CN-121838901-B - Method for externally pushing hydrate dissociation time under high methane concentration condition

CN121838901BCN 121838901 BCN121838901 BCN 121838901BCN-121838901-B

Abstract

The invention provides a method for extrapolating the dissociation time of a hydrate under the condition of high methane concentration, which is based on the principle of sequence consistency and time self-similarity of cage structure rupture events in the dissociation process of the hydrate, and on the premise of consistent initial and final dissociation states, the phase concentration of a methane aqueous solution mainly modulates the absolute rate of dissociation, and the relative time duty ratio of different spatial layers/structural stages in the total dissociation process is kept unchanged. Under the condition that microsecond to tens of microsecond full-course molecular dynamics simulation is not needed, high-efficiency prediction of long-time dissociation behavior of the hydrate under the conditions of high concentration and near balance is realized. Compared with the traditional mode, the method can keep the staged non-uniform characteristic of the dissociation process, obviously reduce the calculation cost, reduce extrapolation deviation and improve the capturing capability and result reliability of dissociation time sudden increase phenomenon under the near-balance high-concentration condition.

Inventors

Guo Muzhi
ZHONG JIE
ZHANG JUN
LIU ZHIYUAN
Dong Bingxia
WANG GUANGYAO

Assignees

中国石油大学(华东)

Dates

Publication Date: 20260508
Application Date: 20260313

Claims (8)

1. A method for extrapolating the dissociation time of a hydrate under conditions of high methane concentration, comprising the steps of: S1, constructing an initial molecular dynamics system model comprising a methane hydrate phase and a methane water solution phase, setting the external temperature, the external pressure and the concentration of the methane water solution phase, and determining the initial solid-liquid interface position, wherein the space layer is divided along the normal direction on the basis of the initial solid-liquid interface, and the space layer at least comprises an unstable boundary layer R 0 , a reference area R 1 and target research areas R target ,R 1 and R target which are effective research areas after R 0 is removed; S2, carrying out reference track molecular dynamics simulation under the condition that the mole fraction of methane in the aqueous solution phase of methane is less than or equal to 0.1%, continuously tracking the dissociation process of R 1 and R target , and respectively recording the characteristic time of complete dissociation of R 1 Characteristic time of complete dissociation of R target ; S3, performing estimated trajectory molecular dynamics simulation under the condition that the mole fraction of methane in the aqueous solution phase of methane is more than or equal to 0.3 percent, simulating until R 1 is completely dissociated, and recording the characteristic time of the complete dissociation of R 1 ; S4, based on the time self-similar assumption of the hydrate dissociation process, the relative time duty ratio of each space layer in the total dissociation process is not changed along with the change of the concentration of liquid-phase methane, and the following scaling relationship is established: ; And calculating the total dissociation time of R target under the condition of high methane concentration As a result of predicting the dissociation time of the target study region R target under the condition of high methane concentration.
2. The method for extrapolating the dissociation time of a hydrate under conditions of high methane concentration according to claim 1, wherein the unstable boundary layer R 0 is a hydrate layer in the range of 0-0.6nm from the initial solid-liquid interface, R 1 and R target are effective investigation regions after the unstable boundary layer R 0 is excluded, R 1 is in the range of 0.6-1.2nm from the initial solid-liquid interface, and R target is in the range of 0.6-1.8nm from the initial solid-liquid interface.
3. The method for extrapolating the dissociation time of a hydrate under conditions of high methane concentration as claimed in claim 1, wherein the initial solid-liquid interface position is determined by any one of the following means: a. The water density, the methane density or the position of the mutation of the partial order parameter profile along the normal direction of the interface; b. the transition positions of the hydrate cage structure distribution from high to low are identified through a crystal nucleus identification algorithm; c. the resulting interface plane is fitted based on the geometric boundaries of the hydrate lattice layer in the initial configuration.
4. The method for extrapolating the time to dissociation of a hydrate under conditions of high methane concentration as claimed in claim 1, wherein the determination of complete dissociation of R 1 and complete dissociation of R target uses at least one of the following structural criteria: a. The number of complete hydrate cage structures in the region is reduced to zero; b. the tetrahedral order parameter of the water molecules in the region is reduced to a liquid threshold value F4= -0.04; c. the characteristic peaks of the crystal lattice of the hydrate in the region disappear, or the density/potential energy in the region reaches and is stabilized on a liquid platform; d. Complete disintegration is determined by molecular connectivity or cage network connectivity to the region.
5. The method for extrapolating the time of dissociation of a hydrate under conditions of elevated methane concentration as claimed in claim 1, wherein the time self-similarity assumption is constrained by the consistency of a sequence of cage structure disruption events during dissociation of the hydrate, said sequence of events comprising at least lattice interface complete cage disruption, partial cage duty cycle increase, cage network connectivity disruption and complete liquefaction of the region, and ensuring that the relative time duty cycle remains unchanged at different methane concentrations with the consistency.
6. The method for extrapolating the time of dissociation of a hydrate under conditions of high methane concentration as claimed in claim 5, wherein the time self-similarity of the reference trajectory molecular dynamics simulation and the estimated trajectory molecular dynamics simulation of the dissociation of the hydrate assumes that the following conditions are satisfied: a. The two adopt the same initial geometric configuration, temperature and pressure conditions, and only the concentration of liquid-phase methane is different; b. The initial dissociation state and the final dissociation state of the two are consistent, and the final dissociation state at least comprises complete dissociation of R target ; c. The molecular dynamics of the reference trajectory simulates the selected aqueous methane solution phase concentration such that R target completes dissociation within an acceptable calculated time period to obtain integrity 。
7. The method for extrapolating the dissociation time of the hydrate under the condition of high methane concentration according to claim 1, wherein the molecular dynamics simulation in S2 and S3 adopts an isothermal and isobaric ensemble, the temperature and the pressure are kept stable through a temperature and pressure control algorithm, the time step is in the order of femtosecond, and a periodic boundary condition is adopted, and the simulation duration covers at least R target complete dissociation in the reference trajectory molecular dynamics simulation and at least R 1 complete dissociation in the estimated trajectory molecular dynamics simulation.
8. The method for extrapolating the dissociation time of the hydrate under the condition of high methane concentration as set forth in claim 1, further comprising performing repeated trace statistics on the reference trace molecular dynamics simulation process by performing a plurality of independent reference trace molecular dynamics simulations under the same temperature, pressure and concentration conditions, respectively Average and dispersion of (2), and based thereon As a result of (a).

