CN114626237B - Method for calculating critical temperature and critical density of alkane fluid in limited space - Google Patents
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- 238000000034 method Methods 0.000 title claims abstract description 33
- 150000001335 aliphatic alkanes Chemical class 0.000 title claims abstract description 25
- VNWKTOKETHGBQD-UHFFFAOYSA-N methane Chemical compound C VNWKTOKETHGBQD-UHFFFAOYSA-N 0.000 claims abstract description 82
- 238000004364 calculation method Methods 0.000 claims abstract description 43
- VYPSYNLAJGMNEJ-UHFFFAOYSA-N Silicium dioxide Chemical compound O=[Si]=O VYPSYNLAJGMNEJ-UHFFFAOYSA-N 0.000 claims abstract description 25
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- 238000011161 development Methods 0.000 abstract description 6
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- OTMSDBZUPAUEDD-UHFFFAOYSA-N Ethane Chemical compound CC OTMSDBZUPAUEDD-UHFFFAOYSA-N 0.000 description 1
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- 229910052710 silicon Inorganic materials 0.000 description 1
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Abstract
The invention relates to a method for calculating critical temperature and critical density of alkane fluid in a limited space, which comprises the following steps: step 1) adopting MATERIALS STUDIO software to build a silicon dioxide model to generate an atomic coordinate file and a force field parameter file; step 2) adopting MATERIALS STUDIO software to establish a methane fluid structure, and generating an atomic coordinate file and a force field parameter file; step 3) defining Monte Carlo simulation calculation parameter keywords in a calculation parameter control file in.conf by using open source software GPU Optimized Monte Carlo; step 4) roughly calculating the temperature and the pressure in a large range; step 5), fine calculation under a small temperature and pressure range; and 6) extracting the gas-liquid phase change density at each temperature according to the rho-P phase diagram of each temperature and pressure in the remote critical region to obtain a rho-T diagram, and carrying out iterative solution by using a linear diameter law and a density scale law to obtain the critical temperature T c and the critical density rho c of methane in the limited space. The invention has reliable principle, high precision, simplicity, convenience and applicability, and has important practical significance and wide application prospect for the development of unconventional oil and gas reservoirs in China.
Description
Technical Field
The invention relates to a molecular simulation method for testing critical properties of a limited space alkane fluid in the petroleum and natural gas industry, in particular to a calculation method for the critical temperature and critical density of the actual alkane fluid in a limited space.
Background
In recent years, with the improvement of the technical level of oil and gas reservoir exploration and development, the proportion of a large number of unconventional oil and gas reservoirs in the oil and gas development field of China is increased, and the research is widely focused by researchers, wherein the fluid phase experimental test is one of hot spots and difficulties. Unconventional oil and gas reservoirs generally have the characteristics of small rock skeleton particles, large specific surface area and micro-nano pore development, and a plurality of complex interface effect problems exist between fluid and reservoir pore media, so that the change rule of the phase state of the fluid in the micro-nano pore media is different from that of the conventional oil and gas reservoir fluid. The conventional oil and gas reservoir fluid phase experimental test is generally carried out according to the national standard of oil and gas reservoir fluid physical property analysis method GB/T26981-2020, and a Canadian DBR fluid phase tester, french Mo Ji or ST company fluid phase tester are mostly adopted as an experimental instrument, and underground oil and gas reservoir fluid is injected into a PVT cylinder of the tester to directly observe the phase change of the fluid; however, the forces between the fluid and the solid wall are weak, and it is difficult to simulate the interaction between unconventional reservoir pore media (nano-scale) and reservoir fluids.
In recent years, researchers apply the nano experiment technical method to the micro-pore medium fluid phase test, the current experiment method mainly comprises an adsorption-desorption method, a Differential Scanning Calorimetry (DSC), a diffusion method, a nano channel chip method and the like [Liu X,Zhang D.A review of phase behavior simulation of hydrocarbons in confined space:Implications for shale oil and shale gas[J].Journal of Natural Gas Science and Engineering,2019:102901],, the manufacturing cost of instruments and equipment related to the experiment method is high, and the methods have advantages and disadvantages, for example, the nano channel chip experiment belongs to a direct experiment test method, and is a recommended experiment test method for researching micro-nano pore channel fluid phase behaviors at present because of being capable of realizing microscale, high temperature and high pressure and visualization, and the saturation pressure determination precision is higher, but is influenced by space, the phase parameters related to the volume are difficult to obtain, and the phase parameters are required to be combined with theoretical calculation.
