Ross, T. S. (New Mexico Institute of Mining & Technology) | Rahnema, H. (New Mexico Institute of Mining & Technology) | Nwachukwu, C. (New Mexico Institute of Mining & Technology) | Alebiosu, O. (ConocoPhillips Co) | Shabani, B. (Oklahoma State University)
Steam injection—a thermal-based enhanced oil recovery (EOR) process—is used to improve fluid mobility within a reservoir, and it is well known that it yields positive results in heavy-oil reservoirs. In theory, steam injection has the potential of being applied in light-oil reservoirs to enable vaporization of in-situ reservoir fluids, but field developments and scientific studies of this application are sparse. Conventional displacement methods like water-flooding and gas-flooding have been applied to some extent, however, oil extraction in such reservoirs relies on recovery mechanisms like capillary imbibition or gravity drainage to recover oil from the reservoir matrix. Furthermore, low-permeability reservoir rocks are associated with low gravity drainage and high residual oil saturation.
The objective of this study is to evaluate the potential of steam injection for light (47°API) oil extraction in naturally-fractured reservoirs. It is theorized that this method will serve as an effective tool for recovery of light hydrocarbons through naturally-fractured networks with the benefit of heat conduction through the rock matrix. This research investigates the application of light-oil steamflood (LOSF) in naturally- fractured reservoirs (NFR).
A simulation model comprised of a matrix block surrounded by fracture network was used to study oil recovery potential under steam injection. To simulate gravity drainage, steam was injected through a horizontal well completed in the upper section of the fracture network, while the production well was completed at the bottom of the fracture network. The simulation included two different porous media: (1) natural fractures and (2) matrix blocks. Each of these porous media was assumed to be homogeneous and characterized based on typical reservoir properties for carbonate formations. This study also analyzed the impact of different recovery mechanisms during steam injection for a light-oil sample in NFR, with reservoir sensitivity examined, based on varying amounts of vaporization, injection rate, permeability, matrix height and capillary pressure. Of these, vaporization was found to be the dominant factor in the application of LOSF in NFR, as described in detail within the results.
A systematic lab study was conducted to investigate the impact of water on asphaltene deposition tendency, with emphasis on percent water cut and ion composition. Two crude oils from Gulf of Mexico with different properties were applied as probe oils to study asphaltene deposition using a capillary deposition flowloop. Distilled (DI) water and a synthetic brine with 6.5% NaCl salinity were used to create water-in-oil emulsions to study the impact of water on asphaltene deposition. For one oil sample, it was observed that adding as few as 2 vol% DI water to the oil/n-heptane mixture could cause as much as 56% reduction on deposition rate. When DI water was replaced by the synthetic brine, the reduction in deposition rate decreased. However, when the synthetic brine also contained ferric ion (Fe3+) or aluminium ion (Al3+), the deposition rate was restored back to the same or an even higher level as the base case without water. ICP analysis revealed that deposits collected from tests with ferric ion or aluminium ion also contains significant amount of those two ions, plus remarkable increases on other divalent ions including Ni and V. In the second oil, adding 10–20% synthetic brine also reduced deposition rate 10–25%. With only 10 ppm ferric ion in the brine, the deposition rate for the second oil was largely restored back to the original level without water.
Oudinot, Anne Y. (Advanced Resources International, Inc.) | Koperna, George J. (Advanced Resources International, Inc.) | Philip, Zeno G. (Halliburton) | Liu, Ning (New Mexico Institute of Mining & Technology) | Heath, Jason E. (New Mexico Institute of Mining & Technology) | Wells, Arthur (US Department of Energy NETL) | Young, Genevieve B.C. (BG Group) | Wilson, Tom (West Virginia University)
Changes that occur with increase in capillary number in the detailed structure of residual oil trapped in water-wet sandstone core samples have been investigated. The technique of using a nonwetting phase that can be solidified and separated from the porous medium has been applied with styrene monomer as the nonwetting phase and 2% CaCl2 brine as the wetting phase. The size distributions of residual oil blobs, obtained under various flow conditions, were measured by both image analysis and Coulter counter techniques. Specific features of blob shapes and dimensions were checked by optical and electron microscopy. The changes in size distribution and shapes of blobs provide insight into the mechanisms of trapping and mobilization of residual oil.
At the conclusion of waterflooding an oil-bearing reservoir, a significant fraction of the original oil still remains in the swept region as trapped residual oil. In water-wet reservoirs, this residual oil, S*or, may typically occupy 25 to 50% of the pore space and provides a main target for tertiary oil recovery. Trapped oil can provides a main target for tertiary oil recovery. Trapped oil can be recovered from a core sample at S*or, by immiscible displacement if the ratio of viscous to capillary forces, expressed in this work as the capillary number Nc = exceeds a critical value. Changes in microscopic distribution of oil within pore spaces can still occur at capillary numbers less than critical. Above the critical capillary number, Nc,(crit), oil is displaced from the core sample. In laboratory investigations, nondimensional relationships between capillary number and the ratio Sor/S*or (residual oil saturation, Sor, normalized with respect to S*or) have been found to be remarkably similar for a variety of sandstones. In addition to the amount of trapped oil. its microscopic distribution within the pore spaces of a reservoir rock is important to gain a better understanding of oil-recovery mechanisms. This knowledge may also be important to the design and implementation of tertiary recovery processes. For example, in modeling the recovery of residual oil, the viscous force required for mobilization of a residual oil blob trapped under water-wet conditions is expected to be inversely proportional to blob length. The technique of using a nonwetting phase, which after flooding to residual saturation can be solidified and then separated from the porous medium to study the microscopic structure of residual porous medium to study the microscopic structure of residual nonwetting phase, was probably first employed by Craze, who referred to the observed capillary structures as irregularly shaped blobs. Blob-size distributions have been measured in the past in sandpacks with styrene monomer as the oleic phase before solidification. The results of this study, although released, have not been made available through publication to the research community at large. A previous study in which styrene polymerization was used has also been cited but is not available. A technique for the study of residual oil structures that involved trapping of melted wax has been used by Morrow and Humphrey. Since Reed and Healy credit the method used by Humphrey to Taber's much earlier unpublished work, it is clear that blobs prepared by this technique have been examined by several investigators. Also, scanning electron micrographs (SEM's) of pore casts of blobs of residual nonwetting phase obtained through solidification of Wood's metal with hot toluene as the wetting phase have been presented by Swanson. Although considerable attention has been paid to the obviously important subject of residual oil structure, the amount of experimentally determined, quantitative information on blob structure and the statistics of blob populations is very limited. To obtain such information, satisfactory techniques for preparing statistically representative blob samples and measuring their size distributions must be devised. Once obtained, the experimentally determined blob-size distributions can be related to measured conditions for mobilization and compared with changes in size distribution predicted by theory.