Published by the DTI Oil & Gas Directorate for the reservoir engineering and IOR community in the UK.
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Three-Phase Flow in WAG Processes: Pore Scale Modelling and Comparison with Micromodel Experiments

Contact: Prof. Dabir Tehrani (dabir.tehrani@pet.hw.ac.uk)

One of the most efficient IOR methods is through the application of the "Water Alternating Gas Injection" (WAG) process. The injection of water followed by gas causes a significant redistribution of the fluids in the reservoir resulting in considerable increase in oil recovery. Oil which otherwise would not be mobile, under either gas injection or water injection alone, is mobilised and may be produced. Many laboratory experiments, simulating the WAG process, have been conducted on cores and a number of field trials have been performed. Numerous analytical and numerical simulations of the process have also been carried out. However, a number of aspects of the underlying mechanisms, which operate during the WAG process are still not clearly understood. Furthermore, the form of the macroscopic parameters required to model the WAG process - principally the relative permeabilities and capillary pressures - are often unknown.

To obtain an improved understanding of the processes and develop tools that can be used for accurate prediction of additional oil recovery using WAG, a project was initiated at Heriot-Watt University in December 1997. In this Phase 1 (completed May 2001), micromodels of different wettability were used to study the basic physics of the WAG displacements. The results from these experimental observations were incorporated into mathematical network models, which were used to generalise the experimental results to more realistic reservoir rock systems. In Phase 1 of the study, the experiments were performed only at one value of pressure, hence one set of interfacial tension (IFT) values. It is known that the fluid IFTs play a central role in the mechanics of three-phase flow through porous media. Recoveries are expected to be higher when the gas/oil IFT is near zero (near miscible WAG). A Phase 2 of this research consortium started in June 2001 to study these further sensitivities again using a combination of micromodel observations and pore-scale network modelling.

An example of a direct comparison between an observed micromodel three-phase fluid distribution and the corresponding network model calculation is shown in Figure 1 for (a) a second gas flood and (b) a fifth gas flood in a WAG cycle. Note the extensive redistribution of fluids in the higher gas cycle as referred to above. The research has produced a number of advances in our understanding of three-phase flow and a list of downloadable papers in the public domain can be found on http://www.pet.hw.ac.uk/research/wag/publs.html.

Fig. 1 - Simulated and experimental micromodel fluid configurations after a. second gas flood, b. fifth gas flood. Key: Calculation, blue = water, red = oil, white = gas: Experiment, blue = water, red = oil, yellow = gas. (Click for larger image)

The current three-year project (WAG2) is supported by: BP Exploration Operating Company, BHP Billiton Ltd, Norsk Hydro A.S.A., PDVSA, Total-Fina-Elf, Shell UK Exploration & Production, Statoil and the UK DTI. The Heriot-Watt research team is: Experimental micromodels - Ali Danesh, Dabir Tehrani, Mehran Sohrabi and Graeme Henderson; Pore-scale modelling - Rink van Dijke, Ken Sorbie and Steve McDougall. For further information, please contact Professor Dabir Tehrani.

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