Dynamics of pore-throat plugging and snow-ball effect by asphaltene deposition in porous media micromodels

Asphaltene deposition in the pore-throats of reservoir rock reduces permeability, which leads to a reduction in oil production. The amount of deposited asphaltene is generally expressed by an equation that sums three types of deposition: surface deposition, pore-throat plugging, and entrainment of the asphaltene. The pore-throat plugging is accelerated by the increase of the amount of deposited asphaltene, which is termed the snow-ball effect. Wang and Civan's model is commonly used to calculate the deposition amount of asphaltene and permeability reduction of rock. The model includes several adjustable parameters to define the rates of each deposition type. In the term for calculating pore-throat plugging, snow-ball effect coefficient is defined as an adjustable parameter as well. The validity of Wang and Civan's model is verified by comparing the measurement results of core experiments with the calculated results of the model. However, since there are many adjustable parameters, it is difficult to confirm consistency between the interpreted deposition behavior obtained from the core flooding tests and the actual deposition behavior. In particular, although pore-morphology alteration can be an important factor for the development of pore-throat plugging, a pore-scale investigation is still limited. In this study, poremorphology alteration by pore-throat plugging was investigated through microfluidic experiments to deeply understand pore-throat plugging behavior. A mixture of crude oil and n-Heptane was injected into a micromodel to conduct real-time monitoring of asphaltene deposition. The area occupancy of deposited asphaltenes in the micromodel was calculated in each deposition type by image analysis. In addition, the streamlines and the velocity profile of the injected fluid were computed using computational fluid dynamics software. Consequently, the different pore-throat plugging behavior was confirmed in the middle and the late stage of the deposition. In the middle stage, the amount of deposited asphaltene increased by 3.8 times due to pore-throat plugging. In the late stage, only a 1.2 times increase was observed. Although the snow-ball effect parameter is constant in Wang and Civan's model, the alteration of the streamlines and coordination numbers of pore-throats suggested reducing the snow-ball effect at the late stage. Based on this finding, different snow-ball effect parameters were introduced in each stage to match the experimental results with the results calculated by Wang and Civan's model. As a result, both the total amount of deposited asphaltene and the breakdown by deposition type showed good agreement. However, when ignoring the deceleration of the snow-ball effect, the calculated amount of porethroat plugging overestimated the experimental result by 90%. In addition, the entrainment rate was ten times larger than the experimental result. This study provides a detailed understanding of the dynamics of pore-throat plugging and suggests the importance of considering the snow-ball effect reduction in the asphaltene deposition model.

Automated summary

Asphaltene deposition in pore-throats reduces permeability and oil production. Wang and Civan's model is commonly used to calculate deposition amount and permeability reduction, but difficulty arises from the numerous adjustable parameters impacting consistency with observed data.