PhD position – Measurement of Streaming Potential in Porous Media

IntroductionOne of the important research results during the last decades has been the deciphering of the fact that the wetting state of the rock is not a static parameter, and that the wetting state can be changed by the injection water. By changing the ionic composition of the injection water in field test and core flooding the oil recovery can be increased significantly. The research community seems to agree to the notion of a connection between increased recovery and changes in the wetting state of the rock. But exactly how and why this leads to increased recovery are still open questions, but of enormous significance, as only economic favorable processes for EOR are implemented by the industry. When water is used during secondary recovery for pressure support, the injection water generally have a different composition and temperature than the formation water present in the reservoir. And therefore the injected water will not in general be in equilibrium with the rock. Injection of disequilibrium water into a reservoir may lead to dissolution/precipitation, changes in surface charge and potential. All these changes may lead to wettability alteration and additional oil produced.

Thus in order to optimize the water chemistry for EOR one strategy would be to quantify the degree of chemical alteration of the rock induced by a specific brine, and then correlate this alteration to oil recovery. During the last year we have through several Petromaks and COREC projects gained an improved understanding of the various dissolution and precipitation processes taking place as seawater-like brines are injected into chalk cores at reservoir temperature. These investigations have mainly been made by measuring any changes of the ionic composition in a core flood, combined with XRD and SEM investigation of the cores before and after any flooding. However, a change in surface charge and potential is much harder to monitor. By using crushed core material, the Zeta potential can be measured in different brines by standard techniques (e.g. Acoustosizer). This technique is, however, limited to low temperature (max. 50°C), which makes the uses in this context rather limited. The Zeta potential is an interfacial property that is of great importance for understanding the behavior of solids in more detail. It gives insight into the charge and adsorption characteristics of solid surfaces. There is thus a need for a technique which can determine any changes in surface properties of core materials at realistic reservoir conditions. Streaming potential measurement is thus a powerful technique for characterizing and monitoring the degree of any modifications or changes of the solid surface. This technique, which has been known for quite some time, has been used for tracking subsurface flow (Revil, Pezard et al. 1999, Revil, Schwaeger et al. 1999), sandstone cores (Vinogradov, Jaafar et al. 2010) and carbonate cores (Jackson and Vinogradov 2012). It has also been suggested that the streaming potential could be used down hole to monitor waterfront movement during petroleum production (Jackson, Gulamali et al. 2012).

In this proposal we therefore suggest to establish new lab routines to measure the streaming potential in sandstone and chalk cores at realistic reservoir conditions and with different brines. Any changes in streaming potential for different brine compositions will be correlated to different levels in oil recovery produced by these brines. The measured streaming potentials will be analyzed by use of the established techniques described in (Hiorth, Cathles et al. 2010).

Primary ObjectiveObtain in situ measurements of the streaming potential of cores at realistic reservoir conditions and relate these changes to oil recovery.

Research Tasks1. Measure the streaming potential for varying single phase fluids at ambient and reservoir

temperature, with the set up shown in the next section. We will start with “simple” brines, NaCl, MgCl2, and Na2SO4, before we introduce seawater-like brines.

2. Model results by the use of surface complexation theory (Hiorth, Cathles et al. 2010).3. Compare streaming potential measurements with published data showing the effect of brine

composition on recovery. Make correlations of how changes to the imposed experimental control parameters (temperature, and brine composition) relate to the changes in the observed variables (oil recovery, and zeta potential).

Experimental set upThe schematic diagram of the experimental setup is shown in Figure 1. The brine saturated core plug is held in a stainless steel core holder with a prescribed confining pressure. The plug is electrically isolated from the core holder by two plastic end pieces. In addition, the tubing and connections are made of non-conductive material. Two AgCl electrodes are positioned in the inlet and outlet line in order to eliminate electrode flow effects. The design of these electrodes is shown in Figure 2. This setup provides an earthed Faraday enclosure to eliminate spurious external electrical currents. To measure the streaming potential by varying imposed hydraulic pressure differences, a brine flooding system is developed which is also isolated from any electric conductive material. The inlet cylinder and outlet cylinder, in Figure 1 are made of glass and containing the injected brine and oil (Isopar H). Isopar-H oil is flooded using a Quizix pump to maintain constant flow rate/ fluid pressure. A data acquisition system is established where the pressure drop and voltage drop (streaming potential) across the core plug are recorded simultaneously. The laboratory setup is shown in Figure 3.

Figure 1: Experimental setup for measuring streaming potential

Figure 2: Schematic diagram of the electrode which is in electrical contact with the flowing brine via a low permeable porous disc.

Figure 3: Experimental setup in the Lab

Timeline and Milestones2013 2014 2015M1 M2 M3M1: Establish lab procedures and measure (reproducible) streaming potential on sandstone and chalk cores at ambient conditions. M2: Measure the streaming potential at realistic reservoir conditions. M3: Cores at various wetting conditions. Interpretation and PhD thesis completed.

PersonnelThe research will be carried out as a PhD project. Aksel Hiorth will be the main supervisor and Merete V. Madland will act as co-supervisor. Svein Myhren (UiS) and Fred Bratteli (IRIS) will assist with establishment of lab procedures, the measurements and interpretation.

References

Hiorth, A., L. M. Cathles and M. V. Madland (2010). "The impact of pore water chemistry on carbonate surface charge and oil wettability." Transport in Porous Media 85(1): 1-21.Jackson, M. D., M. Y. Gulamali, E. Leinov, J. H. Saunders and J. Vinogradov (2012). "Spontaneous Potentials in Hydrocarbon Reservoirs During Waterflooding: Application to Water-Front Monitoring." Spe Journal 17(1): 53-69.Jackson, M. D. and J. Vinogradov (2012). "Impact of wettability on laboratory measurements of streaming potential in carbonates." Colloids and Surfaces a-Physicochemical and Engineering Aspects 393: 86-95.

Revil, A., P. A. Pezard and P. W. J. Glover (1999). "Streaming potential in porous media 1. Theory of the zeta potential." J. Geophys. Res. 104(B9): 21-31.Revil, A., H. Schwaeger, L. M. Cathles and P. D. Manhardt (1999). "Streaming potential in porous media 2. Theory and application to geothermal systems." Journal of Geophysical Research-Solid Earth 104(B9): 20033-20048.Vinogradov, J., M. Z. Jaafar and M. D. Jackson (2010). "Measurement of streaming potential coupling coefficient in sandstones saturated with natural and artificial brines at high salinity." Journal of Geophysical Research-Solid Earth 115.