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.

Pressure-Volume-Temperature (PVT) relationships have long been studied as a basis of understanding the phase behavior of fluid systems. In order to understand the behavior of these fluid systems, smaller quantities usually referred to as samples are obtained and studied under varying PVT conditions in order to define the character of the fluids. PVT experimental measurements provide key data for reservoir engineering and production applications; however the importance of having valid samples may be overlooked during preparation of the laboratory study and interpretation of the results.

Fields in the Niger Delta Region in Nigeria typically comprise of stacked multiple reservoirs, sometimes with more than one culmination in a particular sand unit. Due to the occurrence of numerous hydrocarbon-bearing intervals in these stacked reservoirs, it is often unlikely that an appraisal well downdip of the structures will provide fluid contacts information at all levels in the field. Based on this fact, some gas-bearing reservoirs with relatively small Hydrocarbon In-Place volumes cannot be economically developed owing to the requirement of a dedicated appraisal well to establish the presence or absence of an oil-rim. The presence of an oil rim in a predominantly gas reservoir can go a long way in influencing the long term development philosophy. For example, the presence of an oil rim could lead to delay in gas cap production, and thus affect an operator's ability to meet gas demand contracts. An alternative approach that can verify the presence of oilrim without a dedicated appraisal well will aid early development of these reservoirs.In this paper, the use of compositional gradient model as an alternative way of predicting presence of oil-rim in the Niger Delta reservoirs is presented. The model is calibrated with data from several reservoirs with known fluid contacts, and then applied to gas reservoirs where fluid contacts are unknown. The results show good comparison with log, core and amplitude data when the fluid column is continuous and not interrupted by fluid infusion or geological barrier.The theory of using compositional gradients to establish the presence of oil-rim has been widely investigated around the world. The theory is based on compositional variation along hydrocarbon column where the forces at play are majorly arising from gravity, chemical and thermal forces. In a typical hydrocarbon bearing column under gravitational forces, the mole fraction of lighter components decreases while that of heavy fractions increases from top to base of the column. As a result, fluid composition and properties also vary. This is the basic theory underlying the use of compositional gradient model to predict oil rim presence.In the Niger Delta where the CAPEX for a new well is in the region of $25 million for an onshore well, this approach becomes a cost-effective way of confirming the presence of oil-rim, and therefore aid the profitable and speedy development of the asset.

Gas condensates and volatile fluids require detailed Equation-Of-State (EOS) characterization to account for compositional variations with pressure, volume and temperature changes. Proper characterization is important in determining the in-place volumes of gascondensates and volatile systems. These volumes are important in determining reserves which play an important role in driving a company's share value. The in-place volumes also drive sales contracts as well as surface equipment design.Gas condensate reservoirs also pose a serious challenge to well productivity especially below the dewpoint in tight reservoirs due to condensate banking. As this situation may lead to expensive stimulation projects, it is important to optimally define operating standards early in the life of the asset. Accurate PVT characterization of gas condensate fluid is key to defining these standards. It is therefore important to get these fluids properly characterized so as to make the right investment decisions. EOS characterization of heptanes-plus requires extended analysis beyond the C 7+ fraction. In several instances, we do not have True Boiling Point (TBP) data for these heavy components. In cases where available, experimental data still remains the most reliable source of heptanes-plus properties. Heptanes -Plus in reservoir fluids contains hundreds of different components which are impossible to identify by chemical separation techniques. Even if it were possible to identify them, it would not be possible to measure the critical properties and other EOS parameters for fluids heavier than C 20 . This problem is solved practically by making approximate characterization of the heavier compounds with experimental and mathematical methods. A molar distribution model can be used to estimate the properties of heptanes-plus. A molar distribution model is a relationship between cumulative molar quantity and some expression for Cumulative molecular weight. Different methods are used to describe molar distribution. Some methods employ a consistent separation of fractions (e.g., by SCN) thus leading to the expression of the molar fraction as a direct relationship between mole fraction and molecular weight of individual cuts. Most methods in this category assume that C 7+ molecular weights decrease exponentially. A more general approach uses the threeparameter gamma probability function to describe molar distribution. The Gamma distribution model is used in this work to study the splitting scheme of the C7+ and compare this with experimental data from extended analysis to see how they compare. If the results prove to be acceptable, then the three-parameter gamma model becomes an alternative in the absence of hard data to split heavy ends to improve their characterization.