WO2023071303A1 - 一种定量预测二氧化碳强化气藏开采和封存的方法 - Google Patents

一种定量预测二氧化碳强化气藏开采和封存的方法 Download PDF

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WO2023071303A1
WO2023071303A1 PCT/CN2022/105876 CN2022105876W WO2023071303A1 WO 2023071303 A1 WO2023071303 A1 WO 2023071303A1 CN 2022105876 W CN2022105876 W CN 2022105876W WO 2023071303 A1 WO2023071303 A1 WO 2023071303A1
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density
adsorption
pores
rock
gas
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French (fr)
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周娟
荆铁亚
刘练波
郜时旺
牛红伟
郭东方
赵文韬
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中国华能集团清洁能源技术研究院有限公司
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Priority to DE112022000342.1T priority Critical patent/DE112022000342T5/de
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    • G—PHYSICS
    • G16—INFORMATION AND COMMUNICATION TECHNOLOGY [ICT] SPECIALLY ADAPTED FOR SPECIFIC APPLICATION FIELDS
    • G16C—COMPUTATIONAL CHEMISTRY; CHEMOINFORMATICS; COMPUTATIONAL MATERIALS SCIENCE
    • G16C20/00—Chemoinformatics, i.e. ICT specially adapted for the handling of physicochemical or structural data of chemical particles, elements, compounds or mixtures
    • G16C20/10—Analysis or design of chemical reactions, syntheses or processes
    • G—PHYSICS
    • G06—COMPUTING; CALCULATING OR COUNTING
    • G06F—ELECTRIC DIGITAL DATA PROCESSING
    • G06F30/00—Computer-aided design [CAD]
    • G06F30/20—Design optimisation, verification or simulation
    • E—FIXED CONSTRUCTIONS
    • E21—EARTH OR ROCK DRILLING; MINING
    • E21B—EARTH OR ROCK DRILLING; OBTAINING OIL, GAS, WATER, SOLUBLE OR MELTABLE MATERIALS OR A SLURRY OF MINERALS FROM WELLS
    • E21B41/00—Equipment or details not covered by groups E21B15/00 - E21B40/00
    • E21B41/005—Waste disposal systems
    • E21B41/0057—Disposal of a fluid by injection into a subterranean formation
    • E21B41/0064—Carbon dioxide sequestration
    • E—FIXED CONSTRUCTIONS
    • E21—EARTH OR ROCK DRILLING; MINING
    • E21B—EARTH OR ROCK DRILLING; OBTAINING OIL, GAS, WATER, SOLUBLE OR MELTABLE MATERIALS OR A SLURRY OF MINERALS FROM WELLS
    • E21B43/00—Methods or apparatus for obtaining oil, gas, water, soluble or meltable materials or a slurry of minerals from wells
    • E21B43/16—Enhanced recovery methods for obtaining hydrocarbons
    • E21B43/164—Injecting CO2 or carbonated water
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N15/00—Investigating characteristics of particles; Investigating permeability, pore-volume or surface-area of porous materials
    • G01N15/08—Investigating permeability, pore-volume, or surface area of porous materials
    • G01N15/088—Investigating volume, surface area, size or distribution of pores; Porosimetry
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2253/00—Adsorbents used in seperation treatment of gases and vapours
    • B01D2253/10—Inorganic adsorbents
    • B01D2253/106—Silica or silicates
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2259/00—Type of treatment
    • B01D2259/45—Gas separation or purification devices adapted for specific applications
    • B01D2259/4525—Gas separation or purification devices adapted for specific applications for storage and dispensing systems
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
    • B01D53/02—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols by adsorption, e.g. preparative gas chromatography
    • B01D53/04—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols by adsorption, e.g. preparative gas chromatography with stationary adsorbents
    • B01D53/0454—Controlling adsorption
    • E—FIXED CONSTRUCTIONS
    • E21—EARTH OR ROCK DRILLING; MINING
    • E21B—EARTH OR ROCK DRILLING; OBTAINING OIL, GAS, WATER, SOLUBLE OR MELTABLE MATERIALS OR A SLURRY OF MINERALS FROM WELLS
    • E21B2200/00—Special features related to earth drilling for obtaining oil, gas or water
    • E21B2200/20—Computer models or simulations, e.g. for reservoirs under production, drill bits
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N9/00—Investigating density or specific gravity of materials; Analysing materials by determining density or specific gravity
    • G01N9/36—Analysing materials by measuring the density or specific gravity, e.g. determining quantity of moisture
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02C—CAPTURE, STORAGE, SEQUESTRATION OR DISPOSAL OF GREENHOUSE GASES [GHG]
    • Y02C20/00—Capture or disposal of greenhouse gases
    • Y02C20/40—Capture or disposal of greenhouse gases of CO2

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  • the invention belongs to the technical field of utilization and sequestration of carbon dioxide, in particular to a method for quantitatively predicting the exploitation and sequestration of carbon dioxide-enhanced gas reservoirs.
