Conventional primary and secondary water flooding of Deepwater Gulf of Mexico (GOM) reservoirs typically result in substantial un-recovered oil providing an attractive target for enhanced oil recovery (EOR) processes. One of the challenges of applying EOR gas injection in these offshore reservoirs is the h igh oil asphaltene content. Anadarko Petroleum Corporation and Schlumberger have jointly investigated the effects of gas addition on the phase behavior of oil, especially its effect on asphaltene precipitation and deposition. The study focuses on the experimental results from various tests showing the instability of asphaltenes in oil from various gas injection scenarios. Three common EOR injection gases: nitrogen (N2), carbon dioxide (CO2) and methane (CH4) were studied. N- heptane was (n-C7) also included for comparison of solids phase behavior during depressurization. Most asphaltene laboratory testing use n-C7 at ambient conditions, whereas asphaltene precipitation occurs with change in pressure and temperature during reservoir depletion processes. The study collected PVT and flow assurance data for original live fluid and for additions of N2, CO2, CH4 and n- C7 at high pressure and temperature conditions. Measurements include asphaltene onset pressure (AOP), saturation pressure (Pb), swelling tests and asphaltene deposition tests. Other basic measurements of the corresponding dead oil include SARA analysis, viscosity, density and fluid characterization. Fluids from the field presented a compositional variation with a variety of asphaltene contents from 4 to 15.5%. Results of experimental flow assurance assessments revealed the black oil has high propensity for asphaltene precipitation due to addition of injected gas. The addition of N2, CO2 or CH4 significantly aggravates the asphaltene precipitation condition of these fluids. The comparison between the three gases showed that, when added in the same mole proportion, N2 was the strongest precipitant followed by CH4.

Equilibrium experimental data for methane, ethane, and propane incipient hydrate formation in the presence of diethylene glycol and that for propane in the presence of ethylene glycol were obtained. Fortysix experiments were performed in the temperature and pressure ranges (264 to 283) K and (0.181 to 4.5) MPa, respectively, using the pressure search method. The experimental data were compared with predictions made using a hydrate equilibrium calculation method. The average deviation between the experimental data and calculated values was found to be 4.91%.

The asphaltene content effect on crude oil properties was investigated for a series of deepwater Gulf of Mexico (GOM) fluids with asphaltene contents varying from 4 to 15 wt %. The objective of the study was to conduct flow assurance screening tests on GOM samples collected from different sands and determine properties of the dead oil and the asphaltene stability. Densities, refractive indices, and viscosities were measured at different temperatures in dead oils with three different asphaltene contents. The properties showed defined tendencies with the asphaltene content and with the temperature. The application of the one-third rule in the calculation of properties, such as solubility parameter and viscosity of dead oil systems, was evaluated. This approach also provides an alternative to calculate the refractive index based on densities obtained from an equation of state. The analysis also shows the important role that the asphaltene content plays in determining the viscosity of crude oil and evaluates the possibility of predicting viscosity from refractive index, as proposed by Vargas et al. Another important aspect to evaluate is the prediction of the asphaltene stability in the crude oil by measuring basic dead oil properties, such as density and refractive index. The asphaltene instability trend (ASIST) method was used to predict the asphaltene precipitation onset at reservoir conditions. In this analysis, the asphaltene stability was studied on the heaviest and lightest samples (high and low asphaltene content) by determining the minimum quantity of precipitant required to initiate asphaltene flocculation, followed by measurement of the refractive index of the mixture at the onset conditions. The asphaltene precipitation kinetic effect was also considered in this study.

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