Reactive transport and gel formation in two-phase systems and porous media
Reactive transport phenomena occur in a wide variety of scientific and engineering fields. Coupled mass transfer and chemical reactions are found in chemical reactors, biological cells, soils, etc. An interesting example of reactive flow is found in the application of gel systems in porous rocks, in order to modify the fluid flow properties in the rock, with respect to oil or water. High water-cut during oil and gas production is a world-wide problem, especially in maturing oil fields, and leads to a decline in hydrocarbon production and to water disposal problems. Gel treatments can be applied in the near-well bore region to reduce or block the flow of water into the well. Gels have also been considered for their potential use as a barrier to contaminant transport in groundwater.
A novel type of gelant was introduced by Thompson and Fogler,that can be mixed with oil, and reacts upon contact with water to form a gel in the water phase. This gelant, TMOS or Si(OCH3)4, reacts with water as according to the sol-gel principle:
With respect to the application of the gelant in two-phase systems (in bulk or in porous media) the gelant TMOS is initially mixed with oil or a hydrocarbon. Near the oil-water interface the gelant will transfer to the water phase and react with water to form a gel. This process is shown in Figure 1.
The coupled mass transfer and gel reactions were studied in bulk systems containing both oil and water. Two series of experiments were performed. The first one was done using n-hexadecane (for the oil phase) and normal water (for the water phase). The reactive transport was monitored using a 4.7 Tesla NMR scanner. The second series was done using a mineral oil and heavy water (D2O) with or without a buffer. In this series the effect of pH on the reactive transport mechanisms was analyzed. These experiments were done using a 0.95 Tesla NMR scanner equipped with a binuclear rf insert. The hardware was custom made to allow for fast toggling between both components.
2D images were acquired, using the 4.7 Tesla scanner, to obtain a qualitative view on the process and to obtain a measure for the rate of mass transfer that is directly indicated by the shift of the oil-water interface. The images are T1-weighted to yield an adequate contrast between water and oil/TMOS (see Figure 2). From these 2D images the interfacial tension between both phases can be determined by employing a detailed image analysis and optimization procedure. Our analysis showed that the interfacial tension changes during the reactive transport. This is relevant with respect to the two-phase flow processes in porous media.
Fig. 2. NMR images of the
two-phase bulk system (vertical slice of a cylindrical sample),
acquired at the beginning of the experiment (left frame), and after 15 hours (right frame).
The upper phase represents the oil/TMOS phase (initial f = 40 vol%),
and the lower phase represents the (gelled) water phase.
Additionally, in the bulk systems the concentration of the gelant can be monitored by measuring the T1 of the mixture. For this a calibration of the relaxation time T1 for different concentrations was obtained (see Figure 3). In the second series (with the D2O buffers) it was observed that the change in concentration, i.e. the mass transfer, is driven by the hydrolysis reaction, the rate of which is a strong function of pH and temperature. Figure 4 shows the concentration plots for different pH systems.
NMR experiments on reactive transport in porous materialsBentheim sandstone cores were prepared to contain oil and water well-distributed throughout the porous network. At residual water or residual oil conditions a few pore volumes of mixture, consisting of TMOS and oil, were injected in the core within minutes. Then during shut-in the concentration of TMOS in the oleic phase is determined at several positions along the core (see Figure 6) for about 48 hours. Simultaneously, the T2 of the aqueous phase is measured, which indicates the rate of gelation. Preliminary results were obtained from some experiments in which the phases and components were separated on the basis of chemical shift. However, the 1H spectra suffer from a significant line-broadening in the natural rocks, which hinders the separation of the components in the NMR signal.
Effect of gel placement on the permeability
Parallel to the NMR
experiments with the sandstone cores, as described above, the effect of
on the relative permeabilities was determined. This was done by
differential pressure (see Figure 6) over the cores, while injection
water. The results showed that the relative permeability to oil was
to a factor of 3.2. The relative permeability to water was reduced by a
between 1.9 and 27. For each experiment, the relative permeability to
reduced more than that to oil. No clear dependence of the reduction on
parameters (pH, temperature) was observed within the range considered.
permeability reduction is advantageous for the water shut-off treatments.
A series of beam-bending experiments was performed to study the effect of in situ formed gel (under single phase) conditions on the overall permeability of the sandstone. Beam bending (see Figure 8) is an excellent method to measure the permeability in low-permeable media, such as gels, concrete etc.
H.J., L. Pel, H.P. Huinink, P.L.J. Zitha, Mass transfer and gelation in
sandstone cores of a novel water shut-off chemical, conference paper
presented at the 2006 SPE/DOE Symposium on Improved Oil Recovery, held
Tulsa, Oklahoma, USA, 22-26 April 2006.
H.J., H.P. Huinink, L. Pel, P.L.J. Zitha, Analysis of coupled mass
sol-gel reaction in a two phase system, J. of Applied Physics 100,
Castelijns, H.J., H.P. Huinink, P.L.J. Zitha, Characterization of interfacial effects during reactive transport with MRI methods, Colloids & Surfaces A, in press (2007).
Castelijns, H.J., H.P. Huinink, L. Pel, P.L.J. Zitha, The effect of pH on the couples mass transfer and sol-gel reaction in a two-phase system, J. of Physical Chemistry B, accepted (2007).
Castelijns, H.J., G.W. Scherer, L. Pel, P.L.J. Zitha, Permeability reduction in porous materials by in situ formed silica gel, J. of Applied Physics, accepted (2007).