WAG in heterogeneous reservoirs. The main advantages are associated with GOR reduction and increased oil rates. The foam diverts the injected gases into unswept areas with high oil saturation, increasing sweep efficiency and gas-breakthrough time.7
Foam in porous media reduces the gas mobility by blocking the pores through which the gas flows. It consists of individual lamellae, thin liquid films, each straddling a pore body or a throat and separated from each other at least a pore length.9 These thin films block gas flow paths where they reside.
