Investigation of asphaltene deposition associated formation damage during huff-n-puff injection in shale reservoirs
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Huge amount of shale oil in-place has been explored all over the world. The advanced horizontal drilling and multiple hydraulic fracturing in horizontal well techniques have made the commercial development of shale oil reservoirs possible. The success of shale oil plays in U.S. showed the huge economical potential of shale oil resources. However, one major problem in shale oil development is the extremely low oil recovery factor. The primary oil recovery factor of a shale reservoir is just around 5% which means that most of the shale oil remains in the formation and cannot be produced. Thus, an increasing number of researches are focusing on the IOR (improved oil recovery) and EOR (enhanced oil recovery) techniques in shale oil plays. Among all the EOR techniques, gas injection EOR is believed to have the most potential to be applied in shale plays. Many experimental and simulation study regarding gas injection EOR in shale oil reservoirs showed promising enhanced oil recovery results. One problem during the gas injection EOR process is the asphaltene precipitation and deposition. Such phenomenon was widely reported and studied in conventional reservoirs, while it was not taken into consideration in researches regarding gas injection EOR in shale oils. In order to figure out whether the asphaltene associated formation damage exists in gas injection EOR process in shale reservoirs, how severe this problems could be， and how to evaluate such kind of problems during gas injection EOR process in shale, we carried out both experimental works and simulation studies to answer the questions mentioned above. The effect of gas injection on asphaltene precipitation in shale oil samples was investigated experimentally. The permeability reduction, pore size distribution change, and wettability alternation caused by the gas huff and puff injection in shale rock samples were investigated experimentally. Experimental results confirmed that the gas injection into shale oil can generate large enough asphaltene particles to block the pore and throat inside the shale rocks and asphaltene deposition caused non-negligible permeability damage during gas huff and puff injection in shale. Base on the experimental measurement of the permeability reduction during the gas huff and puff injection process, core-scale experiment model was built up to study the asphaltene precipitation and deposition process in the core experiments. A reservoir scale model was also built up based on the parameters of asphaltene deposition model from experiments and core-scale experiment model to investigate the effect of asphaltene deposition caused formation damage on production and oil recovery in shale reservoirs. Optimization strategies of CO2 huff and puff injection scenarios to minimize asphaltene precipitation and deposition were studied based on the reservoir scale model. Experimental results showed that severe permeability damage was caused by asphaltene during CO2 huff and puff injection (e.g., 48.5%), especially in the first cycle (e.g., 26.8%) in one of the tested Eagle Ford shale core plug. The asphaltene deposition also reduced pore and throat size and made the rock surface to more oil-wet. Analysis of the experiments using simulation approach show that oil recovery factor reduction starts right after the beginning of CO2 huff and puff injection and the effect of asphaltene deposition on oil recovery factor accumulated during the later cycles. The asphaltene deposition was mainly formed in the near surface area of the core plug. As the CO2 concentration is quickly increased in the first cycle and more oil is near the rock surface in the first cycle, asphaltene precipitation and deposition were most significant during the huff period in the first cycle compared with the subsequent cycles. The effects of huff and puff injection operational scenarios on oil recovery factor reduction caused by asphaltene deposition were analyzed based on the reservoir scale model. Reducing huff injection rate, reducing huff injection time and increasing puff producing time are favorable optimization strategy for minimizing asphaltene deposition caused oil recovery reduction. Although by changing other operational scenarios, the amount of asphaltene deposition could be reduced, meanwhile the oil recovery factor was reduced significantly. Thus, we recommend controlling the asphaltene deposition by optimizing the huff injection rate, huff injection time, and puff producing time based on the requirements of industrial production.