  • the object of the present invention is to provide a method for quantitatively predicting the recovery and storage of carbon dioxide-enhanced gas reservoirs.
  • the method of the present invention can simulate the production process consisting of multiple CO2 huff-puff stages, and can quantitatively predict the CH4 in each stage
  • the recovery factor and CO2 storage rate based on which the entire extraction path can be optimized with economical considerations in mind.
  • the present invention provides a method for quantitatively predicting carbon dioxide enhanced gas reservoir exploitation and storage, comprising the following steps:
  • the molecular dynamics method is used to construct the structure model of the rock pores, and the structure is optimized;
  • the system is depressurized, and the residual CH4 density in the pores and the sequestered CO2 density are obtained from the adsorption isotherm;
  • ⁇ rec represents the recovery factor of CH4 ;
  • ⁇ initial is the average density of CH4 in the rock pores under the initial pressure of the reservoir, and
  • ⁇ residual,p is the average density of residual CH4 in the rock pores under the pressure p;
  • ⁇ sec represents the CO 2 storage rate
  • ⁇ CO2 is the average density of pure CO 2 in the rock pores under the initial reservoir pressure
  • ⁇ CO2,p is the average density of CO 2 stored in the rock pores under the pressure p;
  • step C) Repeat step C) multiple times to obtain the residual CH 4 density and the stored CO 2 density ⁇ CO2,p in the pores of each stage, and calculate the CH 4 recovery rate and CO 2 storage of each stage according to formula I and formula II Rate;
  • the structural models of methane and carbon dioxide molecules are constructed first, and the structures are optimized, and then the simulation of the adsorption process in step B) is carried out.
  • the giant canonical Monte Carlo method is used to simulate the adsorption process of CO2 and CH4 gas in the rock pore structure model obtained in the step A), and the adsorption process of CO2 and CH4 gas in the rock pores under different pressures is obtained.
  • the adsorption amount the adsorption isotherm at the same temperature and different pressure is obtained, and the adsorption isotherm model is obtained by curve fitting the data.
  • a structural model of rock pores is constructed according to the data of influencing factors in the collected reservoir;
  • the influencing factors in the reservoir include one or more of water content, salt content and pore characteristics.
  • multiple groups of adsorption isotherms are formed from the adsorption data obtained by multiple groups of different proportions of CO2 and CH4 mixed gas to obtain an adsorption map, and the CH4 recovery rate and CO2 storage rate.
  • the invention provides a method for quantitatively predicting the exploitation and storage of carbon dioxide-enhanced gas reservoirs, comprising the following steps: A) using molecular dynamics to construct a rock pore structure model conforming to the rock characteristics of the target gas reservoir, and optimizing the structure; B) The giant canonical Monte Carlo method is used to simulate the adsorption process of CO2 and CH4 gas in the rock pore structure model obtained in the step A), and the CH4 single component, CH4 / CO2 mixed gas, CH4 under different pressures are calculated, The adsorption isotherm of CO 2 single component in rock pores; C) According to the initial reservoir pressure and depressurization range, the CH 4 density in the pores before and after depressurization is obtained from the adsorption isotherm; then the injection is calculated according to the Peng-Robinson state equation After CO 2 is added to the system pressure, the residual CH 4 density in the pores and the stored CO 2 density are obtained from the adsorption isotherm; the system is depressurized, and the
  • the model in the present invention can restore CO2 to enhance the gas reservoir recovery process, reflect the competitive adsorption behavior of CO2 and CH4 in rock pores during the production process, can quantify the recovery of CH4 and the storage of CO2 , and can quantitatively Evaluating the CO 2 stimulation effect and comparing the CH 4 recovery rate and CO 2 sequestration rate under different injection routes to obtain the optimal production route has guiding significance for the engineering design of CO 2 enhanced gas reservoir recovery.
