Formation Flow Impairment in Carbonate Reservoirs Due to Asphaltene Precipitation and Deposition during Hydrocarbon Gas Flooding

Abstract

AbstractHydrocarbon gas injection has proven to be one of the most efficient Enhanced Oil Recovery (EOR) methods, especially for tight and heterogeneous reservoirs with light to medium API oil, where water flooding is expected to be inefficient. Asphaltene precipitation and deposition, however, might occur due to pressure and fluids compositional changes with the gas injection. This complex phenomenon requires experimental and numerical investigation to understand the conditions at which flow impairment due to asphaltene formation damage may occur, resulting in lowering well flow capacity and in turn lower ultimate oil recovery.In this experimental study, low permeability carbonate rock core samples were flooded with hydrocarbon gas under reservoir conditions. The floods were conducted on core samples of two different lengths representing two different rock types based on average rock permeability and Pore Throat Size Distribution (PTSD). Additionally, these core samples were flooded at two different operating conditions to mimic the average reservoir and the wellbore flowing pressure conditions. As a prelude to these experiments, Asphaltene Onset Pressure (AOP) and Asphaltene Onset Concentration (AOC) of the oil under study with the injection gas were established through NIR, SARA and Titration analysis.Flow impairment due to formation damage by asphaltene precipitation and deposition was analyzed through permeability measurements before and after gas flooding. In all cases permeability reduction was observed. Permeability reduction was found to be function of rock types, reservoir pressure, and length of composite core samples. We assume that pore throat bridging by the larger size asphaltene particles caused higher permeability reduction in the samples of poorer rock types. Experiments conducted at lower pressures showed more damage. This is consistent with the lower AOC at lower pressure. Longer core samples give more time for asphaltene flocculation resulting in more asphaltene formation damage and more permeability reduction. Scanning Electron Microscopic (SEM) images of core plugs before and after the gas flooding process were found to be not conclusive with respect to direct detection of asphaltene deposition in the core samples and further work is planned to positively identify asphaltene deposition in the rock samples.