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GEOHORIZONS |
1 Department of Earth Science and Engineering, Imperial College London, United Kingdom; m.belayneh{at}imperial.ac.uk
2 Department of Earth Sciences, Swiss Federal Institute of Technology, Zürich, Switzerland; present address: Institute of Petroleum Engineering, Heriot-Watt University, Edinburgh, United Kingdom
3 Department of Earth Science and Engineering, Imperial College London, United Kingdom
Mandefro Belayneh is a research associate at the Department of Earth Science and Engineering, Imperial College London, where he obtained his M.Sc. degree and his Ph.D. in structural geology. Prior to joining Imperial, he had industrial experience in Ethiopia. His research interests are studying the links between geological stresses, brittle failure, and fluid flow in the Earth's crust and their applications to fractured and faulted reservoirs.
Sebastian Geiger is a geoscience lecturer at the Institute of Petroleum Engineering at Heriot-Watt University and is currently an academic visitor at the Department of Earth Science and Engineering, Imperial College London. He has been a postdoctoral researcher at the Eidgenössische Technische Hochschule (ETH) Zürich where he also received his Ph.D. in 2004. He holds an M.Sc. degree from Oregon State University. His research interests are the use of numerical simulations to study the complex interplay of hydrodynamics and thermodynamics during multiphase flow in geologically complex systems.
Stephan K. Matthäi is the Governor's Lecturer in Earth Science and Engineering at Imperial College London. He received a Ph.D. from the Australian National University and has postdoctoral experience from Cornell University, Stanford University, and the Swiss Federal Institute of Technology (ETH). He leads an industry consortium on enhanced oil recovery from fractured reservoirs and has created the Complex Systems Platform (CSP), an object-oriented numerical simulation application programming interface applicable to discrete fracture modeling.
Water flooding of fractured reservoirs is risky because water breakthrough can occur early, leading to a prohibitively high water cut. In mixed or oil-wet carbonates, capillary drive is negligible or absent. For this scenario, we investigate fluid-pressure-driven displacement of oil by water in two-phase flow numerical models based on naturally fractured limestone beds mapped along the British Channel coast. These reservoir analogs are represented by unstructured finite-element grids with discrete representations of intersecting fractures. We solve the governing equations for slightly compressible two-phase flow with our original control-volume finite-element method. This permits the direct examination of displacement patterns in fractures and rock matrix.
We find that the irreducible saturation in the fractured carbonate is much higher than the value prescribed to the rock matrix. The shape of water invasion fronts is highly sensitive to the viscosity ratio of oil and water. When the Brooks-Corey relative permeability model is applied to the rock matrix at a viscosity ratio of 1, the total mobility, 
t, is low at intermediate saturations. This stabilizes displacement fronts where a girdle of reduced t develops, but this effect disappears as the viscosity ratio increases.
For an idealized model with a water-wet matrix, we have also evaluated the effect of countercurrent capillary-pressure–driven flow across fracture-matrix interfaces. The rate of this countercurrent imbibition scales with the specific fracture surface area and decays exponentially as intermediate saturation zones develop adjacent to the fractures. The resulting reduced t feeds back into the fluid-pressure-driven displacement process.
This article has been cited by other articles:
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  S. K. Matthai and H. M. Nick Upscaling two-phase flow in naturally fractured reservoirs AAPG Bulletin, November 1, 2009; 93(11): 1621 - 1632. [Abstract] [Full Text] [PDF]
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M. W. Belayneh, S. K. Matthai, M. J. Blunt, and S. F. Rogers Comparison of deterministic with stochastic fracture models in water-flooding numerical simulations AAPG Bulletin, November 1, 2009; 93(11): 1633 - 1648. [Abstract] [Full Text] [PDF]
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 S. K. Matthai, S. Geiger, S. G. Roberts, A. Paluszny, M. Belayneh, A. Burri, A. Mezentsev, H. Lu, D. Coumou, T. Driesner, et al. Numerical simulation of multi-phase fluid flow in structurally complex reservoirs Geological Society, London, Special Publications, January 1, 2007; 292(1): 405 - 429. [Abstract] [Full Text] [PDF]
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