Equation of State of a Complex Fluid Column and Prediction of Contacts in Orocual Field, Venezuela

Abstract

Abstract Diligent management of a high-pressure gas injection program in a field with a complex column of hydrocarbon fluids requires reservoir modeling. It is important and necessary to perform a detailed study of the PVT samples including the evaluation of the fluid distributions and to define an equation-of-state (EOS) to be used in a reservoir model. The Orocual field is located in the northern Monagas state of Venezuela and is owned and operated by Petroleos de Venezuela, S.A. (PDVSA), the national oil company of Venezuela. The field is characterized as a complex structure having more than 3000 feet of vertical hydrocarbon column. Fluids change from light gas in the top of the structure to condensate gas, to a near-critical transition fluid and volatile oil, to under-saturated oil at the bottom of the column. Reservoir compartmentalization adds complexity to the field and structurally equivalent, non-communicating fluid regions exist. The dispersion of some sample characteristics versus depth was thought to be the result of non-representative samples. The ability to integrate the PVT data into the geological model adds another dimension in defining the structural complexities. The paper demonstrates a technique to identify representative samples for use in developing an EOS, and for initializing fluids in place. A method is presented to adjust the component composition versus depth, providing consistent vertical composition distribution and compositional model stability. This method meets the objectives of matching field production observations. Also, a method is presented to quickly initialize a full field model using a 1-D compositional simulator to give full field model stability in an environment of high temperature gradient. The final result is a 7 pseudo-component EOS with a single characterization defining the compositional gradient of the hydrocarbon column from gas to black oil. The method demonstrates that composition relative to depth can be predicted in those parts of the reservoir where samples do not exist, but where production and test data must be matched. Other areas in northern Monagas state have similarly complex systems1. Many areas are only partially developed and can benefit by applying techniques presented in this paper to describe the fluids and to estimate gas-oil contacts. Results of compositional simulation show a single EOS and vertical compositional variation reproduce the complex character of the field hydrocarbon column, matching field measured observations of saturation pressure (ps), gas-oil-ratio (GOR) and fluid densities.