The simulation of gas production from oceanic gas hydrate reservoir by the combination of ocean surface warm water flooding with depressurization

A new method is proposed to produce gas from oceanic gas hydrate reservoir by combining the ocean surface warm water flooding with depressurization which can efficiently utilize the synthetic effects of thermal, salt and depressurization on gas hydrate dissociation. The method has the advantage of high efficiency, low cost and enhanced safety. Based on the proposed conceptual method, the physical and mathematical models are established, in which the effects of the flow of multiphase fluid, the kinetic process of hydrate dissociation, the endothermic process of hydrate dissociation, ice-water phase equilibrium, salt inhibition, dispersion, convection and conduction on the hydrate dissociation and gas and water production are considered. The gas and water rates, formation pressure for the combination method are compared with that of the single depressurization, which is referred to the method in which only depressurization is used. The results show that the combination method can remedy the deficiency of individual producing methods. It has the advantage of longer stable period of high gas rate than the single depressurization. It can also reduce the geologic hazard caused by the formation deformation due to the maintaining of the formation pressure by injected ocean warm water.     

降压开采模拟试验的水合物分解阵面演化过程

水合物分解阵面演化过程与开采安全性和产气效率密切相关,是开采原位监测的重要组成部分。在玻璃砂样品中进行了甲烷水合物降压开采模拟试验,探讨了水合物饱和度对渗流阵面和水合物分解阵面演化过程的影响,结合已有理论模型,分析了水合物分解阵面传播速度的关键影响因素。结果表明:渗流阵面和水合物分解阵面的传播距离均与时间平方根呈近似线性关系,传播速度均随水合物饱和度的增加而减小;水合物分解阵面的传播速度随多孔介质的有效渗透率和降压幅度的增加而变快,随孔隙率的增加而变慢,粗砂质地层更有利于水合物降压分解阵面的传播。     

多孔介质中天然气水合物注热+降压开采的实验研究

为研究注热与降压相结合的开采方式是否更加有利于天然气水合物的开采,在自制的天然气水合物开采模拟实验系统上进行了实验研究.在一维填砂模型中人工生成天然气水合物之后,进行先注热盐水然后再降低压力(注热+降压)的开采模拟实验,分析了开采过程中系统温度、电阻率变化规律以及产气量、能量效率等.结果表明:产气规律具有明显的阶段性,...     

Numerical and experimental investigation of convective drying in unsaturated porous media with bound water

The heat and mass transfer in an unsaturated wet cylindrical porous bed packed with quartz particles was investigated theoretically for relatively low convective drying rates. Local thermodynamic equilibrium was assumed in the mathematical model describing the multi-phase flow in the unsaturated porous media using the energy and mass conservation equations to describe the heat and mass transfer during the drying. The drying model included convection and capillary transport of the free water, diffusion of bound water, and convection and diffusion of the gas. The numerical results indicated that the drying process could be divided into three periods, the temperature rise period, the constant drying rate period and the decreasing drying rate period. The numerical results agreed well with the experimental data verifying that the mathematical model can evaluate the drying performance of porous media for low drying rates. The effects of drying conditions such as the ambient temperature, the relative humidity, and the velocity of the drying air, on the drying process were evaluated by numerical solution.     

1-D Modeling of Hydrate Depressurization in Porous Media

A thermal, three-phase, one-dimensional numerical model is developed to simulate two regimes of gas production from sediments containing methane hydrates by depressurization: the dissociation-controlled regime and the flow-controlled regime. A parameter namely dissociation-flow time-scale ratio, R, is defined and employed to identify the two regimes. The numerical model uses a finite-difference scheme; it is implicit in water and gas saturations, pressure and temperature, and explicit in hydrate saturation. The model shows that laboratory-scale experiments are often dissociation-controlled, but the field-scale processes are typically flow-controlled. Gas production from a linear reservoir is more sensitive to the heat transfer coefficient with the surrounding than the longitudinal heat conduction coefficient, in 1-D simulations. Gas production is not very sensitive to the well temperature boundary condition. This model can be used to fit laboratory-scale experimental data, but the dissociation rate constant, the multiphase flow parameters and the heat transfer parameters are uncertain and should be measured experimentally.     

天然气水合物降压开采沉积物压缩效应数值模拟研究

深海天然气水合物降压开采过程中,沉积物的压缩会改变储层的物理力学特性,进而对天然气的开采效果产生显著影响.为揭示沉积物压缩效应下井周围储层物理力学特性演化规律,本文建立了考虑沉积物压缩效应的理论模型,通过COMSOL模拟研究了不同初始固有渗透率、初始水合物饱和度和井底压力条件下的降压开采中生产井周围储层的物理力学特性演化规律以及开采效果.结果表明:受沉积物压缩的影响,水合物分解区的渗透率随着与井筒距离的增加先增加后减少;产气与产水速率由零立即上升至峰值,然后迅速下降,并且考虑沉积物压缩时的产气与产水速率比不考虑时低;在水合物完全分解区,渗透率的大小与有效应力成负相关关系,未分解区渗透率的大小与水合物饱和度成负相关关系;井底压力越小,有效应力越大,生产井周围储层的渗透率下降越明显;初始水合物饱和度对产气与产水的影响存在拐点,饱和度拐点位于0.25与0.35之间,高水合物饱和度并不代表储层开采效果好,产气速率的高低还与储层的渗透率有关,高水合物饱和度储层的渗透率较低,产气速率较低;储层初始固有渗透率较高时显著促进了开采效果,但储层变形量较大增加了储层的不稳定性.     

