Capillary Entrapment Caused by Small-Scale Wettability Heterogeneities
- P.P. van Lingen (Delft U. of Technology, Centre for Technical Geoscience) | J. Bruining (Delft U. of Technology, Centre for Technical Geoscience) | C.P.J.W. van Kruijsdijk (Delft U. of Technology, Centre for Technical Geoscience)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Engineering
- Publication Date
- May 1996
- Document Type
- Journal Paper
- 93 - 100
- 1996. Society of Petroleum Engineers
- 5.2.1 Phase Behavior and PVT Measurements, 5.4.7 Chemical Flooding Methods (e.g., Polymer, Solvent, Nitrogen, Immiscible CO2, Surfactant, Vapex), 4.3.4 Scale, 5.3.4 Reduction of Residual Oil Saturation, 4.3.3 Aspaltenes, 1.2.3 Rock properties, 5.4.1 Waterflooding
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Small-scale heterogeneities are abundantly present in oil reservoirs. It is well known that these heterogeneities can reduce the displacement efficiency of water flooding due to capillary entrapment of oil. A typical heterogeneous facies is the laminated cross-bed set with alternating high and low permeable laminae. Apart from this heterogeneity in grain-size, the wettability of the laminae can also differ. The laminae can contain different minerals with different wetting characteristics, and wettability can be affected by adsorption of hydrocarbons to the grain surface.
In this paper we focus on the effects of wettability on the entrapment. We measured capillary pressure curves for porous media with various wetting characteristics. We describe a procedure which allows the reader to quantify the trapped oil saturation depending on absolute permeability, relative permeability and capillary pressure curves of the foreset laminae, based on a simplified cross-bed geometry. The example calculations show the sensitivity of various parameters with respect to the magnitude of trapping.
The importance of capillary entrapment in small-scale heterogeneities during water flooding and EOR projects is generally recognized.
A typical heterogeneous facies is the cross-bedded sandstone (Fig. 1a), which is associated with fluvial and deltaic deposits. Cross-bedded reservoirs can be very fine- to coarse-grained and are generally moderately well sorted.
Cross-bed sets consist of foresets and bottomsets (Fig. 1b). The foresets consist of alternating layers of fine-grained and coarse-grained material with sharp boundaries. The bottomset consists of fine-grained material comparable to the fine grained foreset laminae. Typically, the average grain size of the fine-grained and coarse-grained laminae differs a factor 1.5. The coarse-grained laminae are generally more poorly sorted. Tables 1 and 2 give typical length scales of cross-bed sets, which were obtained from outcrop studies at a fluvial deposit in Spain.
Oil displacement from cross-bed sets has received broad attention. Ringrose et al. studied the effects of cross-layer flow and along-layer flow by numerical simulation. They determined oil-pseudo relative permeability curves based on simulation results. Pickup et al. introduced tensorial effects for one phase and two phase flow. The papers show the complexity involved in the quantification of trapped oil in cross-bedded structures.
In this paper we focus on the effects of heterogeneities in wettability and pore-structure on the residual oil saturation in cross-bedded reservoirs. To this end we studied the effects of wettability on the shape of the capillary pressure curve. Subsequently we derived simple analytical formulae relating the residual oil saturation to the dimensionless capillary number. These formulae clearly demonstrate the effects of sorting, wettability and scale. Finally, we validated the results through scaled laboratory experiments.
Although the formulae were derived for simplified geometrical models, they allow the reader to obtain a quick estimate of the importance of trapping under specific field conditions.
Wettability in cross-bedded reservoirs
Apart from the heterogeneity in pore geometry, cross-bed sets may also show variations in the wettability of the laminae. One of the causes for wettability heterogeneities can be variations in the mineralogy of the different laminae. Hartkamp-Bakker reports variations with respect to the amount of quartz, kaolinite and anhydrite in cross-bedded fluvial sandstones. Robin et al. found quartz, feldspar, and illite to remain preferentially water-wet while kaolinite booklets were found to be oil-wet. They also reported that particle size can influence wettability.
Wettability in cross-bed sets can also be influenced by adsorption of hydrocarbon fractions (asphaltenes & resins) to the rock matrix.
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