Condensate Banking Dynamics in Gas Condensate Fields: Compositional Changes and Condensate Accumulation Around Production Wells

Abstract

Abstract A bank of condensate can rapidly build up around a producing well when bottom hole flowing pressure falls below dewpoint in a gas condensate field. This bank grows as reservoir pressure declines both impairing well deliverability of gas and causing loss of heavy components at surface. An understanding of the condensate banking processes is essential in predicting the behaviour of the wells and of the fields. In this paper, a general theoretical treatment of condensate banking dynamics is attempted to show how the compositions of heavy components of a gas condensate change with time around production wells during depletion. A new component relative mobility term is defined and an analytical model is developed for this purpose. The dependence of condensate banking behaviour on the mobility terms and the way it varies with fluid pressure-volume-temperature (PVT)/vapour-liquid-equilibrium (VLE) properties and relative permeabilities is studied in some detail for a range of simple rich and lean gas condensate mixtures. Results have showed that the newly-defined component mobility term is a critical parameter in the behaviour of condensate banking. Two distinct effects are recognized, one dominant in the near wellbore region and one which is more important in the far field. The compositional change of heavy component is shown to be particularly rapid in low permeability high yield reservoirs and where fluids are close the critical point. Natural steady state situations may not exist with regard to changes in mixture compositions. The reservoir permeability and variation of production rate have significant impacts on condensate accumulation near wellbore and on the way of how the condensate bank develops. In summary, the aim of this paper is to introduce a new theoretical treatment of condensate bank formation and to use this as a basis to increased conceptual understanding of the processes involved. Introduction Gas condensate reservoirs are becoming increasingly important with more and more new condensate fields discovered worldwide as exploration drilling encounters greater depth, higher pressure and high temperature conditions. Retrograde condensation occurs during production and banks of condensate may build up around the producing wells. This presents specific problems in the development of such reservoirs which depends on both gas and condensate production profiles. The formation and build-up of the condensate liquid near wellbore is due to the pressure depletion below dewpoint during production. The growing condensate bank progressively impedes flow of gas to the well and causes loss of heavy components at the surface. In terms of reservoir well performance, the condensate banking behaviour close to the wellbore is the dominant factor. Fig. 1 shows a schematic illustration of the liquid build-up and the pressure profiles in the near wellbore region of a gas condensate reservoir. Whitson and Fevang1,2characterised the near wellbore flow behaviour by a condensate blockage. Condensate blockage can reduce well deliverability significantly in some reservoirs although the severity depends on a number of reservoir and well operation parameters.