Oil & Gas - Maximising Recovery Programme (formerly SHARP) IOR Views

Introduction

Approximately one half of the world’s hydrocarbon reserves are contained in fractured reservoirs, with large deposits in the North Sea and the Middle East.  However, often 80 – 95% of the oil in place is left underground, since most of the oil is retained in relatively low permeability rock while fluid flow is confined to the fractures.  There is huge uncertainty associated with designing improved oil recovery schemes in fractured reservoirs for two principal reasons:  first the geological description of the fracture patterns is highly uncertain; and second the basic physical processes and their macroscopic description, particularly when they involve multiphase flow, are still not well established.

In Phase 1 of the research, Professor Blunt and Dr. Stephan Matthäi developed novel simulation software to model flow in fractured reservoirs. Dr. Matthäi’s CSP-based discrete fracture simulator represents the state-of-the art, incorporating higher-order accurate yet unconditionally stable transport algorithms for unstructured volumetric hybrid element meshes of fractures and matrix (1-4).  Professor Blunt’s field-scale streamline dual porosity simulator has demonstrated its accuracy and efficiency compared to conventional grid-based codes and has been applied to synthetic and real field examples (5-8). 

Phase 1 also demonstrated a successful transfer of ideas from research to practical application.  The streamline-based methodology was commercially implemented (9) and has been applied by other groups (10-11).

Figure 1: A 2,000 fracture model (left, generated with the FRED software, Golder Associates Inc., Seattle, USA) showing the fracture aperture distribution and the model just before water breakthrough (right); the green saturation iso-surface represents the height of the Buckley-Leverett shock (0.6). White contour lines enhance gray translucent fluid pressure iso-surfaces on the sides and at the top of the model. From (5).

Phase 2 - Upscaling

It is still not clear, however, how to upscale fracture flow: that is to find the relevant approach to describe multiphase flow at the large scale (km) – because of the complex interactions between fractures and variable rock types, the behaviour is much richer than at the small (cm) scale – and to relate the parameters in the transport equations to laboratory and field measurements.  Furthermore, the field-scale demonstration of the software has thus far been limited.

To illustrate the problem, Figure 1 shows a discrete fracture simulation using CSP.  The average behaviour is complex and requires a richer description than available in current dual porosity/dual permeability simulators.  One way to approach this is to divide a discrete fracture model conceptually into blocks (Figure 2) – representing simulation cells in a full field model – and compute the average transmissivity, mobility, fractional flow and fracture/matrix transfer that give the same overall response.

To guide this effort we will develop transfer functions for different displacement processes based on approximate analytical solutions to the flow equations.  We will presume that the functional forms of the transfer are similar for complex fracture geometries but with different physical coefficients.  These transfer functions will then be input into full-field simulation – Figure 3.

The overall aim of the research is a fracture-to-field methodology for designing optimal recovery strategies.

Figure 2:  Example of a partitioned model with 30 discrete fractures.  The subdivision into grid blocks permits a direct comparison of results obtained from the unstructured hybrid finite-element model (as shown) and its regular gridded equivalent with effective media properties and computed using a conventional reservoir simulator.

Figure 3:  Horizontal slices through a full-field simulation of a Chinese oil field showing the fracture (left) and matrix (right) saturations after 1,800 days of waterflooding.  The top figure (a) shows the single-rate model, the middle figure (b) the three-rate model and the bottom figure (c) a water-wet case.  From (9)

We are currently looking for further sponsors. For more information contact: Professor Martin Blunt, Department of Earth Science and Engineering, Imperial College London, SW7 2AZ, UK.  Tel: 00 44 20 7594 6500; email m.blunt@imperial.ac.uk or visit www.imperial.ac.uk/ese.

References

1. Matthäi, S. K., Aydin, A., Pollard, D. D. and Roberts S. G.: “Simulation of transient well-test signatures for geologically realistic faults in sandstone reservoirs,” SPEJ (1998) 3(1), 62-76.
2. Matthäi, S. K. and Belayneh, M., “Fluid flow partitioning between fractures and a permeable rock matrix,” Geophysical Research Letters, 31:, 7602-6 (2004).
3. Matthäi, S. K., “Fluid flow and (reactive) transport in fractured and faulted rock.” J. Geochem. Expl., 78-79, 179-82 (2003).
4. Matthäi, S. K., Mezentsev, A and Belayneh, M.: “Two-Phase Flow Experiments with Fractured Rock Represented by Unstructured 3D Hybrid Control-Volume Finite-Element (CVFE) Meshes,” SPE 93341, proceedings of the SPE Reservoir Simulation Symposium, Houston Texas, 31 January – 2 February (2005).
5. Di Donato, G., Huang, W. and Blunt, M. J.: “Streamline-Based Dual Porosity Simulation of Fractured Reservoirs,” SPE 84036, proceedings of the SPE Annual Meeting, Denver, Colorado, 5-8 October (2003).
6. Di Donato, G. and Blunt, M. J.: “Streamline-based dual-porosity simulation of reactive transport and flow in fractured reservoirs,” Water Resources Research, 40, W04203, doi:10.1029/2003WR002772 (2004).
7. Huang, W., Di Donato, G. and Blunt, M. J.: “Comparison of streamline-based and grid-based dual porosity simulation,” Journal of Petroleum Science and Engineering, 43, 129-137 (2004).
8. G Di Donato, H Lu, Z Tavassoli and M J Blunt, “Multi-rate transfer dual porosity modeling of gravity drainage and imbibition,” SPE 93144, proceedings of the SPE Reservoir Simulation Symposium, Houston, Texas U.S.A., 31 January 2005 – 2 February (2005).
9. Thiele, M. R., Batycky R. P., Iding, M. and Blunt, M. J.: “Extension of Streamline-Based Dual Porosity Flow Simulation to Realistic Geology,” proceedings of the ninth European Conference on the Mathematics of Oil Recovery, Cannes, France, September (2004).
10. Al-Huthali, A. and Datta-Gupta, A.: “Streamline simulation of counter-current imbibition in naturally fractured reservoirs,” Journal of Petroleum Science and Engineering 43(3-4) 271-300 (2004).
11. Moreno, J., Kazemi, H. and Gilman, J. R.: “Streamline Simulation of Counter-Current Water-Oil and Gas-Oil Flow in Naturally Fractured Dual-Porosity Reservoirs,” SPE 89880, proceedings of the SPE Annual Meeting, Houston, Texas, September 26 – 29, (2004).