Hydrocarbon_kinetics

Data Structure: Hydrocarbon_kinetics

Description

Defines the assignment and processing data associated with Hydrocarbon kinetic definition

Usage

Hydrocarbon_kinetics     NUM=ival   where ival  is the data structure number

Description

Overview

This data structure defines the assignment and processing data associated with Hydrocarbon kinetic definition. It is used in conjunction with:

•Kerogen_kinetics which defines the data associated with each kerogen type

•Group_data - > Kerogen_assignment which assigns Kerogen_kinetics data to specific element groups

The data defined includes:

•Vitrinite maturation model and data: currently "EasyRo" is the only model available (Sweeney and Burnham, 1990)

•Oil - > Gas cracking data (Pepper and Dodd, 1995)

•Hydrocarbon expulsion data Kerogen_gas_sorption and Kerogen_oil_sorption (Pepper and Corvi, 1995(b))

•Hydrocarbon density for porosity update and computation of the rate of volume change for evaluation of pore pressure field

Oil - > Gas cracking (Pepper and Dodd, 1995)

Initial oil and generated oil may be transformed to gas. This secondary cracking can be incorporated by using a staggered update procedure where, after updating the kerogen to oil and gas transformation, the oil-gas transformation is evaluated using a similar multicomponent parallel reaction law defined either by a Gaussian distribution ( Oil_gas_cracking_data )

or alternatively by a table of parallel reaction energies ( Oil_gas_cracking_table )

The kinetics of the cracking process, and hence kinetic modelling parameters, are highly dependent on oil composition, primarily the saturate to aromatic ratio of the generated oil. The HI of the source rock, reflecting indirectly the saturate to aromatic ratio of the generated oil, can be correlated with the rate constants for oil cracking, thus allowing prediction of cracking rates based on knowledge of a simple routinely performed geochemical measurement. Consequently an option to use HI dependent parameters is also available.

The type of model to be used is defined via Oil_gas_cracking_model with valid types being:

•"Gaussian" - Gaussian distribution defined by the user

•"Table" - Table of parallel reaction constants defined by the user

•"GaussHI" - Gaussian distribution dependent on the initial hydrocarbon index ( HI0mghc /gc ). For the "GaussHI" model the activation energy and variance are defined by the logarithmic relationships provided by Corvi and Dodd (1995) i.e.

where the default parameters are:

•Premultiplier A0 =1014(1/s)

•Activation energy constant a1 =233.6(kJ/mol)

•Activation energy constant b1 =7.5(kJ/mol)

•Variance constant a2 =32.5

•Variance constant b2 =8.55. These parameters may be re-set by defining Oil_gas_cracking_hi_data

Hydrocarbon Expulsion (Pepper and Corvi, 1995(b))

Quantities of oil and gas available, i.e. after the processes of oil and gas generation and oil-gas cracking, are compared with threshold quantities required for expulsion. The oil and gas expulsion threshold (OET and GET, respectively) quantifies the maximum concentration of oil and gas that can be retained relative to the residual organic carbon at that time giving

The change in amount of oil ( cOE) and gas ( cGE) expelled at time t is then defined as

Due to a lack of firm experimental evidence, Pepper and Corvi (1995b) assume global constants:

•aG =0.02g/gC

•aO =0.10g/gC

Porosity Update due to Kerogen Transformation

Geomechanically, the kerogen is contained within the material matrix and defined by the TOC as a weight percentage of the original rock matrix. The total potential porosity associated with the kerogen may therefore be defined as

The volume fraction associated with the kerogen derived from the fractional TOC ( TOCf) as:

The transformation of kerogen to hydrocarbon results in a change in porosity of the source rock equal to the change in volume fraction of kerogen, which is considered part of the matrix. This transformation is assumed to occur when the hydrocarbon is expelled from the kerogen and transferred to the fluid phase so that

Pore Pressure Volume Generation

It is assumed that the pore pressure is sufficient that the converted oil and gas are both in a liquid state and that they can be considered as a single hydrocarbon phase with density ρHC. This may either be constant or governed by a simple empirical law as a function of pressure and temperature. After conversion it is initially adsorbed into the Kerogen and both porosity change and pore pressure generation are assumed to occur when the hydrocarbon is expelled from the Kerogen. The rate of volume change is then defined as:

Default hydrocarbon densities for the five kerogen types are shown in the table below:

References

•Sweeney. J. J. and Burnham. A. K. (1990), Evaluation of a simple model of vitrinite reflectance based on chemical kinetics. AAPG Bulletin. 74. 1559-1570.

•Pepper, A.S. and Corvi, P.J. (1995(a)), Simple kinetic models of petroleum formation. Part I: oil and gas generation from kerogen. Marine and Petroleum Geology. 12(3) 291-319. 1995(a)

•Pepper, A.S., Dodd, T.A. (1995), Simple kinetic models of petroleum formation. Part II: oil-gas cracking. Marine and Petroleum Geology. 12(3) 321-340.

•Pepper, A.S. and Corvi, P.J. (1995(b)), Simple kinetic models of petroleum formation. Part III: Modelling an open system. Marine and Petroleum Geology. 12(4) 417-452.