Theoretical comparison of two setups for capillary pressure measurement by centrifuge.

There are several approaches for the calculation of capillary pressure curves in porous media including the centrifuge method. In this work, a new installation of centrifuge test is introduced and compared with the traditional setup. In the first setup, which is a standard approach in labs, the core face closest to the rotational axis is open to the non-wetting phase, while the farthest face is open to the wetting phase where strictly co-current flow is generated in rotations; labeled Two-Ends-Open (TEO). In the second setup, which is proposed as a new approach, only the outer radius surface is open and is exposed to the light non-wetting phase; labeled One-End-Open (OEO). This setup strictly induces counter-current flow. The two systems and their corresponding boundary conditions are formulated mathematically and solved by a fully implicit numerical solver. The TEO setup is validated by comparison with commercial software. Experimental data from the literature are used to parameterize the models. It is mathematically, and with examples, demonstrated that the same equilibrium is obtained in both systems with the same rotational speed, and changing the installation does not influence the measured capillary pressure. This equilibrium state is only dependent on the rotational speed, rock capillary pressure properties, and fluid densities, not the installation geometry, relative permeabilities, or fluid viscosities. However, the dynamic transition trend and saturation profiles were found to be dependent on the applied installation. It was observed that the OEO setup takes almost identical equilibration time as the TEO setup for mixed-wet states, although it needed much longer time in water-wet states. The presence of threshold capillary pressure significantly increased the time scale of the OEO setup. Also, it was found that in contradiction to the TEO setup, the dynamic saturation profile in OEO was rarely influenced by viscosity ratio. To conclude, the performed history matching analysis demonstrated that the OEO setup can be applied for the calculation of counter-current relative permeability from the production data with reasonable accuracy.

Permeability Evolution of Shear Failing Chalk Cores under Thermochemical Influence.

Development of petroleum reservoirs, including primary depletion of the pore pressure and repressurization during water injection naturally, leads to changes in effective stresses of the formations. These changes impose mechanical deformation of the rock mass with subsequent altering of its petrophysical properties. Besides mechanical compaction, chalk reservoirs on the Norwegian Continental Shelf also seem susceptible to mineralogical and textural changes as an effect of the injecting fluid's chemical composition and temperature. Understanding such chemical and thermal effects and how they interplay with the mechanical response to changes in effective stresses could contribute to improved prediction of permeability development during field life. This article presents results from mechanical testing of chalk cores of medium-porosity (32%) outcrop chalk (Niobrara Formation, Kansas) in triaxial cells. The experimental setup allows systematic combinations of fluctuating deviatoric stress, temperature (50 and 130 °C), and injecting fluid (calcite-equilibrated sodium chloride, calcite-equilibrated sodium sulfate, and reactive synthetic seawater) intended to replicate in situ processes, relevant to the North Sea chalk reservoirs. Deviatoric loading above yield resulted in a shear failure with a steeply dipping fracture of the core and a simultaneous increase in permeability. This occurred regardless of the brine composition. The second and third deviatoric loadings above yield did not have the same strong effect on permeability. During creep and unloading, the permeability changes were minor such that the end permeability remained higher than the initial values. However, sodium sulfate-injected cores retained most of the permeability gain after shear fracturing compared to sodium chloride and synthetic seawater series at both temperatures. Synthetic seawater-injected cores registered the most permeability loss compared to the other brines at 130 °C. The results indicate that repulsive forces generated by sulfate adsorption contribute to maintain the fracture permeability.

Metal on metal oxide nanowire Co-catalyzed Si photocathode for solar water splitting.

We report a systematic study of Si|ZnO and Si|ZnO| metal photocathodes for effective photoelectrochemical cells and hydrogen generation. Both ZnO nanocrystalline thin films and vertical nanowire arrays were studied. Si|ZnO electrodes showed increased cathodic photocurrents due to improved charge separation by the formation of a p/n junction, and Si|ZnO:Al (n(+)-ZnO) and Si|ZnO(N(2)) (thin films prepared in N(2)/Ar gas) lead to a further increase in cathodic photocurrents. Si|ZnONW (nanowire array) photocathodes dramatically increased the photocurrents and thus photoelectrochemical conversion efficiency due to the enhanced light absorption and enlarged surface area. The ZnO film thickness and ZnO nanowire length were important to the enhancements. A thin metal coating on ZnO showed increased photocurrent due to a catalyzed hydrogen evolution reaction and Ni metal showed comparable catalytic activities to those of Pt and Pd. Moreover, photoelectrochemical instability of Si|ZnO electrodes was minimized by metal co-catalysts. Our results indicate that the metal and ZnO on p-type Si serve as co-catalysts for photoelectrochemical water splitting, which can provide a possible low-cost and scalable method to fabricate high efficiency photocathodes for practical applications in clean solar energy harvesting.