Report
Teaser, summary, work performed and final results
Periodic Reporting for period 2 - MILEPOST (Microscale Processes Governing Global Sustainability)
Teaser
The subsurface plays a key role in addressing the security of water, food and energy supplies, e.g. as a source of potable water, location for nutrients fluxes for food sources and efficient recovery of hydrocarbons. However, we do not have sufficient knowledge to predict the...
Summary
The subsurface plays a key role in addressing the security of water, food and energy supplies, e.g. as a source of potable water, location for nutrients fluxes for food sources and efficient recovery of hydrocarbons. However, we do not have sufficient knowledge to predict the behaviour of subsurface systems. The field of reactive transport modelling is an important tool for addressing this extremely complex interplay of flow, transport and reactions occurring over various temporal and spatial scales in the subsurface. The capture of scale dependence remains the most difficult challenge, where complex localised reactions coupled with transport processes can result in pore scale gradients affecting scale dependence of reaction rates and accounting for large discrepancies between field and laboratory rates. State-of-the-art tools (e.g. X-ray microtomography and on-chip porous media) are not sufficient to understand reactive flow, as they do not provide real-time mapping of propagation of fronts (e.g. temperature, pressure, concentration) that are critical to refine and validate simulations.
The MILEPOST project addresses the key engineering global challenge of understanding reactive transport in porous networks, and in doing so addresses key societal challenges related to energy, food and water supplies. Global population continues to rise and is expected to surpass 8 billion by 2030, and consequently, we can anticipate an associated global increase in the requirements of around 50% for energy and food, and 30% in relation to fresh water. This demands the sustainable management of the subsurface in a large number of engineering applications, including decarbonising our fossil energy by carbon dioxide capture and storage (CCS), remediation of contaminants in groundwater aquifers, protection of subsurface water resources, enhancing recovery from hydrocarbon reservoirs, precision agriculture, soil irrigation and fluxes of nutrients to crops. We anticipate that the new knowledge generated by our research will benefit society and will have impacts across energy, environment, water and food sectors. For example, the developed tools could be used to optimise the selection of enhanced oil and gas recovery opportunities in the North Sea, where on average only 48% of the oil originally in the reservoirs is recovered, and a meagre 1% increase in recovery would mean extra revenue of > £17 billion. The UK Royal Academy of Engineering report on ‘Global Water Security – an engineering perspective’ stated that 1.2 billion urban dwellers rely on groundwater for their water supply. However, pollution of groundwater supplies represents a significant contribution to water scarcity, with a number of major cities having to switch from groundwater supplies because of pollution. Water security has indeed a significant impact on economic stability, e.g. the drought in Australia reduced its GDP by 1%.
The ambition of the MILEPOST project is to progress beyond the state of the art to print 3D replicas of porous cores that enable monitoring the properties within the pores. Our unique approach is to develop for the first time three-dimensional instrumented replicas of porous structures, so we can gain much needed dynamic data at the pore scale that can be incorporated into validated simulations coupling flow and reactive transport processes.
The integration of novel tools to replicate porous networks with integrated sensors in order to obtain real-time mapping of dynamic parameters and models validated using digital tools will provide an unprecedented new understanding of fundamental pore-scale processes of flow, transport, and reaction and improve dramatically our ability to develop and upscale high-fidelity reactive transport models. It is anticipated that the MILEPOST project will transform our ability to analyse and predict the behaviour of a wide range of pore-scale processes governing the macroscopic behaviour of complex subsurface systems
Work performed
The MILEPOST project is structured into three themes and this section summarizes the work performed under each theme during this reporting period.
THEME 1: Novel tools to manufacture porous networks with integrated sensors
The ambition of this theme is to develop a reliable process for the manufacturing of three-dimensional replicas (3D micromodels) of porous subsurface structures containing an arrangement of pores and throats with the dimensions not exceeding a few tens of μm. Within this theme, we also aim to develop a technique suitable for embedding miniaturised sensors into the replicas in order to measure in-vivo propagation of various fronts (such as pressure, temperature and pH) inside complex 3D structures at the pore scale.
3D printing of microfluidic devices from transparent plastics:
We initially performed a detailed literature survey on the techniques suitable for 3D printing using transparent materials, such as glass and plastics. This review showed that 3D printing of glass for the manufacture of enclosed porous networks needed for this project faces several challenges. This includes a lack of transparency, low build resolution and surface quality, and relatively high dimensions of the printed features.
We also used two 3D printers available at Heriot-Watt University in order to investigate two printing methods of micromodels from transparent plastics. The first printer (CONNEX 500) enabled high resolution manufacture of transparent objects from a material called VAROCLEAR RDG 810 with a resolution of 600 and 1600 dpi in the XY and Z axis, respectively. Our investigation has shown that the CONNEX 500 machine is not suitable for the printing of our enclosed porous network structures because this printer uses a support material that always remains inside channels after the printing, and its removal is very difficult, particularly when the diameter of printed features is < 1mm, as shown in Figure 1 (a). More satisfactory results were obtained using a low cost FormLabs FORM2 printer that uses a resin called CLEAR V2. Using this machine, we were able to manufacture simple micromodels, as shown in Figure 1(b), comprising internal (resin free) channels with a diameter of approximately 1mm.
