Several factors, including the oil composition, reservoir pressure and temperature, and the properties of asphaltene, influence asphaltene precipitation from reservoir oil. Asphaltene starts to precipitate and plug the reservoir pore space under certain reservoir conditions, reducing flow capacity of the wells and the amount of recoverable oil. It is necessary to understand the mechanism of asphaltene precipitation and the resulting effects on well performance in order to build a representative forecasting model for reservoir management. The goal of this study is to model the effects of asphaltene precipitation on fluid flow and oil production from the vuggy, fractured reservoirs of the Taratunich Field.
Asphaltene precipitation has presented serious problems in the development of this field, which is located in the Bay of Campeche, Mexico. Asphaltene deposition has been reported both at the tubings and surface facilities. Special laboratory fluid and core studies and analysis of the well and reservoir performance data have been conducted in search of methods to mitigate or prevent this problem. Subsurface oil samples were collected for use in the laboratory analysis of the fluid to determine the composition, phase behavior and onset pressures of asphaltene and wax precipitation. Asphaltene was precipitated when the reservoir pressure declined below certain level. Laboratory flow tests were designed to measure also the rock flow capabilities as a function of pressure. Cores representing different rock types were used and different pressure levels were chosen spanning the range of asphaltene precipitation, which was determined from the fluid characterization study.
The analysis of the well and reservoir performance data revealed that lithology types and pore throat size play an important role in where asphaltene precipitates in the reservoirs. The results of the experiments showed that oil permeability decreases as the core pressure declines below the onset pressure of asphaltene precipitation. A dual porosity, single permeability (DPSP) numerical flow model was constructed with these experimental results, which were converted into tables of pore volume and transmissibility modifications. The reservoir simulator, Eclipse100, allowed the effects of asphaltene precipitation to be modeled through changes in pore volume and transmissibility with reservoir pressure. Since fracture system is the only conduit for fluid flow in DPSP flow models, the changes were applied to fracture system. The various experiments and their results, interpretation of the well and field performance data, as well as the construction of the simulation model are presented in this paper.