The characteristic of unconventional reservoir micro-nano pore development determines that research means should be considered from a micro-nano scale research tool, and molecular simulation is not at the right as an authority in a certain field in the micro-nano scale research tool. The molecular simulation method calculates thermodynamic parameters from microscopic state distribution, and can embody the influence of micro-nano scale interface effect. In recent years, molecular simulation is widely applied to research on interfacial effects of fluids in the oil and gas field and interfacial characteristics in micro-nano pores, partial researchers research on density distribution of different alkanes in a limited state and interfacial properties [Wang R,F Peng,Song K,et al.Molecular dynamics study of interfacial properties in CO2 enhanced oil recovery[J].Fluid Phase Equilibria,2018:S0378381218301225], such as density distribution, interfacial tension, minimum miscible pressure and the like of a mixed fluid of non-hydrocarbon gas and alkanes in a limited state, while critical properties (critical temperature and critical pressure) of a limited space hydrocarbon fluid are in an exploration stage, pitakbunkate et al [Pitakbunkate T,Balbuena P B,Moridis G J,et al.Effect of confinement on pressure/volume/temperature properties of hydrocarbons in shale reservoirs[J].SPE Journal,2016,21(02):621-634] research on methane and ethane phase diagrams in an organic matter lamellar by adopting a giant regular ensemble, but the calculation amount is huge by setting orthogonal calculation modes of dozens of temperature points and dozens of pressure points at each temperature point, and in addition, phase envelope division in the research has strong subjectivity. Therefore, the calculation method of the critical properties of the alkane fluid in the limited space is necessary to sublimate into a set of theoretical method, and a foundation is laid for the follow-up accurate, standard and rapid simulation of the single-component alkane fluid and the actual hydrocarbon fluid in the limited space.
Disclosure of Invention
The invention aims to provide a calculation method for simulating and testing the critical properties of alkane fluid in a limited space, which has reliable principle, high precision, simplicity, convenience and applicability, obtains the phase change of the oil-gas fluid in a micro-pore medium of an unconventional oil-gas reservoir by simulating the critical temperature and critical density of the oil-gas fluid in micro-nano pores, and has important practical significance and wide application prospect for the development of the unconventional oil-gas reservoir in China.
In order to achieve the technical purpose, the invention adopts the following technical scheme.
The method for calculating the critical temperature and critical density of the alkane fluid in the limited space sequentially comprises the following steps:
(1) Model building and writing of a calculation file in.conf:
step 1) adopting MATERIALS STUDIO software to build a silicon dioxide model, generating an atomic coordinate file and a force field parameter file, wherein the process is as follows:
The unit cell parameters used were siθ2_quatetz. Xsd in MATERIALS STUDIO software crystal library: a=0.4913 nm, b=0.4913 nm, c= 0.54052nm, α=90°, β=90° and γ=120°. And (0) surface cutting is carried out on the silica original unit cell, then a slit model (slit model interlayer spacing is nano aperture) is constructed by a supersporulation and layer structure building method, and a structure atomic coordinate file BOX0.pdb and a topology file BOX0.psf are derived after the model is constructed. According to Clayff force field, the action parameters of Si atom, bridging O atom, hydroxyl O atom and H atom are respectively defined in a force field parameter file Par.
Step 2) adopting MATERIALS STUDIO software to establish an alkane fluid structure (taking methane as an example) and generating an atomic coordinate file and a force field parameter file, wherein the process is as follows:
And constructing a methane united atomic model by using MATERIALS STUDIO software, wherein the methane united atomic model uses a carbon atom to replace methane, 3000 methane molecules are set to be fluid particle sources, and a fluid atomic coordinate file BOX1.Pdb and a topology file BOX1.Psf are derived after the model is constructed. Methane uses TraPPE-UA force field, and the action parameters of methane are written in the Par.inp in the step 1).
Step 3) performing Gibbs comprehensive Monte Carlo simulation by using open source software GPU Optimized Monte Carlo, and defining Monte Carlo simulation calculation parameter keywords in a calculation parameter control file in.conf, wherein the process is as follows:
Writing a Monte Carlo algorithm type 'GEMC' in a calculation parameter control file in.conf; translational frequency DisFreq, rotational frequency RotFreq, exchange frequency SwapFreq/INTRASWAPFREQ; simulating the values of the target Temperature 'and the target Pressure'; the addition mode of coulomb effect and electrostatic effect; analog output step number setting.
