Phase behavior and miscible process analysis of Cusiana Field hydrocarbon system using a reduced component equation of state

Abstract

Cusiana Field is located in the foothill belt of Llanos basin in eastern Colombia. This field has large reserves of light hydrocarbons in Mirador (Tertiary), Barco and Guadalupe (Cretaceous) formations. The development of this field beginning with Mirador formation is underway, and includes a gas cycling project. A complex hydrocarbon system has been found in the reservoirs. Because of vertical compositional variation, fluids grade from retrograde gas to black oil as they pass through critical conditions. Phase behavior of these fluids has an important effect on the in-place volume determination and on field development strategies. Previous reservoir analysis and predictions have been done using a 10-component equation of state (EOS), characterized with the help of an extensive laboratory measurement program, including PVT depletion experiments, separation-type tests, and gas injection tests. We developed a reduced component equation of state to reduce time and resources required for full-field compositional simulations using the 10-component EOS. Three different lumping schemes were tried, two of them with six components and one with seven. A validation process was carried out to check the consistency of this reduced component EOS. This process consisted of two steps: The first step was the match of a large set of "experimental" type data generated with the 10-component EOS. The second step was the matching of the saturation and compositional path followed by reservoir oil during a simulated gas injection displacement process. A coreflood experiment and a two-dimensional cross-section of the reservoir were simulated with a one-dimensional and a two-dimensional model, respectively. These models, previously developed by British Petroleum, matched laboratory and field data using the 10-component EOS. Here, we replaced the 10-component EOS with our reduced component EOS and compared the results. All three reduced EOS successfully went through the first step of validation, matching the "experimental" PVT data, but only the seven-component EOS got a good match of the oil saturation and compositional path in the second step. This seven-component EOS reduced the simulation time to about 68% for the one-dimensional case and to 60% for the two-dimensional case. Although multicontact miscibility is not reached during the gas displacement process, vaporization of oil by injected lean gas proved to be an efficient oil recovery mechanism.