Miscible and immiscible displacement processes have received much attention at pore scale. Several techniques have been implemented to visualise CO2 and carbonated water injection in porous media. Some of these are:
1. Use of X-ray CT and PET for high resolution visualisation of fluid flow in porous media.
2. Materials like glass, acrylic and polycarbonates have been widely used to create micromodels to help study micro displacement. Micromodels help to observe and investigate the motion of fluids and menisci, in terms of microgeometry and physical characteristics of liquids, gases and solids present (Sajadian & Tehrani, 1998).
3. Optical methods based on the use of projection methods like Mach-Zehnder Interferometry are also common. Various dyes and indicators have been used to trace fluid movement in porous media and provide a better understanding of fluid flow during CO2 injection process.
Micromodel studies can help understand the microscopic and macroscopic behaviour of displacement processes and help model flow mechanisms on larger scales. Micromodels can be used to study complex interaction between effects of viscous, capillary and gravity forces with pore geometry and topology.
Micromodels can be used to observe effects of change in pore geometry, density difference, wettability, initial saturation and a combination of variables like capillary number (Ca) and bond number (Nb). Although it has been difficult to isolate individual variables, an interrelationship between them has been illustrated by various studies. Various studies conducted using 2-D models and micromodels for visualisation of fluids and recovery processes in porous media are mentioned below:
• Micromodel studies began as early as in 1952 when Chatenever and Calhoun studied two-phase flow in porous media (Chatenever & Calhoun Jr, 1952). Kimber and Caudle studied the distribution of gas and oil, before and after two-phase flow by use of photographic enlarger. This study helped considerably improve visibility at a microscopic level and overall model view (Kimbler & Caudle, 1957).
• McKellar and Wardlaw introduced the use of mirror glass, with silver and copper bonded to the glass. This technique helped develop a wide variety of patterns including repetitive designs. It also facilitated the development of heterogeneous
CHAPTER 3. CO2 AS A DISPLACING FLUID FOR EOR
mercury porosimetry to examine effects of wettability, heterogeneity and network topology in various network types (McKellar & Wardlaw, 1982).
• Chatzis and Dullien presented a critical analysis of “Pore doublet model” for interpretation of trapping of one phase by another during displacement. Their studies in micromodels showed that imbibition is strongly linked to the flow of wetting phase as bulk fluid films which result in snap-off of the non-wetting phase at pore constrictions (Chatzis & Dullien, 1983).
• Sajadian and Tehrani used homogenous and heterogeneous network micromodels to study gravity drainage at 35 bar (Sajadian & Tehrani, 1998). Dastyari et al. used 2-D glass etched micromodels with and without fractures to compare free and forced gravity drainage (Dastyari, Bashukooh, Shariatpanahi, Haghighi, & Sahimi, 2005).
• Hatiboglu and Babadagli used 2-D models to study co- and counter-current type transfer between matrix and fracture due to diffusion (Hatiboglu & Babadagli, 2005). Sohrabi et al. have studied carbonated water injection in glass micromodels using n-decane, dead oil and live oil (Sohrabi et al., 2015). Riazi et al. used glass micromodels to study mechanisms involved in CO2 injection and storage (Riazi, Sohrabi, Bernstone, Jamiolahmady, & Ireland, 2011).
• Hele-Shaw cell has widely been used in visualisation experiments. Hele-Shaw cell consists of two flat plates (at least one of them is transparent) parallel to each other, sealed on the edges and separated by a spacer. Table 3.1 shows some of the studies conducted in CO2-water system. In this thesis, polycarbonate was used to prepare cells of varying thickness, and glass beads represented the porous media in the cell.
Several studies have been conducted using pH indicators for visualisation of CO2 and water system (Emami-Meybodi, Hassanzadeh, Green, & Ennis-King, 2015). Some of these studies use surrogate/analog fluids as a representative of one of the phases. In this thesis, we have used dyes and pH indicator to colour oil and water phases respectively.
Dyes and indicators were used to observe the movement of fluids in porous media and visualise various phenomena that occur during CO2 injection.
Author Setup used Fluids1 Visualisation technique used
(Kneafsey & Pruess,2010) Hele-Shaw cell CO2- water pH indicator in water (Backhaus, Turitsyn, & Ecke,2011) Hele-Shaw cell Water- PG PG used as analog fluid
(Kneafsey & Pruess,2011) Hele-Shaw and bead pack CO2- water pH sensitive dye in water (MacMinn, Neufeld, Hesse, & Huppert,2012) Bead pack MEG- water MEG as a surrogate fluid
(Soroush et al.,2012) Hele-Shaw cell Water- brine Dyed brine as a surrogate fluid (Faisal, Chevalier, & Sassi,2013) Hele-Shaw cell CO2- water pH indicator in water
(Agartan et al.,2015) Bead pack Water- PG Dyed water phase. PG as a surrogate fluid
Table 3.1: Visualisation studies conducted in CO2 – water system Some limitations in visualisation studies mentioned above are:
1. Etched glass micromodels have been used to study fluid system where heterogeneity provided by etched pores is limited. Use of glass beads opposed to etched glass provides more complex pore distribution.
2. Micromodel studies are limited to visualising processes at a microscopic scale and are usually narrowed to a small section of pores.
3. pH indicators used in these studies are not exclusively water soluble.
1PG stands for Propylene glycol and MEG stands for Monoethylene glycol
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4. One of the phases (Oil, water or CO2) has to be digitally coloured to distinguish from other fluids in porous media.
Work done in this thesis aims to overcome these limitations by:
1. Using glass beads to represent complex porous media. Glass beads of varied size distribution and mixed wettability were used.
2. Polycarbonate cells of height 13.8 cm were prepared to visualise fluid system in pores. Size of the cell represents a bigger scale opposed to microscopic visualisation.
3. Dyes were used to represent the oil phase, and pH indicator was used in the water phase to visualise the dissolution of CO2 as a change in colour from blue to yellow.