Laser manufacturing of glass microfluidic devices:
The high transparency, thermal stability, hardness and chemical resistance of glass often makes this the preferred material over others (e.g. silicon, transparent plastics, or photoresist) for the manufacture of microfluidic devices. Unfortunately, conventional manufacturing of glass microfluidic devices is a complex, time-consuming, multi-step process that involves the combination of photo-lithography, etching and bonding. All these processes require the use of multiple tools and a cleanroom; and hence, the fabrication of glass microfluidic devices is expensive. During this project, we developed a novel technique that enables the rapid manufacturing of enclosed porous network micromodels (microfluidic devices) using two borosilicate (Borofloat®33) glass plates. The process uses an ultrashort pulsed laser (Trumpf TruMicro 5x50) which is the only tool used to manufacture the entire device. The same laser is used for: (i) drilling the inlet/outlet ports in the top glass plate, (ii) generating a microfluidic pattern directly on the surface of the bottom glass plate (by laser ablation), and (iii) enclosing the microfluidic pattern by micro-welding the two glass plates together. The manufacturing of the micromodels using this method can be completed with 2-4 hours, depending on the device complexity. Figure 2 shows an example of the micromodel that was manufactured using this novel technique. We have published more details of this manufacturing process in a refereed journal paper that is publically accessible [Wlodarczyk et al., Micromachines vol. 8(7), p. 407 (2018)].
Development of miniature optical fibre-based pressure sensors:
We have used a commercial FISO miniature fibre opt
Final results
During this reporting period, significant progress has been made under each of the three Themes for this project, and more details for each Theme are provided below. It should be noted as well that in addition to the individual achievements under each theme, the unique integration in this project of micromodel fabrication (THEME 1), dynamic testing (THEME 2) and numerical modelling (THEME 3) will enable progress beyond state of the art in our understanding of reactive transport at the pore and core level.
THEME 1: Novel tools to manufacture porous networks with integrated sensors
A significant progress beyond the state of the art already achieved in this project is the development of a novel technique for the manufacture of two-dimensional (2D) microfluidic devices (micromodels) from inexpensive borosilicate glass substrates. This novel technique, which uses only one tool (an ultrafast laser), has allowed us to fabricate tens of the micromodels comprising different pore network patterns, which later were used in various experiments, as described in THEME 2 (Section 1.2). The developed laser-based method has been shown to be suitable for the rapid prototyping of glass microfluidic devices. In contrast to the conventional techniques used for the manufacture of glass microfluidic devices, which involve a combination of photolithography, etching and bonding, our method does not require the use of multiple tools, projection masks, dangerous chemicals (e.g. HF acid) and clean rooms.
So far, we have demonstrated that our fabrication method is capable of producing 2D glass micromodels in which a pore network structure is enclosed between two glass plates. We are planning to further develop our laser-based method and use it for the manufacture of three-dimensional (3D) micromodels, e.g. transparent replicas of core plugs. Our next step will be to improve the developed technique and use it for the manufacture of multi-level microfluidic devices comprising complex 3D internal structures. Moreover, we are also planning to embed various mineral particles into the micromodels in order to study chemical reactions during the flow of various fluids in porous media at the pore level. Since our micromodels are manufactured by welding individual glass plates together, we have free access to the microfluidic patterns during the manufacturing process. This should allow us to insert small pieces of minerals and enclose them inside the microfluidic patterns.
Expected results until the end of the project also include the novel embedment of miniature sensors into the pore network structures. As mentioned in Section 1.2, we are planning to embed commercial FISO pressure sensors in the first instance. In the meantime, we are also developing our own sensors that will be capable of measuring a wider range of pressures. We are also proposing to embed these home-built sensors inside our micromodels. Finally, within this theme, we also aim to develop miniature pH sensors. These sensors along with the pressure sensors will be used to measure for the first time the dynamic chemical and physical changes during the flow of different fluids in various pore network patterns.
THEME 2: Dynamic studies of manufactured porous networks
As part of this theme, we have designed, built and commissioned a fluid flow visualisation rig capable of injecting a wide range of fluid types (such as brine, hydrocarbons and viscous oils) and observe multiphase-flow phenomena. During the current reporting period, we have focused on studying hydrodynamic aspects of CO2 storage like phase trapping and front propagation inside inert porous media. As THEME 1 progresses, micromodels with embedded sensors and reactive minerals will be made available for dynamic studies in THEME 2. This will allow us to visualize and study in real-time multiphase flow in reactive as well as inert porous media.
Moreover, for the next reporting period, micromodels with embedded sensors will be dynamicall