Physics of Oil Entrapment in Water-Wet Rock
- K.K. Mohanty (U. of Minnesota) | H.T. Davis (U. of Minnesota) | L.E. Scriven (U. of Minnesota)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Engineering
- Publication Date
- February 1987
- Document Type
- Journal Paper
- 113 - 128
- 1987. Society of Petroleum Engineers
- 5.4.7 Chemical Flooding Methods (e.g., Polymer, Solvent, Nitrogen, Immiscible CO2, Surfactant, Vapex), 5.2.1 Phase Behavior and PVT Measurements, 4.1.2 Separation and Treating, 5.4.1 Waterflooding, 5.7.2 Recovery Factors, 5.3.1 Flow in Porous Media, 5.1.4 Petrology, 5.3.4 Reduction of Residual Oil Saturation, 5.1 Reservoir Characterisation, 5.2 Reservoir Fluid Dynamics, 4.1.5 Processing Equipment, 4.3.4 Scale
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Displacement of oil from an initially oil-filled porous rock by water consists of advancement of menisci and rupture of oil connections. In displacements controlled by capillarity, which are typical of oil reservoir floods, these pore-level events are governed by the local pore geometry, pore topology, and fluid properties, but the pressure field initiates these pore-level events and integrates them with the externally imposed Darcy flow.
This paper reports the physics of the pore-level events and their integration on a computationally simple model of rock: a square network of pores. The novelty of the approach lies in keeping track of the evolution of the displacement front and in constructing an approximation of the entire pressure field that carries the information essential for predicting the evolution.
The result gives insight into the state of the residual oil saturation and its dependence on pore geometry and the capillary number, Nca, of displacement. As the capillary number increases, the residual oil saturation decreases and the residual oil blobs tend to be smaller. As the pore size distribution becomes wider, the decrease of residual oil saturation with capillary number becomes smoother.
Displacement of oil from water-wet porous sedimentary rock by waterflooding usually leads to entrapment of a considerable fraction of oil. The processes involved are controlled by the interplay of capillary and viscous forces1 and, in some cases, buoyancy and inertial forces.
Understanding entrapment phenomena and displacement mechanisms is important for designing EOR techniques. If, as in some important processes, entrapped oil blobs are mobilized by lowering interfacial tension (IFT), then the length distribution of blobs is a key factor in determining the recovery efficiency.2,3 It is desirable to know how this distribution is created and to develop a theoretical basis for correlating the distribution with the morphology of the reservoir rock and the conditions of waterflooding. For example, knowledge of the displacement mechanism permits one to predict the integrity of the trailing edge of an oil bank.
Understanding of the mechanisms of two-phase flow in porous media has progressed considerably in the last few decades. Studies in this area can be regarded as either macroscopic or microscopic. In macroscopic studies, averaged quantities, such as relative permeabilities and residual saturations, are. measured in samples that are large compared with pore scale. When correlated with average flow parameters, such as capillary number, the measurements can be interpolated and, with greater risk, extrapolated. In microscopic studies, pore-level events in model systems are visualized and measured to understand why an oil phase becomes disconnected into ganglia or blobs,4-7 how oil/water menisci move,8,9 why ganglia or blobs remain trapped,1,2,10 and what is required to mobilize them.1,2,11 The results have guided the design of macroscopic studies and the choice of correlating parameters. What has been lacking is a means of directly linking important macroscopic quantities to experiments at the microscopic level.
Recently, pore-level physics has been combined with percolation theory12,13 and population balances14 to predict macroscopic quantities for tertiary oil recovery. The need for a statistical physics of flow in porous media is reflected by Fatt's15 pioneering attempts at network modeling. Recently, highly simplified pictures of pore-level physics have been combined with a stochastic approach called percolation theory13 and another known as population balance equations14 in attempts to correlate better the macroscopic quantities needed to interpret corefloods and to design EOR processes. This paper addresses the issues of more accurately modeling the pore-level events and the chaotic nature of rock and relating them to displacement on a macroscopic scale.
For example, unanswered questions remain about the mechanism of oil disconnection. The disconnection process, as surmised by Reed and Healy,16 should be strongly influenced by the local pressure field near a flood front, which is unsteady even in a steady displacement process. To date, no attempt has been made to estimate that field or its effect. The increased relative permeability near the flood front estimated by Morrow and Boonraum7 for gas displacement is an important quantity but does not appear to be adequate for determination of oil-disconnection events.
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