  • Fig. 1 is the schematic flow sheet of method among the present invention
  • Fig. 2 is an adsorption isotherm diagram of CH4 in rock pores in different molar ratios CH4 / CO2 mixed gas in one embodiment of the present invention
  • Fig. 3 is an adsorption isotherm diagram of CO2 in rock pores in different molar ratios CH4 / CO2 mixed gas in one embodiment of the present invention
  • the invention provides a method for quantitatively predicting carbon dioxide enhanced gas reservoir exploitation and storage, comprising the following steps:
  • the system is depressurized, and the residual CH4 density in the pores and the sequestered CO2 density are obtained from the adsorption isotherm;
  • ⁇ rec represents the recovery factor of CH4 ;
  • ⁇ initial is the average density of CH4 in the rock pores under the initial pressure of the reservoir, and
  • ⁇ residual is the average density of residual CH4 in the rock pores under the pressure p;
  • ⁇ sec represents the CO 2 storage rate
  • ⁇ CO2 is the average density of pure CO 2 in the rock pores under the initial reservoir pressure
  • ⁇ CO2,p is the average density of CO 2 stored in the rock pores under the pressure p;
  • step C) Repeat step C) multiple times to obtain the residual CH 4 density and the stored CO 2 density ⁇ CO2,p in the pores of each stage, and calculate the CH 4 recovery rate and CO 2 storage of each stage according to formula I and formula II Rate;
  • the 3D modeling sketch software Sketch tool of the material performance simulation software Materials Studio can be used to draw the three-dimensional molecular structure models of methane and carbon dioxide molecules, and the structure optimization can be carried out through the open source software package LAMMPS to obtain the minimum energy conformation.
  • the initial cube simulation box is constructed, the dimensions of the box in the x, y, and z directions are all 10nm, and periodic boundary conditions are set in the three directions.
  • 8 kerogen molecules were randomly put into the simulation box, and then a series of NVT and NPT kinetic simulations were performed on the simulation system from high temperature to low temperature with reference to Table 1, thus obtaining the kerogen matrix model.
  • the present invention by combining the data of some influencing factors to jointly construct the structural model of rock pores, the present invention can take the factors in the real reservoir into consideration in the construction of the rock model, and finally reflect it on the spectrum of the adsorption isotherm, so that the simulated The calculation result is more accurate.
  • the influencing factors are such as water content, salt content, rock pore shape and other characteristic data.
  • the water content data of the reservoir can be collected.
  • it can be constructed according to the water content data.
  • Water-bearing rock pore model Based on the constructed water-containing rock pore model, the adsorption amount of CO 2 /CH 4 gas in (water-containing) rock pores is simulated, and the adsorption isotherm map (under water-containing condition) is obtained. At this time, the map contains The effect of water volume.
  • the present invention adopts the giant canonical Monte Carlo method to simulate the adsorption process of CO2 and CH4 gas in the rock pore structure model obtained in the step A), and obtains CO2 and CH4 gas at different pressures at the same temperature.
  • the amount of adsorption in the rock pores (expressed as gas density), the adsorption isotherm at the same temperature and different pressures is obtained, and the adsorption isotherm model is obtained by curve fitting the data.
  • multiple sets of adsorption isotherms are formed from the adsorption data obtained from multiple sets of mixed gas with different ratios, and finally the spectrum is obtained.
  • the reservoir temperature is set to 338.15K.
  • the calculated spectra are shown in Figures 2-3.
  • CO 2 swallowing According to the Peng-Robinson equation of state, the system pressure after injecting CO 2 is 26.1MPa (the mole fraction ratio of CH 4 and CO 2 in rock fractures after injection is 1:3), and the remaining CH4 density and sequestered CO2 density.
  • CO 2 discharge reduce the system pressure to 20MPa, and obtain the residual CH 4 density in the pores and the sequestered CO 2 density from the spectrum.
  • step 5 calculates according to the Peng-Robinson state equation and obtains that the system pressure after injecting CO 2 is 28.4MPa, and step 6 depressurizes to 20MPa.
  • CH4 recovery rate is defined as the ratio of the number of CH4 molecules released from the rock nanopores during the mining process (production amount) to the number of CH4 molecules in the pores under the initial pressure condition (gas storage).
  • ⁇ rec represents the recovery factor of CH4 ;
  • ⁇ initial is the average density of CH4 in the rock pores under the initial pressure of the reservoir, and
  • ⁇ residual is the average density of residual CH4 in the rock pores under the pressure p.
