The inflow performance of gas wells in gasicondensate fields may be impaired when condensate banks form near the wellbore as a result of the pressure dropping below the dewpoint. This impairment may be alleviated

sPE/37/ySummary.This paper describes a practical application of 'thermodynamic modeling that indicated that tie PVT properties of the reservoir fluid in the Birha field were strongly @@dependent, explaining the presence of significantly undersaturated oil only 200 m [655 ft] below a gas cap.. Field te.its confi~:d the thermodynamic model to be correct. Furthemno:e, this paper discusses the options for field development, including the consideption of possible benefits of (developed) miscibility when gas is injected.

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Gravity forces may generate and stabilise variations in composition along the hydrocarbon column in thick reservoirs. From the condition of thermodynamic equilibrium in a hydrocarbon column in the field of gravity one may determine with an equation of state the variations in composition with depth. The developed calculational procedure uses the Soave and Peng-Robinson procedure uses the Soave and Peng-Robinson equation of state. Results are shown for various hydrocarbon systems. Aromatics content, overall pressure and the introduction of binary interaction pressure and the introduction of binary interaction coefficients in the equation of state are seen to have strong influence on the predictions of gravity segregation. Observed compositional variations and corresponding variations in bubble points/dew points as well as the occurrence of natural points as well as the occurrence of natural miscibility between cap gas and reservoir oil can be explained. Introduction In thick reservoirs variations in composition along the hydrocarbon column have been observed (Refs. 1–9). The mole fractions of the lighter hydrocarbons decrease, whereas the heavy fraction increases from the top to the bottom of the reservoir. These variations may affect reservoir fluid properties considerably. In the Brent reservoir properties considerably. In the Brent reservoir (Refs. 1–2), for instance, PVT measurements on bubble-point pressures/dew-point pressures of fluid samples show these pressures to be well below local reservoir pressures at the sampling interval. Fluid samples may even show miscibility between cap gas and reservoir oil under initial reservoir conditions. In studying recovery processes, it is essential to have understanding of the underlying mechanisms. An explanation for the phenomena observed may be that gravity forces induce and stabilise the compositional differences. In the literature the influence of gravity on the spatial distribution of components in a reservoir fluid system that is in thermodynamic equilibrium has been analysed before by Sage and Lacey (Ref.10). They made calculations assuming ideal solution behaviour. Since the simplifying assumptions made are too restrictive, we initiated a study on this subject with the aim of achieving a more satisfactory solution. The condition of thermodynamic equilibrium is equivalent to the requirement that for each component in the system the sum of the chemical potential and the gravity potential should be potential and the gravity potential should be constant throughout the column. This leads to a set of non-linear equations. The chemical potentials can be calculated from an equation potentials can be calculated from an equation of state. In this paper the theory of gravity segregation and the method of calculation are discussed. Results for a number of hydrocarbon systems are presented together with a discussion and general conclusions of this work. GRAVITY SEGREGATION Process description Process description When a multi-component system is in true thermodynamic equilibrium in a gravity field, the system should be isothermal and for each component in the system the sum of the chemical potential and the gravity potential should be constant (Refs. 10–11). Compared with the usual vapour-liquid equilibrium condition, which requires the chemical potential for each component to be equal in both potential for each component to be equal in both phases, i.e. constant in the whole system, there phases, i.e. constant in the whole system, there is now an additional term due to gravity.

The Hydrocarbon Field Planning Tool (HFPT), recently developed by Shell, provides capabilities for rigorous integrated subsurface-surface production forecasting in the medium to long term (1–30 years). HFPT can be used for gas, gas-condensate, oil and mixed gas-oil fields. HFPT models allow business optimisation by making more efficient use of the existing assets and by reducing investment costs in the new fields. In the simulation, HFPT uses a pressure-balanced solution of the integrated system: from the reservoir(s), through wells and surface facilities, to the delivery point. A wide range of fluid models is available, from simple gas-condensate and black oil PVT models to multi-component models with EOS flash calculations. HFPT provides optimisation functionality for maximising the returns in oil and gas fields, while accommodating operational preferences for production allocation and network constraints. It can also model injection networks and optimal lift gas distribution. Introduction The need for an integrated approach to dynamic field modelling has now been accepted by many players in the oil industry [1]. Issues which can be analysed in an integrated model, and which cannot be adequately addressed in a stand-alone reservoir model (or multiple stand-alone models), include:Pressure interaction between surface and subsurface.Pressure interference between different reservoirs and wells connected to a shared surface facility. An example is a high-pressure well backing out a low-pressure well.Mixing of dissimilar fluids from different reservoirs in the production network.Influence of facility constraints, e.g. separator limits, on a set of reservoirs connected to a shared facility.Production optimisation in the overall system against a set of common criteria. A number of field studies, performed using integrated subsurface-surface models, have already been reported, e.g. [2][3], showing benefits of such models. Over the past ten years, a wide range of applications from commercial vendors have appeared on the market which allow modelling of the subsurface and surface in an integrated way. Most of the available tools are designed either for a very simple reservoir description or for a simple surface description, or both. Hydrocarbon Planning Tool (HFPT) has been developed by Shell to fulfil the need for rigorous integrated subsurface-surface production modelling. It has been designed for accurate medium to long term forecasting, for optimising of production from existing fields, for analysing near-field potential in mature fields and for developing new fields. Currently, HFPT focuses mainly on medium to long term forecasting which is dominated by subsurface behaviour. However, surface and process facilities models can also be modelled in great detail, when needed. Requirements for Integrated modeling An integrated subsurface-surface model consists of the following main data modules, illustrated in Figure 1:PVT model.Subsurface model.Surface production system model.Processing facilities model.Overall integration and control, contracts, optimisation targets, development planning.