Description

Method for externally pushing hydrate dissociation time under high methane concentration condition Technical Field The invention relates to the technical field of molecular simulation, in particular to a method for extrapolating the dissociation time of hydrate under the condition of high methane concentration. Background Natural gas hydrates are widely existing in submarine sedimentary formations, permafrost regions and deep-sea oil and gas environments, and can be formed and dissociated during natural gas exploitation, submarine pipeline transportation, well bore and gas production system operation. The formation and dissociation of the hydrate can obviously influence the flow safety, the recovery efficiency and the engineering risk assessment, so that the accurate characterization of the dissociation dynamics rule of the hydrate under the conditions of different temperatures, pressures and liquid-phase methane concentration has important significance. At present, molecular dynamics simulation is an important means for researching hydrate dissociation mechanism because of being capable of analyzing microscopic processes such as hydrate cage structure rupture, interface evolution, gas diffusion and the like on a molecular scale. However, as the methane concentration in the liquid phase increases and approaches equilibrium concentrations, the hydrate dissociation time increases significantly non-linearly, often extending from nanosecond scale to microsecond or even longer scale. Since classical molecular dynamics simulation is limited by affordable computational costs, direct simulation of complete dissociation of the target region at high methane concentrations typically requires a large amount of computational resources, and it is difficult to perform enough repetitive trajectories to obtain reliable statistics, thus limiting systematic studies on dissociation kinetics in near-equilibrium high concentration intervals. In addition, when constructing a "hydrate-water solution" interface model, rapid transient collapse often occurs near the initial interface due to excessive interfacial energy, forming an unstable boundary layer. If the boundary layer is incorporated directly into the dynamics statistics, non-steady state disturbances are introduced and the comparability and repeatability between different trajectories are reduced. The prior method generally lacks effective isolation of the transient effect and standardized definition of a target research area, and further weakens the reliability of dissociation dynamics data under the condition of high concentration. On the other hand, the dissociation of the hydrate does not proceed at a uniform rate. The phenomenon of slow phase alternation, which is dominated by the structurally complete cage, and fast phase alternation, which is dominated by the partially open cage/defect interface, often occurs in the actual trajectory, resulting in that the dissociation time cannot be linearly extrapolated from the initial short-time data. Thus, there is a need for a method that automatically and robustly extrapolates the complete dissociation time of a target investigation region at high methane concentrations, especially near equilibrium conditions, using available local dissociation information without the need for ultra-long direct Molecular Dynamics (MD) simulation. Disclosure of Invention In view of the above, the invention aims to provide a method for externally pushing the dissociation time of a hydrate under the condition of high methane concentration, which is based on the dynamic similarity scaling principle, so as to solve the problems that the dissociation time is too long under the condition of high methane concentration in the existing hydrate dissociation research, the direct molecular dynamics simulation calculation cost is too high, the adequate repeated sampling is difficult to carry out, and the traditional fitting extrapolation generates systematic underestimation in a near-balance high concentration interval. The invention utilizes the principle of stability and time self-similarity of a cage structure rupture event sequence in the hydrate dissociation process, namely, on the premise that initial dissociation state is consistent with final dissociation state, the concentration of liquid-phase methane mainly changes the absolute rate of dissociation, and the relative time duty ratio of different spatial layers/structural stages in the total dissociation process is kept unchanged. By acquiring the space layer dissociation time proportion in the low-concentration reference track and only explicitly simulating the dissociation time of the reachable subareas in the high-concentration estimation track, the invention can reliably calculate the complete dissociation time of the target research area under the high-concentration (especially near-equilibrium) condition without performing microsecond or even longer-scale whole-course simulation, thereby redu