  • the CO2 storage rate is defined as the ratio of the number of CO2 molecules adsorbed in rock pores during CO2 injection to the theoretical maximum CO2 storage capacity.
  • the formula for calculating the CO2 storage rate is as follows,
  • ⁇ sec represents the CO 2 storage rate
  • ⁇ CO2 is the average density of pure CO 2 in the rock pores under the initial reservoir pressure
  • ⁇ CO2,p is the average density of CO 2 stored in the rock pores under the pressure p;
  • the present invention first locates the initial density of methane in the pores on the map according to the initial pressure of the reservoir; after the depressurization process of the first stage is completed, the residual methane density in the pores after the pressure reduction can be obtained through the map. Then carry out the CO 2 injection process (CO 2 swallow) in the process of CO 2 swallowing and puffing, and calculate the pressure of the system after CO 2 injection through the Peng-Robinson state equation. According to the pressure, the residual methane density in the pores after CO 2 injection can be obtained through the map and CO2 storage density.
  • the third stage is the CO 2 exhalation process, the system is depressurized, and the residual methane density and CO 2 storage density in the pores after depressurization can be obtained from the map. After each stage is completed, the residual methane density and CO2 storage density in the pores can be obtained through the map, based on which the methane recovery rate and CO2 storage rate of each stage can be calculated.
  • the simulation method of the present invention can greatly reduce the cost required for actual mining trial and error. Based on this method, the CH recovery rate and CO storage rate corresponding to different CO injection paths can be quantitatively calculated by establishing a map, which is convenient for prediction The optimal gas reservoir development path and the most suitable CO2 storage method. At the same time, the method in the present invention can quantitatively evaluate the effect of CO2 production increase.