(2) Rough calculation and fine calculation under each temperature and pressure:
Step 4) rough calculation under wide temperature and pressure ranges: and determining a large range of calculated temperature and pressure in the limited space according to the critical property of the methane body phase, and performing simulation. Inquiring saturated vapor Pressure, critical Temperature and critical Pressure of methane phase through NIST website, determining a Temperature calculation interval of 100K-200K, a Pressure calculation interval of 0.2 MPa-3.2 MPa (2 bar-32 bar), setting each Temperature and Pressure at a Temperature and Pressure position in a parameter control file in.conf calculated in step 3), specifically, setting the Temperature once every 10K from 100K until 200K is finished, and 11 Temperature values in total; at each temperature value, the pressure starts at 2bar, and a calculated pressure point is set every 2bar, to 32bar end.
Step 5) fine calculation under a small temperature and pressure range: and simulating to obtain a phase diagram under a large temperature and pressure range, determining a phase change region according to the gas-liquid phase, and then performing fine simulation under a small pressure difference. Simulating to obtain a rho-P phase diagram of methane in a silicon dioxide slit at a temperature calculation interval of 100K-200K and a pressure calculation interval of 0.2 MPa-3.2 MPa, determining a gas-liquid phase pressure transformation interval at each temperature, determining a phase transformation pressure interval at each temperature in a remote critical area, and then performing fine simulation under a small pressure difference (the pressure step length is 0.1 bar).
(3) Determination of methane critical temperature and critical density in confined space:
Step 6) fine simulation to obtain rho-P phase diagrams under each temperature and pressure of a far critical region, extracting gas-liquid phase densities under each temperature to obtain rho-T diagrams, regarding methane under a limited space as a special substance, iteratively solving by using a linear diameter law and a density scale law, and predicting critical temperature and critical density:
ρlip-ρvap＝B(T-TC)β
Wherein β=0.325;
ρ lip: a liquid phase density;
ρ vap: a gas phase density;
ρ c: critical density;
t c: a critical temperature;
T: calculating the temperature;
A. b: a and B are constants for saturated vapor and fluid densities.
And calculating to obtain the critical temperature T c and critical density rho c of methane in the limited space.
Compared with the prior art, the invention has the following beneficial effects:
(1) The present invention uses open source software GPU Optimized Monte Carlo (GOMC) to perform Gibbs ensemble Monte Carlo simulation, and fluid phase change can be obtained for a given temperature according to a series of pressure simulations; the simulation calculation speed is high, and the calculation and analysis cost is low.
(2) According to the phase change diagram calculation method, the phase change diagram under a large temperature difference is obtained through the calculation of the universal temperature pressure boundary, the phase change region is subjected to fine simulation after the phase change region is determined, and the huge workload of point-by-point calculation under a small temperature difference is reduced.
(3) According to the invention, fluid in a limited space is regarded as a specific substance, and the critical temperature and critical density are obtained by using the linear diameter law and the density scale law to carry out iterative solution, so that the subjective influence of artificial division of an envelope curve is reduced.
Drawings
FIG. 1 is a schematic diagram of an analog architecture.
FIG. 2 is a phase diagram of limited methane at a wide range of temperature pressures.
Fig. 3 is a fine simulated phase diagram of a restricted methane phase change zone.
FIG. 4 is a graph of a restricted methane far critical zone ρ -T.
Detailed Description
It should be noted that the following detailed description is exemplary and is intended to provide further explanation of the invention as well as to facilitate a person skilled in the art to understand the invention. It should be understood that the invention is not limited to the precise embodiments, and that various changes may be effected therein by one of ordinary skill in the art without departing from the spirit or scope of the invention as defined and determined by the appended claims.
Examples
The calculation method for simulating and testing the critical temperature and critical density of the alkane fluid in the limited space, which takes methane as an example, comprises the following steps:
Step 1) SiO2_quatetz. Xsd in MATERIALS STUDIO software crystal library was used, and the unit cell parameters included: a=0.4913 nm, b=0.4913 nm, c= 0.54052nm, α=90°, β=90° and γ=120°. The silica original unit cell was subjected to (0 0.1) surface cutting, then a slit model was constructed by supersporulation and layer structure building methods, and the layer spacing was adjusted to 4nm, as shown in fig. 1 (a). And (3) deriving a silicon dioxide slit structure, and generating a slit atomic coordinate file BOX0.pdb and a topology file BOX0.psf through VMD conversion. The quartz adopts Clayff force field, and the non-bond interaction parameters of Si atoms, bridging O atoms, hydroxyl O atoms and H atoms are respectively defined in a force field parameter file Par.
