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Heavy and Thermal Oil Recovery Research

Fractured and Low Permeability Porous Media

The CAT Scanner allows horizontal and vertical imaging of cores.

A common thread running through the experimental portion of this work is the use of the SUPRI X-ray computed tomography (CT) scanner to image oil, water, and gas saturation distributions. Thus, we obtain the position and shapes of displacement fronts in porous media. The CT equipment developed at Stanford is unique in that it allows both multi-energy scans for imaging of three phases and vertical or horizontal positioning.

Of substantial interest is the pore structure and flow characteristics of diatomite, a low permeability (0.1 to 10 mD) and high porosity (0.4 to 0.7) siliceous shale. We desire a better understanding of flow in the matrix of this rock and matrix to fracture transfer of fluids. Flow studies are naturally divided into experiments that probe behavior in the matrix, flow in fractures, and matrix/fracture interactions. At Stanford, we have designed and tested apparatus that allows visualization of displacement patterns and macroscopic fluid flow pathways in unfractured and fractured low-permeability media. The entire length of a core (up to 14 in) is imaged simultaneously with negligible X-ray CT artifacts. Experiments can be conducted under (i) free imbibition, (ii) forced imbibition, (iii) controlled imbibition, and (iv) counter-current imbibition conditions. Controlled imbibition is possible using high-pressure syringe pumps that can control either injection pressure or injection rate. To establish the similarity in flow mechanisms between diatomite and other rocks, we study simultaneously sandstone and to a lesser extent chalk.

A core with pressure gauges being scanned to track saturation fronts in real time.

Experiments are being conducted under flow and initial fluid saturation conditions that are relevant to the field and also under quasi-static conditions. Ideally, reservoir rock samples with varying wettability will be characterized with respect to pore size, shape and frequency; then rock from the same or a similar sample will be subjected to imbibition/displacement tests. This work is necessary to further develop our understanding of oil/water location and transport in diatomite given different dynamic and quasi-static conditions. In particular, we will have measured the recovery efficiency and residual oil saturation under different dynamic conditions of imbibition. Once a sufficient base of spontaneous imbibition data is collected, we will test a semi-analytical technique for obtaining relative permeability and capillary pressure from imbibition experiments.

A micromodel used in imbibition studies.

Matrix to fracture interactions of porous media are much studied, but not well understood. In companion work, we study imbibition and oil production mechanisms to better understand matrix/fracture transport in fractured porous media. We use both the CT scanner with cores of prescribed fracture geometry and etched-silicon micromodels that allow direct visualization of flow processes under a microscope. Micromodels will be redesigned so that both countercurrent and cocurrent imbibition can be visualized. Hence, we will observe directly oil production from the matrix into the fracture and water imbibition into the matrix. Water flow rate through the fracture will also be varied to simulate fractures that are nearer or farther from a well. Companion experiments with similar matrix-fracture geometry will be conducted and monitored in the CT scanner. When complete, we will have a much clearer understanding of how factors such as flow rate in the fracture, fractional flow of water in the fracture, and fracture geometry affect matrix to fracture transfer of oil. We intend to develop more accurate formulations for the corresponding terms used in reservoir simulation.

References

Akin, S., Castanier, L. M. and Brigham, W. E. (1999). "Effect of Temperature on Heavy Oil/Water Relative Permeabilities." SPE 54120, 1999 SPE International Thermal Operations Symposium, Bakersfield, CA, 17-19 Mar. 1999.

Hornbrook, J. W., Castanier, L. M. and Pettit, P. A. (1991). "Observation of Foam/Oil Interactions in a New, HIgh-Resolution Micromodel." SPE 22631, 66th Annual Technical Conference and Exhibition of the Society of Petroleum Engineers, Dallas, TX, October 6-9, 1991.

Kovscek, A. R., Patzek, T. W. and Radke, C. J. (1995). "A Mechanistic Population Balance Model for Transient and Steady-State Foam Flow in Boise Sandstone." Chemical Engineering Science 50(23): 3783-3799.

Kovscek, A. R., Johnston, R. M. and Patzek, T. W. (1997). "Evaluation of Rock/Fracture Interactions During Steam Injection Through Vertical Hydraulic Fractures." SPE Production and Facilities May: 100-105.

Kumar, M. and Beatty, F. D. (1995). "Cyclic Steaming in Heavy Oil Diatomite." SPE 29623, SPE 65th Western Regional Meeting, Bakersfield, CA, March 8-10, 1995.

Maini, B. B., Sarma, H. K. and George, A. E. (1993). "Significance of Foamy-Oil Behaviour in Primary Production of Heavy Oils." Jour. of Canadian Pet. Tech. 32(9): 50-54.

Schembre, J. M., Akin, S., Castanier, L. M. and Kovscek, A. R. (1998). "Spontaneous Water Imbibition into Diatomite." SPE 46211, 1998 Western Regional Meeting of the Society of Petroleum Engineers, Bakersfield, CA, May 10-13, 1998 SPE.

Smith, G. E. (1988). "Fluid Flow and Sand Production in Heavy-Oil Reservoirs Under Solution-Gas Drive." SPE Production Engineering May: 169-180.

Energy Resources Engineering Department, School of Earth Sciences, Stanford, CA
kovscek@stanford.edu

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