  • the method of the present invention is applicable to complex reservoir conditions, for example, when the reservoir contains water, the influence of parameters such as water content can be reflected in the map.
  • the method in the present invention can be applied to the enhanced exploitation of various gas reservoirs, such as the exploitation of shale gas, coalbed methane and tight rock formation gas as well as conventional gas reservoirs.

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Abstract

本发明提供一种定量预测二氧化碳强化气藏开采和封存的方法，通过将GCMC和PR-EOS结合的方式，实现多次连续注采过程的模拟还原，包括初次降压过程和多次CO 2吞-吐过程。本发明采用GCMC算法实现不同温度压力下气体分子在岩石孔隙内的吸附模拟，得到储层温度下甲烷/二氧化碳在岩石孔隙中的吸附等温线图谱，以该图谱作为注采过程中岩石孔隙内气体密度变化的参照；通过PR-EOS计算注CO 2后体系压强，实现连续注采。本发明中的模型能够定量地评估CO 2的增产效果，通过对比不同注入路径下的甲烷采收率和二氧化碳封存率以获得最优的开采路径，对CO 2强化气藏开采的工程设计具有指导意义。

Description

一种定量预测二氧化碳强化气藏开采和封存的方法
本申请要求于2021年10月29日提交中国专利局、申请号为202111276874.6、发明名称为“一种定量预测二氧化碳强化气藏开采和封存的方法”的中国专利申请的优先权，其全部内容通过引用结合在本申请中。
技术领域
本发明属于二氧化碳的利用和封存技术领域，尤其涉及一种定量预测二氧化碳强化气藏开采和封存的方法。
背景技术
目前，对于CO 2强化气藏开采的效果和机理研究尚处于初步阶段，不同CO 2注入路径和注入量对气藏开采和CO 2封存的影响不明确，实验和现场试验的难度大、成本高、耗时长。因此，有必要通过模拟CO 2强化气藏开采过程，预测不同注入路径下的气藏采收率和CO 2封存率。
部分学者采用分子动力学方法(MD)研究了岩石孔隙中CO 2置换CH 4的行为。由于这种方法在模拟尺度上的局限性，已有的模型仅可预测单次降压过程或者单次注入CO 2过程。而由于气藏岩石的低孔隙度、低渗透率特性，需要通过多次连续的CO 2吞吐过程来实现更充分的开采，以获得更高的CH 4采收率和CO 2封存率。因此，需要一种更加准确的方法和模型，来计算不同注入路径下的CH 4采收率和CO 2封存率以获得最优的开采路径。
发明内容
本发明的目的在于提供一种定量预测二氧化碳强化气藏开采和封存的方法，本发明中的方法能够模拟由多个CO 2吞-吐阶段构成的开采过程，并且可以定量预测各个阶段的CH 4采收率和CO 2封存率，基于此可以在考虑经济性的前提下优化整个开采路径。
本发明提供一种定量预测二氧化碳强化气藏开采和封存的方法，包括以下步骤：
A)基于目标气藏的岩心特性，采用分子动力学方法构建岩石孔隙的结构模型，并进行结构优化；
B)采用巨正则蒙特卡洛方法模拟CO 2和CH 4气体在所述步骤A)得到的岩石孔隙结构模型中的吸附过程，计算得到不同压强下CH 4单组分、CH 4/CO 2混 合气、CO 2单组分在岩石孔隙中的吸附等温线；
C)根据储层初始压强和降压幅度，从吸附等温线得到降压前后孔隙中CH 4密度；
然后根据Peng-Robinson状态方程计算得到注入CO 2后体系压强，从吸附等温线得到孔隙中残余CH 4密度和封存CO 2密度；
将体系降压，由吸附等温线得到孔隙中残余CH 4密度和封存CO 2密度；
根据孔隙内CH 4和CO 2密度变化，按照式I和式II，计算CH 4采收率和CO 2封存率；
Figure PCTCN2022105876-appb-000001
其中，η rec表示CH 4的采收率；ρ initial为储层初始压强下，岩石孔隙中CH 4的平均密度，ρ residual,p为压力p下岩石孔隙中残余CH 4的平均密度；
η sec表示CO 2的封存率；ρ CO2为储层初始压强下，岩石孔隙中纯CO 2的平均密度；ρ CO2,p为压力p下岩石孔隙中封存的CO 2的平均密度；