Step 2) a methane co-atomic model was constructed using MATERIALS STUDIO, which uses one carbon atom instead of methane, and sets 3000 methane molecules as the fluid particle source, as shown in fig. 1 (b). And (3) deriving a fluid structure, and generating a fluid atomic coordinate file BOX1.Pdb and a topology file BOX1.Psf through VMD conversion. Methane uses TraPPE-UA force field, and the interaction parameter of adding methane is written in the Par.inp in the step 1).
Step 3) writing a Monte Carlo algorithm type 'GEMC' in the calculation file in.conf; translational frequency DisFreq, rotational frequency RotFreq, exchange frequency SwapFreq/INTRASWAPFREQ; simulating the values of the target Temperature 'and the target Pressure'; the addition mode of coulomb effect and electrostatic effect; the total number of simulated steps was set to 5.0×10 6 steps, with the first 1.0×10 6 steps for equilibration and the last 4.0×10 6 steps for statistical analysis.
Step 4) inquiring the saturated vapor Pressure, the critical Temperature and the critical Pressure of methane through an NIST website, determining a Temperature calculation interval of 100K-200K in a limited space, wherein the Pressure calculation interval is 0.2 MPa-3.2 MPa (2 bar-32 bar), setting each Temperature and each Pressure in a calculation parameter control file in.conf in step 3), and simulating, specifically, setting the Temperature from 100K, once every 10K until 200K is finished, and 11 Temperature values in total; at each temperature value, the pressure starts at 2bar, and a calculated pressure point is set every 2bar, to 32bar end.
Step 5), simulating to obtain a rho-P phase diagram of methane in a 4nm quartz slit at a temperature calculation interval of 100K-200K and a pressure calculation interval of 0.2 MPa-3.2 MPa, as shown in figure 2; it can be seen that at a temperature of 140K, methane undergoes a gas-liquid change between 2bar and 4 bar; at a temperature of 145K, methane undergoes a gas-liquid change between 3bar and 5 bar; similarly, the gas-liquid change is obvious under the far critical condition, so that the phase change pressure interval under the temperature of 140K-160K is determined, and then the fine simulation under the small pressure difference (the pressure step length is 0.1 bar) is performed.
Step 6) fine simulation is carried out to obtain a rho-P phase diagram under each temperature and pressure in a remote critical region, as shown in fig. 3, then the gas-liquid phase change density under each temperature is extracted to obtain a rho-T diagram, as shown in a phase change simulation value in fig. 4, and a linear diameter law and a density calibration law are adopted to predict critical temperature and critical density:
ρlip-ρvap＝B(T-TC)β
Wherein β=0.325;
ρ lip: a liquid phase density;
ρ vap: a gas phase density;
ρ c: critical density;
t c: a critical temperature;
T: calculating the temperature;
A. b: a and B are constants for saturated vapor and fluid densities.
The calculated methane critical temperature 169.49K and critical density 228.94kg/m 3 in the 4nm quartz slit are obviously different from the bulk critical point (bulk critical point in FIG. 4) as shown by the limited phase critical point in FIG. 4.
Further, the structure of the fluid substance is changed, and the critical temperature and critical density of different monocomponent alkanes in the nano holes with given aperture can be obtained by simulation by adopting the method, so that the influence of the nano pore medium on alkanes with different lengths and the critical property deviation characteristic of each monocomponent alkane are clarified.
Furthermore, the aperture of the nano-pore is changed, and the critical temperature and critical density of alkane in different nano-pores can be obtained by simulation by adopting the method, so that the minimum aperture which is not influenced by the nano-structure is obtained by comparing with the bulk value.
Furthermore, the silica is changed to construct nano holes for other minerals, and the method can be used for simulating and obtaining the critical temperature and critical density of the alkane in the nano holes of different minerals so as to analyze the influence of different reservoir minerals on the critical property of the alkane.
Furthermore, the fluid is constructed according to the composition of the oil and gas well fluid, and the critical temperature and critical density of the oil and gas in the nano pore medium can be obtained by simulation by adopting the method, so that a reference basis is provided for modifying the micro-nano oil and gas phase state equation.
The foregoing description is only a preferred embodiment of the present invention, and is not intended to limit the present invention in any way, and any simple modification, equivalent variation, etc. of the above embodiment according to the technical matter of the present invention fall within the scope of the present invention.
