Method and system for quantitatively evaluating kerogen swelling oil in shale
A method and system for quantitatively evaluating kerogen swelling oil in shale is provided. The method includes: establishing different types of kerogen molecular models, and loading each of the kerogen molecular models into a graphene slit-type pore composed of a lamellar structure; performing energy minimization (EM), relaxation and annealing to obtain a kerogen slit-type pore; loading a shale oil molecule into the kerogen slit-type pore to obtain an initial model of swelling and adsorption of shale oil in kerogen; assigning a value to a force field of the shale oil molecule and the kerogen molecule in the swelling and adsorption model to obtain a density of the kerogen and the shale oil; plotting a density curve of the kerogen and the shale oil; calculating kerogen swelling oil mass; determining per-unit kerogen swelling oil mass; and determining the kerogen swelling oil mass in different evolution stages.
1 . A method for quantitatively evaluating kerogen swelling oil in shale, comprising:
establishing, by a molecular modeling software, different types of kerogen molecular models;
loading each of the kerogen molecular models into a graphene slit-type pore composed of a lamellar structure to obtain an initial model;
subjecting each initial model to energy minimization (EM) and relaxation to obtain a compacted kerogen aggregate model, comprising:
subjecting the initial model to EM and 200 ps relaxation at 75° C. under 20 MPa by using a molecular dynamics software to obtain a compacted kerogen aggregate model;
simulating an annealing process of the compacted kerogen aggregate model to obtain a kerogen slit-type pore, comprising:
subjecting the compacted kerogen aggregate model to 200 ps relaxation for warming; and
subjecting the kerogen aggregate model, after the relaxation warming, to 2 ns simulation, cooling and pressurization by using an isothermal-isobaric ensemble (NPT ensemble) at 800° C. under normal pressure to obtain the kerogen slit-type pore;
loading shale oil molecules into the kerogen slit-type pore to obtain an initial model of swelling and adsorption of shale oil in kerogen;
assigning a value to a force field of the shale oil molecules and kerogen molecules in the initial model of swelling and adsorption of the shale oil in the kerogen to obtain a density of the kerogen and the shale oil;
plotting a density curve of the kerogen and the shale oil according to the density of the kerogen and the shale oil;
calculating kerogen swelling oil mass according to the density curve of the kerogen and the shale oil, comprising:
calculating the kerogen swelling oil mass by
Q
oil
=
∫
L
o
1
L
o
2
S
model
·
ρ
oil
dL
according to the density curve of the kerogen and the shale oil,
where Q oil is the kerogen swelling oil mass; L o1 is a start position of an intersection between the kerogen density curve and the shale oil density curve; L o2 is a stop position of the intersection between the kerogen density curve and the shale oil density curve; S model is a cross-sectional area of the swelling and adsorption model of the shale oil in the kerogen; ρ oil is the shale oil density curve;
determining per-unit kerogen swelling oil mass according to the kerogen swelling oil mass;
determining the kerogen swelling oil mass in different evolution stages according to the per-unit kerogen swelling oil mass, comprising:
deriving kerogen mass m x in different evolution stages;
calculating the kerogen swelling oil mass in different evolution stages by
Q
s
=
Q
w
·
m
k
·
f
s
according to the per-unit kerogen swelling oil mass Q w and the kerogen mass m k in different evolution stages;
where Q w is the kerogen swelling oil mass, m k is the kerogen mass in different evolution stages, f s is a swelling ratio reduction coefficient in different evolution stages, and
m
k
=
m
f
·
F
t
+
m
s
=
(
HI
0
/
1000
)
·
F
t
+
(
1
-
HI
0
·
0.083
/
100
)
,
where m f is a mass of a convertible part of the kerogen, with a unit of g; m s is a mass of a non-convertible part of the kerogen, with a unit of g; HI 0 is an original hydrogen index, with a unit of mg/g TOC; 0.083 is a carbon conversion coefficient of the original hydrogen index, dimensionless; F t is a conversion rate, dimensionless;
carrying out a pyrolysis gas chromatography (PY-GC) test on an immature shale sample from a target region, and calibrating chemical kinetics parameters of a primary cracking of kerogen in the target region based on results of the pyrolysis gas chromatography test and principles of chemical kinetics so as to determine a conversion rate F t of the target region according to a burial history and thermal history of the target region;
determining an original hydrogen index HI 0 of the target region according to geochemical data of the target region; and
obtaining a type of kerogen, a kerogen mass and a current evolution stage of the target region to determine a kerogen swelling oil mass in the target region based on the conversion rate and the original hydrogen index of the target region.
2 . The method according to claim 1 , wherein the determining per-unit kerogen swelling oil mass according to the kerogen swelling oil mass comprises:
deriving the mass of the kerogen; and
determining the per-unit kerogen swelling oil mass according to the kerogen adsorbed oil mass and the kerogen mass.