D)重复步骤C)多次，得到各阶段的孔隙中残余CH 4密度和封存CO 2密度ρ CO2,p，按照式I和式II，计算得到各阶段的CH 4采收率和CO 2封存率；
E)根据各阶段的CH 4的采收率和CO 2的封存率，优化气藏开采路径。
优选的，先构建甲烷和二氧化碳分子的结构模型，并进行结构优化，然后在进行步骤B)中吸附过程的模拟。
优选的，采用巨正则蒙特卡洛方法模拟CO 2和CH 4气体在所述步骤A)得到的岩石孔隙结构模型中的吸附过程，得到了不同压强下CO 2和CH 4气体在岩石孔隙中的吸附量，得到相同温度不同压强下吸附等温线，通过对数据进行曲线拟合，得到吸附等温线模型。
优选的，根据采集储层中的影响因素数据，构建岩石孔隙的结构模型；
所述储层中的影响因素包括含水量、含盐量和孔隙特征中的一种或几种。
优选的，所述步骤B)中，由多组不同比例的CO 2和CH 4混合气得到的吸附数据形成多组吸附等温线，得到吸附图谱，根据吸附图谱计算各阶段CH 4采收率和CO 2封存率。
本发明提供了一种定量预测二氧化碳强化气藏开采和封存的方法，包括以 下步骤：A)采用分子动力学方法构建符合目标气藏岩石特性的岩石孔隙的结构模型，并进行结构优化；B)采用巨正则蒙特卡洛方法模拟CO 2和CH 4气体在所述步骤A)得到的岩石孔隙结构模型中的吸附过程，计算得到不同压强下CH 4单组分、CH 4/CO 2混合气、CO 2单组分在岩石孔隙中的吸附等温线；C)根据储层初始压强和降压幅度，从吸附等温线得到降压前后孔隙中CH 4密度；然后根据Peng-Robinson状态方程计算得到注入CO 2后体系压强，从吸附等温线得到孔隙中残余CH 4密度和封存CO 2密度；将体系降压，由吸附等温线得到孔隙中残余CH 4密度和封存CO 2密度；根据孔隙内CH 4和CO 2密度变化，按照式I和式II，计算CH 4采收率和CO 2封存率；D)重复步骤C)多次，得到各阶段的孔隙中残余CH 4密度和封存CO 2密度ρ CO2,p，按照式I和式II，计算得到各阶段的CH 4采收率和CO 2封存率；E)根据各阶段的CH 4的采收率和CO 2的封存率，优化气藏开采路径。本发明中的模型能够还原CO 2强化气藏开采过程，体现开采过程中CO 2与CH 4在岩石孔隙中竞争吸附的行为，可以量化CH 4的采出和CO 2的封存情况，能够定量地评估CO 2的增产效果，通过对比不同注入路径下的CH 4采收率和CO 2封存率以获得最优的开采路径，对CO 2强化气藏开采的工程设计具有指导意义。
附图说明
为了更清楚地说明本发明实施例或现有技术中的技术方案，下面将对实施例或现有技术描述中所需要使用的附图作简单地介绍，显而易见地，下面描述中的附图仅仅是本发明的实施例，对于本领域普通技术人员来讲，在不付出创造性劳动的前提下，还可以根据提供的附图获得其他的附图。
图1为本发明中方法的流程示意图；
图2为本发明一个实施例中不同摩尔比CH 4/CO 2混合气体中CH 4在岩石孔隙中的吸附等温线图谱；
图3为本发明一个实施例中不同摩尔比CH 4/CO 2混合气体中CO 2在岩石孔隙中的吸附等温线图谱；
图2～3中，箭头指示随着降压过程、CO 2吞、CO 2吐过程中，孔隙内残余的CH 4和封存的CO 2平均密度演化过程。
具体实施方式
本发明提供了一种定量预测二氧化碳强化气藏开采和封存的方法，包括以下步骤：
A)采用分子动力学方法构建符合目标气藏岩心特性的岩石孔隙的结构模型，并进行结构优化；
B)采用巨正则蒙特卡洛方法模拟CO 2和CH 4气体在所述步骤A)得到的岩石孔隙结构模型中的吸附过程，计算得到不同压强下CH 4单组分、CH 4/CO 2混合气、CO 2单组分在岩石孔隙中的吸附等温线；
C)根据储层初始压强和降压幅度，从吸附等温线得到降压前后孔隙中CH 4密度；
然后根据Peng-Robinson状态方程(PR-EOS)计算得到注入CO 2后体系压强，从吸附等温线得到孔隙中残余CH 4密度和封存CO 2密度；
将体系降压，由吸附等温线得到孔隙中残余CH 4密度和封存CO 2密度；
根据孔隙内CH 4和CO 2密度变化，按照式I和式II，计算CH 4采收率和CO 2封存率；
Figure PCTCN2022105876-appb-000002
其中，η rec表示CH 4的采收率；ρ initial为储层初始压强下，岩石孔隙中CH 4的平均密度，ρ residual为压力p下岩石孔隙中残余CH 4的平均密度；
η sec表示CO 2的封存率；ρ CO2为储层初始压强下，岩石孔隙中纯CO 2的平均密度；ρ CO2,p为压力p下岩石孔隙中封存的CO 2的平均密度；
D)重复步骤C)多次，得到各阶段的孔隙中残余CH 4密度和封存CO 2密度ρ CO2,p，按照式I和式II，计算得到各阶段的CH 4采收率和CO 2封存率；
E)根据各阶段的CH 4的采收率和CO 2的封存率，优化气藏开采路径。
具体步骤如下：
