NUCLEATION AND CEMENTATION EXPERIMENTS IN SEE-THROUGH POROUS ROCK ANALOGUES

Nucleation and cementation processes in reservoir rocks are of foremost interest for reservoir prediction especially in tight- and unconventional reservoirs. We developed a transparent see-through microreactor for nucleation and cementation experiments under the microscope. It allows image sequences and high resolution analyses in a controlled system of a porous rock analogue. Varying grain sizes and materials served as input for different pore sizes and surface charges, respectively.

The first experimental series focuses on diffusion-controlled cementation, using potash alum, halite and sodium nitrate as precipitating substances. Halite plugging is well known from the Bunter reservoir rocks in Europe, overlying the Zechstein evaporites. Potash alum and sodium nitrate are used as analogues for cements such as calcite. Material transport is driven by a temperature gradient from a reservoir into a small 0.5 mm thin rock volume. All materials show a pore size dependent nucleation from the fluid phase, although different for the substances. While halite nucleates in large pores, alum prefers small pores as nucleation and cementation sites. Fluid and solid properties such as pH and surface charge also have an effect on potential nucleation and cementation sites. Results are compared to our second experimental series on advection-controlled cementation, using potash alum and sodium nitrate. Here, a supersaturated solution is pumped at different flow rates into a small microfracture placed in a transparent microreactor. Crystals preferentially grow against the transport direction, a process not yet described in natural cemented fractures.