Fluid Characterization of an Iranian Carbonate Oil Reservoir Using Different PVT Packages

Abstract

Abstract Precise and accurate characterization of a reservoir fluid is a very important factor in reservoir simulation studies. PVT experiments are usually expensive and performed in limited conditions. Therefore Computer EOS based PVT packages are used widely for the prediction and evaluation of fluid properties at reservoir, well and surface conditions over a wide range of temperature, pressure and composition1,2,3,4. In this work different PVT packages that are available in National Iranian Oil Company (NIOC) have been used to complete fluid analysis and experiment simulation of an Iranian oil reservoir and PVT model preparation for the reservoir simulators. This reservoir is a medium size, highly fractured carbonate reservoir located in southwest of Iran. The carbonate formation has dense matrix with improved permeability by presence of fracture network. This reservoir was initially highly undersaturated and after 5 years of oil production the oil pressure reached to the saturation pressure and gas injection was started into the reservoir5. In this work different equation of states, different regression methods and correlations were analyzed for the prediction of fluid properties. Considering the results of this work in general good agreement was obtained for the experimental and calculated data for used packages. However, the method used for tuning the equation of states is critical in getting good agreement. Introduction The prediction of the phase behavior and fluid properties of hydrocarbon systems is important in several different aspects of the petroleum industry. These include the development of fields containing black-oils, volatile oils, gas condensates, and natural gas, as well as the design of gas injection processes, and hydrocarbon processing units6. The reservoir under study is an initially highly undersaturated oil reservoir where located in southwest of Iran. This field is a well-fractured carbonate reservoir and contains light oil with gravity of 35 °API and very low H2S impurity5. There are six bottom hole reservoir fluid sample from the various depths and scatter wells in which all of the PVT data were evaluated. For this purpose some PVT experiments have been performed on the fluid samples of the reservoir and six complete PVT experiment study is available. In order to determine the accuracy of the bottom hole samples and variation of the fluid properties vs. depth several plots were drawn with respect to pressure and depth of sampling. After testing the consistency and determination of the experimental data validity, PVT packages have been run several times in order to tune the PVT models with different experimental data. After matching runs, the best suitable sample with minimum laboratory errors was selected. The results of the experimental data and calculated data have been compared with each other by calculating the relative error, variance, average of absolute deviation from the mean and standard deviation. Also the properties of the reservoir fluid were calculated in different temperatures and pressures for example in the gas-oil contact, datum level and water oil level depths. Description of the reservoir The field under study is a medium size, initially highly under-saturated located in SouthWest of Iran. The existence of hydrocarbon reservoir proved in Asmari and Bangestan formations in this field by drilling well No.1 in 1968. The Asmari formation is a well-developed fractured carbonate oil reservoir and Bangestan formation is a dry gas reservoir with a little amount of condensate5.