Claims (5)
1. The method for calculating the critical temperature and critical density of the alkane fluid in the limited space is characterized by comprising the following steps in sequence:
step 1) adopting MATERIALS STUDIO software to build a silicon dioxide model, generating an atomic coordinate file and a force field parameter file, wherein the process is as follows:
The unit cell parameters used were siθ2_quatetz. Xsd in MATERIALS STUDIO software crystal library: a=0.4913 nm, b=0.4913 nm, c= 0.54052nm, α=90°, β=90° and γ=120°; cutting the surface of a silica original unit cell (0 0.1), constructing a slit model by a supersporulation and layer structure building method, deriving a structure atom coordinate file BOX0.pdb and a topology file BOX0.psf, and respectively defining the action parameters of Si atoms, bridging O atoms, hydroxyl O atoms and H atoms in a force field parameter file Par.inp according to a Clayff force field;
and 2) adopting MATERIALS STUDIO software to establish a methane fluid structure, generating an atomic coordinate file and a force field parameter file, wherein the process is as follows:
Establishing a methane united atomic model by using MATERIALS STUDIO software, wherein the methane united atomic model uses a carbon atom to replace methane, sets 3000 methane molecules as fluid particle sources, derives a fluid atomic coordinate file BOX1.Pdb and a topology file BOX1.Psf, adopts a TraPPE-UA force field, and writes the action parameters of the methane in the Par.inp in the step 1);
Step 3) using open source software GPU Optimized Monte Carlo to define Monte Carlo simulation calculation parameter keywords in a calculation parameter control file in.conf, the process is as follows:
Writing a Monte Carlo algorithm type 'GEMC' in a calculation parameter control file in.conf; translational frequency DisFreq, rotational frequency RotFreq, exchange frequency SwapFreq/INTRASWAPFREQ; simulating the values of the target Temperature 'and the target Pressure'; the addition mode of coulomb effect and electrostatic effect; setting the analog output steps;
step 4) rough calculation under a wide temperature and pressure range, wherein the process is as follows:
According to saturated vapor Pressure, critical Temperature and critical Pressure of methane phase, determining a Temperature calculation interval of 100K-200K and a Pressure calculation interval of 2 bar-32 bar, setting each Temperature and Pressure in a Temperature and Pressure part in a parameter calculation control file in. Conf in the step 3) for simulation, setting the Temperature once every 10K from 100K until 200K is finished, and 11 Temperature values in total; at each temperature value, the pressure starts from 2bar, and a calculated pressure point is set every 2bar until 32bar ends;
step 5) accurate calculation under a small temperature and pressure range, wherein the process is as follows:
simulating to obtain a rho-P phase diagram of methane in a silicon dioxide slit at a temperature calculation interval of 100K-200K and a pressure calculation interval of 2 bar-32 bar, determining a gas-liquid phase pressure change interval at each temperature, determining a phase change pressure interval at each temperature in a far critical area, and performing fine simulation under a small pressure difference, namely, a pressure step length of 0.1 bar;
Step 6) fine simulation is carried out to obtain a rho-P phase diagram under each temperature and pressure of a remote critical zone, then the gas-liquid phase density under each temperature is extracted to obtain a rho-T diagram, and iterative solution is carried out by using a linear diameter law and a density scale law to obtain a methane critical temperature T c and a critical density rho c in a limited space:
ρlip-ρvap＝B(T-TC)β
wherein β=0.325;
ρ lip: a liquid phase density;
ρ vap: a gas phase density;
ρ c: critical density;
t c: a critical temperature;
T: calculating the temperature;
A. B: constants for saturated vapor, fluid density.
2. The method for calculating critical temperature and critical density of constrained space alkane fluid according to claim 1, wherein the critical temperature and critical density of different monocomponent alkanes in a given pore size nanopore are simulated by modifying the fluid structure.
3. The method of claim 1, wherein the pore size of the nanopores is modified to simulate the critical temperature and critical density of the alkane in different nanopores.
4. The method of claim 1, wherein the silica is modified to create nanopores for other minerals to simulate the critical temperature and critical density of the alkane in the nanopores of different minerals.
5. The method for calculating critical temperature and critical density of confined space alkane fluid according to claim 1, wherein the fluid is constructed according to the composition of the well stream, and the critical temperature and critical density of the oil and gas in the nanoporous medium are obtained by simulation.
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