PVT Characterization of a Gas Condensate Reservoir and Investigation of Factors Affecting Deliverability

Abstract

Abstract This paper is a study aimed at the effect of Condensate blockage and tubing size on well deliverability. Gas sales contracts are usually based on the capacity of the system to deliver a specified rate over an agreed period of time. The results of this study are important to determine if a contract can be satisfied or in determining how it can be met. The EOS model is based on data collected from a gas condensate field, offshore Nigeria. This paper highlights the PVT properties most important in predicting gas condensate reservoir behaviour and the approach used in building the EOS model. Proper EOS characterization is key in modeling the behavior of a gas condensate reservoir. As a starting point, the PVT report is obtained and a material balance analysis is carried out on the reported measured data. The reported data used in the equation of state (EOS) model are the Constant Volume Depletion (CVD), Constant Composition Expansion (CCE) and the hydrocarbon analysis during CVD. These are used in the study to do a material balance analysis based on existing correlations. Apart from the subsurface samples, surface samples where obtained and physically recombined, a mathematical recombination was carried out as a quality check on the physical recombination. The material balance serves as a means of validating the PVT data before it is used for an EOS characterization. EOS characterization is carried out using the Soave-Redlich-Kwong (SRK) EOS which in this case is used to fit the measured PVT data through non-linear regression. The heavy fraction i.e. C7+ is split into SCN components from C7 to C30+ before the regression is carried out. In this study the fraction was split into thirty-four components. After a match is obtained, pseudoization is carried out to reduce the number of components in order to reduce simulation run time. EOS characterization is carried out in phazecomp, a state of the art EOS modeling tool. The EOS is tuned so as to match measured data as contained in the PVT report. This characterized fluid is then used as the basis for input into the single well numerical simulation model by which deliverability is studied. The effect of condensate blockage is studied with relative permeability as a variable to see if a desired rate can be maintained as is expected by gas sales contracts. If there is substantial blockage effect from the result of the simulation then it is an indication that a condensate bank forms in the near well bore region leading to loss in productivity. This becomes a useful way to decide what kind of mechanism will be employed for depletion, for example gas injection to keep the pressure above dew point pressure could prevent condensate blockage. The effect of tubing size on well deliverability is also studied so as to get an optimal tubing size to see out a gas contract which is a tubing size that makes it possible to satisfy contractual obligations.