1、首先构建甲烷、二氧化碳分子的结构模型，并进行结构优化。具体实施时，可以利用材料性能模拟软件Materials Studio的3D建模草图软件Sketch工具绘制甲烷、二氧化碳分子的三维分子结构模型，通过开源软件包LAMMPS进行结构优化，得到最小能量构象。
2.采用分子动力学方法构建岩石孔隙的结构模型，并进行结构优化。下面 以岩石中的有机质，即干酪根的构建过程为例，
构建初始的立方体模拟盒子，盒子在x、y、z方向的尺寸均为10nm，且在三个方向上均设置周期性边界条件。首先向模拟盒子中随机投入8个干酪根分子，接着参照表1对模拟体系进行从高温到低温的一系列NVT和NPT动力学模拟过程，由此得到干酪根基质模型。在干酪根基质模型的基础上，可以构建不同形状和大小的干酪根孔隙模型。
表1干酪根基质模型的构建流程
Figure PCTCN2022105876-appb-000003
在本发明中，通过结合一些影响因素数据共同构建岩石孔隙的结构模型，本发明可以将真实储层中的因素考虑进岩石模型的构建当中，最后体现在吸附等温线的图谱上，从而使模拟计算结果更加准确。
所述影响因素如含水量、含盐量、岩石孔隙形状等特性数据等，例如，当储层含水时，可采集储层的含水量数据，在构建岩石模型时，可以根据含水量数据，构建含水的岩石孔隙模型。再基于构建好的含水的岩石孔隙模型，模拟CO 2/CH 4气体在(含水的)岩石孔隙中的吸附量，得到(含水情况下的)吸附等温线图谱，此时图谱中已包含了含水量的影响。将含水情况下的吸附等温线图谱与干燥情况下的吸附等温线图谱进行对比，可以得到含水量对气体吸附量的影响规律(例如：含水量的增大会使得CH 4吸附量减少；含水量增大，对CO 2-CH 4混合气体吸附中CO 2的吸附量的影响较小)。
3、气体吸附模拟。采用巨正则蒙特卡洛方法(GCMC)分别计算储层温度下CH 4单组分、CH 4/CO 2混合气、CO 2单组分在岩石孔隙中的吸附等温线， 进一步获得图谱。
具体的，本发明采用巨正则蒙特卡洛方法模拟CO 2和CH 4气体在所述步骤A)得到的岩石孔隙结构模型中的吸附过程，得到了相同温度不同压强下CO 2和CH 4气体在岩石孔隙中的吸附量(以气体密度表示)，得到相同温度不同压强下吸附等温线，通过对数据进行曲线拟合，得到吸附等温线模型。
进一步的，由多组不同比例混合气得到的吸附数据构成多组吸附等温线，最终得到图谱。
储层温度设置为338.15K。设置了三组不同摩尔分数比的CH 4/CO 2混合气(y CO2＝0.25；y CO2＝0.5；y CO2＝0.75)，为后续设置不同CO 2注入工况作为基础。计算得到的图谱如图2～3所示。
得到图谱之后，进行多个CO 2吞-吐阶段构成的开采过程的模拟计算，具体如下：
4、降压：根据储层初始压强和降压幅度，从图谱得到降压前后孔隙中CH 4密度。假定储层初始压强为30MPa，降压降至20MPa，实现首个降压阶段。根据压强可以在图谱中跟踪岩石孔隙内CH 4密度变化量和变化趋势。
5、CO 2吞：根据Peng-Robinson状态方程计算得到注入CO 2后体系压强为26.1MPa(注入后岩石裂缝中CH 4与CO 2的摩尔分数比为1:3)，从图谱得到孔隙中残余CH 4密度和封存CO 2密度。
6、CO 2吐：将体系压强降至20MPa，由图谱得到孔隙中残余CH 4密度和封存CO 2密度。
7、第二轮CO 2吞-吐过程：重复步骤5和6。其中步骤5中根据Peng-Robinson状态方程计算得到注入CO 2后体系压强为28.4MPa，步骤6降压至20MPa。
8、根据实际情况需要，可进行多轮CO 2吞-吐过程；根据孔隙内CH 4和CO 2密度变化，计算各阶段CH 4采收率和CO 2封存率。
为了量化并比较各中间过程的CH 4采收率和CO 2封存效率，以储层初始压力(30MPa)作为参照，引入CH 4采收率η rec和CO 2封存率η sec两个参数。其中，CH 4采收率定义为开采过程中从岩石纳米孔隙中释放出的CH 4分子数(采出量)与初始压力条件下孔隙内CH 4分子数(气储量)的比值。CH 4采收率的 表达式如下，
Figure PCTCN2022105876-appb-000004
其中，η rec表示CH 4的采收率；ρ initial为储层初始压强下，岩石孔隙中CH 4的平均密度，ρ residual为压力p下岩石孔隙中残余CH 4的平均密度。
假定最终可通过注CO 2将地层压力恢复至原始地层压力(30MPa)，将该压力下可封存的CO 2量作为理论上的最大CO 2封存量ρ CO2。CO 2封存率定义为CO 2注入过程中岩石孔隙内吸附的CO 2分子数与理论上最大CO 2封存量的比值。CO 2封存率的计算公式如下，
Figure PCTCN2022105876-appb-000005
η sec表示CO 2的封存率；ρ CO2为储层初始压强下，岩石孔隙中纯CO 2的平均密度；ρ CO2,p为压力p下岩石孔隙中封存的CO 2的平均密度；
