Experimental Study of Hysteresis Behavior of Foam in Porous Media

Abstract

Gas injection was introduced to the petroleum industry in the early 1950s. Nevertheless, the process efficiency is impacted by the low density and viscosity of the gas, which decrease sweep efficiency. Foam for Enhanced Oil Recovery (EOR) can overcome the downside of the viscous fingering by increasing the apparent viscosity of the

Foam EOR
improves the sweep efficiency by reducing gas mobility and creating a stable
displacement front. In the field application, the surfactant concentration and
flow rate vary in the reservoir, influencing dramatically the foam mobility.
However, the variations of surfactant concentration and flow rate do not relate
monotonously to the foam properties. In some cases, the foam properties depends
on the history of the flow, i.e., a hysteresis effect. But hysteresis in foam
flood has not been well characterized and understood. This study aims to
understand hysteresis behavior of foam in porous media. To this end two series
of experiments have been conducted: 1) Hysteresis behavior due to flow rate
variations and 2) Hysteresis behavior due to surfactant concentration
variations. In the flow rate experiments, several shear-thinning experiments at
different volume fractions of gas (foam quality) are conducted in order to
understand the effect of gas fraction and total velocity on foam generation
mechanisms. In the surfactant concentration experiment, experiments have been
performed at different surfactant concentrations and at different volume
fractions of gas (foam quality). Results showed that a transition from weak to
strong foam is more pronounced in high-quality regimes (gas fractional flow
above 90%) than low-quality regimes (gas fractional flow below 80%).
Remarkably, no hysteresis behavior has been observed in low-quality regimes,
while hysteresis behavior occurred in high quality regimes. Furthermore, the
effect of surfactant concentration on hysteresis behavior has been also
investigated at high- and low-quality regimes. Contrary to some previous works,
hysteresis behavior does not occur for surfactant variation. Remarkably, the
apparent viscosity remains almost constant in lowquality regime for different
surfactant concentrations. These results have important implications of the
injection strategy and the economics of foam EOR. The surfactant concentration
could be decreased and less gas could be injected, and in the same time, the
foam performance could be maintained.

gas. Importantly, the structure of the foam evolves with time due to gas diffusion between bubbles (coarsening). In a bulk foam, the coarsening behaviour is well defined, but there is a lack of understanding of coarsening behaviour in confined geometries, especially in porous media. Nonnekes et al [2014] predicted numerically and analytically that coarsening will cause the foam lamellae to move to low energy configurations in the pore throats, resulting in greater capillary resistance when trying to restart flow. This study describes foam coarsening in a porous medium and the implications for foam propagation. Foam coarsening experiments have been conducted in both a micromodel and in a rock core. The micromodel is etched with an irregular hexagonal pattern, with a Gaussian distribution of pore diameters. Foam was generated by coinjecting surfactant solution and nitrogen gas into the micromodel. Once steady state flow had been achieved, the flow was stopped. The coarsening behaviour of the foam was recorded using time-lapse photography. The core flood coarsening experiments were carried out using a Bentheimer Sandstone core. Foam was produced by coinjecting surfactant solution and nitrogen at the base of the core. Once a steady state flow was achieved, the flow was stopped and the core sealed off. When flow restarted, the additional driving pressure required to reinitiate flow was measured, and this could be attributed to the stable configuration of the coarsened foam. The microfluidic results found that the bubbles coarsened rapidly (t < 10 minutes) to the size of the pores. At the completion of coarsening the majority of the lamellae were located in the pore throats with minimum length. Because of the effect of the walls, the behaviour did not conform to the unconstricted coarsening growth laws. Furthermore, results on coreflood showed that coarsening is a rapid process, in agreement with microfluidic results. An increase in the additional pressure required to re-initiate flow was observed for the first 1 – 5 minutes of flow stoppages, while the pressure peaks did not increase for durations above 5 min. The implications of this behaviour for the field scale are also discussed.