PVT and Hydrocarbon migration

PVT relationship controls fractionation of hydrocarbon composition during migration (Larter and Mills, 1991, England, 2002). Let's look at a typical marine source rock, under the pressure and temperature conditions where hydrocarbons are generated, oil and gas are mostly mixed as a single undersaturated oil phase. As this mixed fluid migrates to shallower depths, oil and gas may start to separate as pressure drops below the "bubble point" for that composition. Depending on the volume of the fluids, seal capacity and trap capacity, and pressure and temperature at the prospect, the final hydrocarbon columns in a prospect may be a gas field (with condensate), an oil field (with dissolved gas) or both (oil field with a gas cap) as can be seen in the figure below.

This figure shows the fill and spill migration path and the end results of hydrocarbon phases in traps at various depths from a simple medium GOR oil source rock. Note that when the oil spilled form trap B and subsequently trap C reaches trap D, the gas exsolves from the oil and forms a gas gap. Similarly, because of its limited size, trap F contains only gas because it can not hold all the exsolved gas. Also worth noting is that Fields A, B and F are all charged directly from the source but the end results are different because of PVT and trap size relationships.

What can we do ?

The work flow for predicting the composition of hydrocarbons is fairly straightforward:

1. GOR of fluids expelled from source rocks is a function of the source facies. It can be estimated based on the quality (Hydrogen Index) and (to a lesser degree than previously thought) maturity of the source rock. KinEx is a tool ideal for this purpose, it can account for the composition of expelled hydrocarbons from a mixture or organo-facies ( Pepper and Corvi, 1995 ). Alternatively, maximum GOR may also be estimated from near by field data.
2. PT conditions at the prospect can be estimated based on nearby well data and/or using a simple 1D basin modeling tool, like Genesis, if we need to examine charge at a different geological time.
3. Using the interactive flash calculator built into Trinity, the estimated charge volumes, reservoir geometry (structure map) and properties as well PT conditions are used to predict the hydrocarbon phase and column heights. Because of the uncertainties involved with the input parameters, the interactive nature of the program allows quick what-if scenarios to be considered to provide a range of possibilities.

The Trinity flash calculator can be used in conjunction with seismic DHI models to validate the property of hydrocarbon fluids and the filling extent in a prospect. When PT and GOR data are available from nearby fields, the flash calculated can be used to back calculate the composition of hydrocarbon charge and then estimate the phase and volumes for prospects without having the required source rock information. PVT modeling is extremely important for petroleum plays where a certain type of fluid (oil or gas) is economical while the other is not. For more detailed information on PVT modeling, ZetWare users may review the reference material under Trinity support.

References:

• England, W. A., 2002, Empirical correlations to predict gas/gas condensate phase behavior in sedimentary basins : Organic Geochemistry, v. 33, p.665-673.
• Larter, S., and N. Mills, 1991, Phase-controlled molecular fractionations in migrating petroleum charges: in W. A. England, and A. J. Fleet, ed., Petroleum migration: Geological Society Special Publication No. 59: , Geological Society, p.137-147.
• Pepper, A. S., 1991, Estimating the petroleum expulsion behavior of source rocks: a novel quantitative approach: in W. A. England, and A. J. Fleet, ed., Petroleum migration: The Geological Society Special Publication 59: London, p.9-31.
• Pepper, A. S., and P. J. Corvi, 1995, Simple kinetic models of petroleum formation. Part I: oil and gas generation from kerogen : Marine and Petroleum Geology, v. 12, p.291-319.