根据上述过程，结合图2～3的图谱，得到表2中的数据。
表2本发明中的方法得到的CH 4采收率和CO 2封存率
Figure PCTCN2022105876-appb-000006
9、根据CH 4开采和CO 2封存需求，优化气藏开采路径。
本发明首先根据储层初始压强，在图谱上定位孔隙内甲烷的初始密度；在第一阶段降压过程完成后，通过图谱可以获得降压后孔隙内残余甲烷密度。随后进行CO 2吞-吐过程中的CO 2注入过程(CO 2吞)，通过Peng-Robinson状态方程计算CO 2注入后体系的压强，根据压强可以通过图谱得到CO 2注入后孔隙内残余甲烷密度和CO 2封存密度。第三阶段为CO 2吐过程，对体系进行降压，从图谱中可以得到降压后孔隙中的残余甲烷密度和CO 2封存密度。每一 阶段完成后均可通过图谱获得孔隙内甲烷残余密度和CO 2封存密度，基于此可以计算各个阶段的甲烷采收率和CO 2封存率。
通过本发明模拟的方式可以大幅减少实际开采试错所需要的成本，基于本方法，可以通过建立图谱的方式定量计算不同CO 2注入路径对应的CH 4采收率和CO 2封存率，便于预测最优的气藏开采路径以及最合适的CO 2封存方式。同时，本发明中的方法可以定量评估CO 2的增产效果。
进一步的，本发明中的方法适用于复杂的储层条件，例如储层含水时，可以将含水量等参数的影响在图谱中体现。本发明中的方法可以应用至各类气藏的强化开采，例如页岩气、煤层气和致密岩层气以及常规气藏的开采。
以上所述仅是本发明的优选实施方式，应当指出，对于本技术领域的普通技术人员来说，在不脱离本发明原理的前提下，还可以做出若干改进和润饰，这些改进和润饰也应视为本发明的保护范围。

Claims (5)

  1. 一种定量预测二氧化碳强化气藏开采和封存的方法，包括以下步骤：
    A)基于目标气藏的岩心特性，采用分子动力学方法构建岩石孔隙的结构模型，并进行结构优化；
    B)采用巨正则蒙特卡洛方法模拟CO 2和CH 4气体在所述步骤A)得到的岩石孔隙结构模型中的吸附过程，计算得到不同压强下CH 4单组分、CH 4/CO 2混合气、CO 2单组分在岩石孔隙中的吸附等温线；
    C)根据储层初始压强和降压幅度，从吸附等温线得到降压前后孔隙中CH 4密度；
    然后根据Peng-Robinson状态方程计算得到注入CO 2后体系压强，从吸附等温线得到孔隙中残余CH 4密度和封存CO 2密度；
    将体系降压，由吸附等温线得到孔隙中残余CH 4密度和封存CO 2密度；
    根据孔隙内CH 4和CO 2密度变化，按照式I和式II，计算CH 4采收率和CO 2封存率；
    Figure PCTCN2022105876-appb-100001
    其中，η rec表示CH 4的采收率；ρ initial为储层初始压强下，岩石孔隙中CH 4的平均密度，ρ residual,p为压力p下岩石孔隙中残余CH 4的平均密度；
    η sec表示CO 2的封存率；ρ CO2为储层初始压强下，岩石孔隙中纯CO 2的平均密度；ρ CO2,p为压力p下岩石孔隙中封存的CO 2的平均密度；
    D)重复步骤C)多次，得到各阶段的孔隙中残余CH 4密度和封存CO 2密度ρ CO2,p，按照式I和式II，计算得到各阶段的CH 4采收率和CO 2封存率；
    E)根据各阶段的CH 4的采收率和CO 2的封存率，优化气藏开采路径。
  2. 根据权利要求1所述的方法，其特征在于，先构建甲烷和二氧化碳分子的结构模型，并进行结构优化，然后在进行步骤B)中吸附过程的模拟。
  3. 根据权利要求1所述的方法，其特征在于，采用巨正则蒙特卡洛方法模拟CO 2和CH 4气体在所述步骤A)得到的岩石孔隙结构模型中的吸附过程， 得到了不同压强下CO 2和CH 4气体在岩石孔隙中的吸附量，得到相同温度不同压强下吸附等温线，通过对数据进行曲线拟合，得到吸附等温线模型。
  4. 根据权利要求1所述的方法，其特征在于，根据采集储层中的影响因素数据，构建岩石孔隙的结构模型；
    所述储层中的影响因素包括含水量、含盐量和孔隙特征中的一种或几种。
  5. 根据权利要求1所述的方法，其特征在于，所述步骤B)中，由多组不同比例的CO 2和CH 4混合气得到的吸附数据形成多组吸附等温线，得到吸附图谱，根据吸附图谱计算各阶段CH 4采收率和CO 2封存率。
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CN113821937A (zh) * 2021-10-29 2021-12-21 中国华能集团清洁能源技术研究院有限公司 一种定量预测二氧化碳强化气藏开采和封存的方法

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