Microfluidics for Carbonate Rock IOR: Some Lessons From Fabrication, Operation and Image Analysis

After successful implementation of Lab-on-a-chip technology in chemical and biomedical applications, the field of petroleum engineering is currently developing microfluidics as a platform to complement traditional core flooding experiments. Potentially microfluidics can offer a fast, efficient and low-cost method to screen many variables like flooding brine composition, reservoir temperature and aging history. We addressed this potential for the waterflooding of carbonate rocks. Using thermostated glass micro- models with rock-inspired 'dual depth' pore geometries as a basis, we explored i) introduction of calcite chemistry into the chip, ii) monitoring of changes in fluid composition and iii) quantification the residual oil from visualization. The development aspects of these efforts are the scope of the present paper. Glass microchannels were functionalized by firmly attaching calcite nanoparticles to the inner surfaces and expanding them via seeded growth. Optical microcopy allowed to monitor the calcite distribution if the particles were not covered with crude oil (CRO). The calcite coating density could be steered, but also showed variability within the chip and between chips. Measurement of residual oil saturation (ROS) from optical microscopy is more challenging in the presence of calcite particles, due to their darker appearance. However, comparing the dynamic intensity of individual pixels with those in CRO- and brine-filled chips, allowed us to not only accommodate the calcite-induced optical differences, but also to find a correlation between calcite coverage and ROS. Our developed data analysis scheme, based on mask overlaying and image registration, also allows a sensitive monitoring of ROS, for both short (30 s) and long times (8 days) using low (2X) magnification Additionally captured high-resolution (10X) images reveal vital details underlying the global oil displacement. Implementation of more on-chip characterization tools could significantly contribute to a better mechanistic understanding of the IOR process. Non-invasive add-ons like Raman microscopy or chemically inert dyes could be implemented directly. Incorporation of electrode sensors will only be efficient if the chips can be re-used, which is currently not the case. This paper uses results from published works and unpublished data to reflect on the development of the microfluidics-for-testing IOR technologies. Our identification of possibilities and technical challenges in IOR on calcite-coated chips should be helpful in future designs of microfluidic research studies.