Laboratory investigation of asphaltene-induced formation damage

The purpose of this study is to evaluate the degree of formation damage caused by asphaltene deposition in the pore throats in case of oilfield operation. Many wells in the Samara region oilfields are operated under high reservoir drawdown, with downhole pressure lower than the bubble point. Such wells’ operating conditions lead to a change in oil composition (light components are extracted from oil while asphaltenes are precipitated and deposited) in the near wellbore, and the productivity of the wells declines due to asphaltene deposition. The study procedure presented in the paper included the following methods: high-pressure microscopy with grain size analysis (the visual method), the near infrared light scattering method and the gravimetric method to measure asphaltenes onset pressure in oil. Formation damage was measured by the filtration method. Asphaltene concentration in oil after filtration was measured by the photocolorimetric analysis. Microcomputed tomography of the core sample was provided to visualize formation damage. In addition, fluid flow in the pore space was simulated before and after asphaltene deposition using a dynamic simulator. In the paper, reservoir oil of one of the Russian oilfields was investigated. The main results of this paper are the following: asphaltene onset pressure in oil at the reservoir temperature (48 °C) was measured as equal to 6.8 MPa which is slightly higher than the bubble-point (6.5 MPa). Oil was flowed through the core sample of the field at three different specific backpressures (at constant flow rate) and formation damage was estimated. The studies have shown that decrease in permeability of the core is caused by asphaltene deposition in the pore space. In this case, a decrease in the amount of asphaltenes in oil emerging from the core sample is observed which was proved by the spectrophotometric analysis. Via microcomputed tomography, a 3D model of the rock matrix and the pore space of the initial and damaged core sample was constructed and a decrease in porosity after formation damage was estimated. Based on the obtained 3D model of the core, computer simulation of fluid flow (in a dynamic simulator) in the initial and damaged core was performed, and the flow parameters (velocity and streamlines) were calculated. The proposed methodology including a set of physical methods to study a core before and after formation damage combined with fluid flow simulation enables predicting potential complications under the field operation.