Compositional Grading - Theory and Practice

Abstract

Abstract This paper quantifies the potential variation in composition and PVT properties with depth due to gravity, chemical, and thermal forces. A wide range of reservoir fluid systems have been studied using all of the known published models for thermal diffusion in the non-isothermal mass transport problem. Previous studies dealing with the combined effect of gravity and vertical thermal gradients on compositional grading have either beenof a theoretical nature, without examples from reservoir fluid systems, orproposing one particular thermal diffusion model, usually for a specific reservoir, without comparing the results with other thermal diffusion models. We give a short review of gravity/non-isothermal models published to date. In particular, we show quantitative differences in the various models for a wide range of reservoir fluids systems. Solution algorithms and numerical stability problems are discussed for the non-isothermal models which require numerical discretization, unlike the exact analytical solution of the isothermal gradient problem. A discussion is given of the problems related to fluid initialization in reservoir models of complex fluid systems. This involves the synthesis of measured sample data and theoretical models. Specific recommendations are given for interpolation and extrapolation of vertical compositional gradients. The importance of dewpoint on the estimation of a gas-oil contact is emphasized, particularly for newly-discovered reservoirs where only a gas sample is available and the reservoir is near saturated. Finally, we present two field case histories - one where the isothermal gravity/chemical equilibrium model describes measured compositional gradients in a reservoir grading continuously from a rich gas condensate to a volatile oil; and another example where the isothermal model is grossly inconsistent with measured data, and convection has apparently resulted in a more-or-less constant composition over a vertical column of nearly 5000 ft. Introduction Composition variation with depth can result for several reasons:Gravity segregates the heaviest components towards the bottom and lighter components like methane towards the top33,34,39.Thermal diffusion (generally) segregates the lightest components towards the bottom - i.e. towards higher temperatures - and heavier components towards the top (towards lower temperatures)3,39.Thermally-induced convection creating "mixed" fluid systems with more-or-less constant compositions, often associated with very-high permeability or fractured reservoirs10,18,31.Migration and "equilibrium" distribution of hydrocarbons is not yet complete, as the times required for diffusion over distances of kilometers may be many 10's of millions of years.Dynamic flux of an aquifer passing by and contacting only part of a laterally-extensive reservoir may create a sink for the continuos depletion of lighter components such as methane.Asphaltene precipitation during migration may lead to a distribution of varying oil types in the high- and low-permeability layers in a reservoir36.Asphaltene precipitation in the lower parts of a reservoir ("tar mats") caused by non-ideal thermodynamics and gravitational forces14,32.Biodegredation varying laterally and with depth may cause significant variation in, for example, H2S content and API gravity.Regional (10–100's km) methane concentrations that may lead to neighboring fields having varying degrees of gas saturation, e.g. neighboring fault blocks which vary from saturated gas-oil systems to strongly-undersaturated oils.Multiple source rocks migrating differentially into different layers and geological units.