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Abstract One of the major problems during gas injection in unconventional reservoirs is asphaltene precipitation and deposition. Asphaltenes can reduce the pore throat in the reservoir and plug the surface and subsurface equipment during the production process, thus, result in oil production reduction with significant financial consequences. The impact of carbon dioxide (CO2) gas injection on asphaltene deposition in unconventional reservoirs still poorly investigated. This research investigates the impact of CO2 gas injection on asphaltene aggregation in ultra-low-permeability pore structures, mainly present in unconventional shale resources. First, the minimum miscibility pressure (MMP) of crude oil with CO2 was determined using the slim tube technique. Then, several CO2 injection pressures were selected to conduct the filtration experiments using a specially designed filtration apparatus. All pressures selected were below the MMP. Various sizes of filter paper membranes were used to study the effect of pore structure on asphaltene deposition. The results showed that asphaltene weight percent was increased by increasing the pressure and a significant asphaltene weight percentage was observed on smaller pore size structures of the filter membranes. The visualization tests revealed the process of asphaltene precipitation and deposition and showed that asphaltene particles and clusters were precipitated after one hour and fully deposited in the bottom of the test tube after 12 hours. High-resolution photos of filter paper membranes were presented using microscopy imaging and scanning electron microscopy (SEM) analysis; these photos highlighted the asphaltene particles inside the filter paper membranes and pore plugging was observed. The study's findings will contribute to a better understanding of the main factors influencing the stability of asphaltene particles in crude oil under immiscible CO2 injection pressure, particularly in nano pores, which are predominant in shale unconventional resources.
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Fabrication and verification of a glass–silicon–glass micro-/nanofluidic model for investigating multi-phase flow in shale-like unconventional dual-porosity tight porous media
Unconventional shale or tight oil/gas reservoirs that have micro-/nano-sized dual-scale matrix pore throats with micro-fractures may result in different fluid flow mechanisms compared with conventional oil/gas reservoirs. Microfluidic models, as a potential powerful tool, have been used for decades for investigating fluid flow at the pore-scale in the energy field. However, almost all microfluidic models were fabricated by using etching methods and very few had dual-scale micro-/nanofluidic channels. Herein, we developed a lab-based, quick-processing and cost-effective fabrication method using a lift-off process combined with the anodic bonding method, which avoids the use of any etching methods. A dual-porosity matrix/micro-fracture pattern, which can mimic the topology of shale with random irregular grain shapes, was designed with the Voronoi algorithm. The pore channel width range is 3 μm to 10 μm for matrices and 100–200 μm for micro-fractures. Silicon is used as the material evaporated and deposited onto a glass wafer and then bonded with another glass wafer. The channel depth is the same (250 nm) as the deposited silicon thickness. By using an advanced confocal laser scanning microscopy (CLSM) system, we directly visualized the pore level flow within micro/nano dual-scale channels with fluorescent-dyed water and oil phases. We found a serious fingering phenomenon when water displaced oil in the conduits even if water has higher viscosity and the residual oil was distributed as different forms in the matrices, micro-fractures and conduits. We demonstrated that different matrix/micro-fracture/macro-fracture geometries would cause different flow patterns that affect the oil recovery consequently. Taking advantage of such a micro/nano dual-scale ‘shale-like’ microfluidic model fabricated by a much simpler and lower-cost method, studies on complex fluid flow behavior within shale or other tight heterogeneous porous media would be significantly beneficial.
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- Award ID(s):
- 1722647
- PAR ID:
- 10176517
- Date Published:
- Journal Name:
- Lab on a Chip
- Volume:
- 19
- Issue:
- 24
- ISSN:
- 1473-0197
- Page Range / eLocation ID:
- 4071 to 4082
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/C9LC00847K
