Geoffrey R. Tick , University of Alabama
Crude oil mobility and the trapping mechanisms in a reservoir are controlled by pore geometry, grain size, sorting, viscosity, capillary action, and interfacial tension existing between the oil-water and solid media. The use of surfactant to lower interfacial tension will increase the mobility of oil thereby enhancing the recovery potential from the reservoir. Morphology of oil blobs and pattern of distributions will impact the resulting available oil-water-solid surface contact area thus controlling the interfacial processes for oil mobilization and recovery. The purpose of this research is to analyze three dimensional (3D) distributions of crude oil particles and understand the interfacial phenomena controlling oil mobilization at the pore scale in order to optimize innovative techniques for tertiary extraction of oil.
Several columns were packed with homogeneous, mildly heterogeneous, and heterogeneous mixed porous media, respectively (uniformity coefficients of 1, 5.8, and 10.6) and saturated with water to simulate a water-wetting-reservoir, and then separately flooded with light (23.4˚API), heavy (14.8˚ API) and extra-heavy (4.2˚API) crude oil. The columns were flushed with an anionic surfactant in two different episodes. Synchrotron X-Ray microtomography (SXM) at Argonne National Laboratory was used to capture high-resolution (<10 μm) 3D images of the oil distribution within the columns before and after the surfactant flooding events.
Results and Discussion
The heterogeneity of the porous media (i.e. grain-size distribution) was a dominant control on the oil-blob-size-distribution and mean blob volume present as residual saturation (initial distribution condition). The light and the heavy oil blobs showed relatively homogenous distribution in the homogenous medium. Both fractions showed significant change in morphology and heterogeneity after the 2 pore-volume (PV) and 5-PV surfactant flooding events, and became more laminar and flattened, coating the media grain boundaries to a greater extent. Additionally, both fractions showed an increase in number of blobs and total blob surface area, and a reduction in the total blob-volume after 2-PV of surfactant flooding. The light-oil distribution showed a 200% increase in total surface area after the 2-PV surfactant flooding event. The initial residual saturation distribution, in the mildly heterogeneous porous medium, showed highest heterogeneity for both the light and the heavy oil fractions. The distribution of this medium exhibited the smallest mean blob volume, for both fractions compared to the other systems. Total oil blob surface area for both fractions (i.e. light and heavy) either slightly increased or continuously decreased in the mildly heterogeneous medium after surfactant flooding. However, both fractions showed an increase in total surface area within the highly heterogeneous medium after the 2-PV surfactant flood and a slight decrease after the 5-PV flood, mainly due to high recovery process during this later flooding episode.
The extra-heavy oil exhibited an interconnected distribution of blobs, primarily as a large ganglia mass after all three stages of flushing (i.e. residual establishing aqueous flush, and the two successive surfactant floods). The extra-heavy-oil distribution (homogenous media) showed only a 7% increase in surface area after 2 PVs of surfactant flooding and a 1.4% decrease in surface area after the 5-PV flood. However, this oil fraction within the mildly heterogeneous porous media showed small increase in total surface area (7% and 3.5%, respectively) after the 2-PV and 5-PV surfactant flood. The extra-heavy-oil within the highly heterogeneous porous medium showed a gradual reduction in the total surface area after each successive surfactant flood mainly due to greater recovery and blob disintegration process after each surfactant flood.
Both the light and heavy oil experiments showed complete recovery from the homogeneous medium after 5 PVs of surfactant flooding, whereas the extra-heavy oil experiment showed minimal recovery after the 5-PV surfactant flood. The mildly heterogeneous porous medium produced a limited recovery after the 2-PV flood (i.e. 1.3% and 10% for the light and heavy oil), and greater recovery after the 5-PV flood (i.e. 23% and 41% recovery for the light and heavy oil fractions). The highly heterogeneous porous medium showed no recovery after the 2-PV surfactant flood; however, significant recovery resulted after the 5-PV flood (i.e. 44% for light oil and 34% gross recovery for heavy oil). Relatively low (6%) net recovery was achieved for extra-heavy oil from the homogeneous sand after the 5-PV surfactant flood, due to the formation of a continuous-oil-phase attributed to a change to an oil wet media system by low pH condition. Limited contact of extra-heavy oil with surfactant solution resulted in less interfacial activity. Negligible net (extra-heavy oil) recovery was achieved from the mildly-heterogeneous-sand mainly due to low associated permeability. The extra-heavy oil within the highly heterogeneous sand experiment yielded an average of 20% recovery after each surfactant flooding event, and, the initially heterogeneous blob distribution, changed significantly to a more homogeneous distribution as flooding progressed. Under these conditions, a stable-spontaneous-emulsion yielded consistent and notable oil recovery.
The results of these experiments demonstrate the high extraction potential of light and heavy oil fractions from the homogeneous medium during tertiary surfactant induced recovery. The extraction potential of extra-heavy oil fraction distributions within these types of reservoirs is expected to be severely limited even under significant surfactant flooding under these conditions. These studies reveal that it is important to characterize reservoirs (porous media systems) based on both permeability and quantitative grain size distribution parameters. Due to the fact that permeability of the mildly heterogeneous medium is the lowest, it can be concluded that for higher permeability media (i.e. homogeneous and highly heterogeneous media) more surfactant solution comes into contact with the oil phases yielding increased surface area. The ability to quantify in-situ blob distribution patterns resulted in a comprehensive understanding of the pore-scale processes affecting oil recovery and aid in the development of enhanced recovery from particular oil-porous-media systems based on surfactant chemistry and characterization of emulsion formation.
Acknowledgment is made to the Donors of the American Chemical Society Petroleum Research Fund for support (or partial support) of this research.