不同水合物饱和度下注热水开采实验研究

为研究不同水合物藏饱和度对注热开采动态的影响,采用自制的一维天然气水合物(NGH)开采模拟实验装置,模拟地层多孔介质的条件,首先在填砂模型中生成不同饱和度的NGH,然后进行注热盐水分解实验。结果表明:不同饱和度的NGH注热分解产气都可划分为三个阶段,不同的是NGH饱和度越高,水合物分解阶段的产气速率越大,且该阶段持续的时间越长;NGH饱和度越高,注热分解阶段电阻率增大的幅度越大,系统各测点温度升高的幅度越小。注热分解过程中产水速率围绕注水速率而上下波动,且NGH饱和度越高,产水速率波动幅度越大;在实验研究范围内,随初始水合物饱和度的升高,注热开采的能量效率逐渐升高。因此,从能量效率来说,高饱和度的水合物藏更适宜于注热开采。     

水平井降压法和热激法水合物开采对海底边坡稳定性的影响

天然气水合物广泛赋存在深海沉积物孔隙中, 被认为是具有巨大开发潜力的未来绿色能源之一, 引起全球的关注. 深海水合物开采将造成含水合物储层的强度劣化, 可能产生孔压积聚, 诱发海床失稳. 本文基于边坡稳定极限平衡分析框架, 引入考虑水合物开采热-流-化学耦合过程的数值分析模型, 研究水合物开采对海底边坡稳定性的影响. 采用TOUGH+HYDRATE热-流-化学耦合分析程序, 模拟了采用水平井降压法和热激法开采深海水合物的过程, 分析了水合物分解锋面扩展和瞬态孔压演变的规律, 并通过SLOPE/W程序采用极限平衡分析方法计算水合物开采过程及停采后的海底边坡安全系数, 分析开采井位置和开采方法对海底边坡稳定性的影响. 研究表明, 对于存在致密盖层的细砂储层陡坡, 单水平井降压开采过程中, 由于孔压降低, 土体有效应力增加, 边坡稳定性显著提高, 当开采井布设在坡体中部时, 边坡稳定性提高最为明显; 停采后, 由于水合物分解导致土体黏聚强度降低, 且孔压逐渐回升到静水压状态, 导致边坡稳定性下降, 最危险滑弧通过水合物分解区. 若采用双水平井热激法开采, 开采过程与停采后的最危险滑弧始终通过水合物分解区, 由于开采过程中温度升高, 井周孔压显著上升, 导致边坡安全系数明显下降, 存在诱发滑坡的风险.      

Effect of decomposition of a gas hydrate on the gas recovery from a reservoir containing hydrate and gas in the free state

The problem of gas recovery in the process of methane hydrate dissociation in a reservoir saturated with gas-hydrate mixture is considered. The mathematical model of hydrate decomposition into gas and water is generalized to include the negative temperature interval and takes ice formation into account. The solution of the problem is represented in the self-similar approximation. It is shown that there exists a transition hydrate decomposition regime in which water and ice are formed simultaneously. A comparative analysis of the recovery is carried out on the basis of relations derived for the masses of recovered gas in different hydrate dissociation regimes. It is shown that an anomalous increase in the recovered gas volumes is observed in the transition hydrate dissociation regime.__________Translated from Izvestiya Rossiiskoi Academii Nauk, Mekhanika Zhidkosti i Gaza, No. 1, 2005, pp. 132–142. Original Russian Text Copyright © 2005 by Tsypkin.     

海洋天然气水合物降压开采地层井壁力学稳定性分析

降压开采天然气水合物使其分解会导致储层孔隙度、渗透率、孔隙压力和岩层骨架有效应力发生改变, 同时降低沉积物的胶结程度, 使地层的抗剪强度和承载能力降低, 从而引起井壁失稳、海底滑坡、海底面沉降等工程问题. 为此, 在地下多相非等温数值模拟软件TOUGH+Hydrate框架内, 基于扩展的三维Biot固结理论, 考虑水合物分解相变、传热(T)、流动(H)、岩土体变形(M)等过程及其相互耦合作用, 建立了新的水合物开采传热-流动-力学(THM)耦合数学模型, 并开发有限元程序对其进行数值求解. 以中国南海神狐海域GMGS1航次SH2站位水合物储层条件为研究对象, 构建了垂直井降压开采THM耦合地层井壁稳定性分析模型, 预测了水合物开采过程中储层温-压-力场和水合物分解区的演化规律, 揭示了地层优势出砂区域和海底面沉降趋势. 结果表明: 储层降压导致地层有效应力增大, 进而引起井周地层发生沉降, 且地层的沉降主要发生在降压开采前期, 最大沉降位置位于井壁周围, 向储层内部延伸地层沉降量快速减小; 水合物分解导致井周地层力学强度降低, 加剧了储层的沉降; 井筒降压造成射孔段井壁应力集中最为明显, 从而造成井壁破坏的潜在风险, 这些区域正是水合物开采出砂防治的关键区域.      

Recovery of Methane From Gas Hydrates in a Porous Medium by Injection of Carbon Dioxide

This paper presents a mathematical model for methane hydrate–carbon dioxide replacement by injection of carbon dioxide gas into a porous medium rich in methane and its gas hydrate. Numerical solutions describing the pressure and temperature variation in a reservoir of finite length are obtained. It is shown that the replacement process is accompanied by a decrease in pressure and an increase in temperature of the porous medium. It is established that during the time of complete replacement of methane from a reservoir decreases with increasing permeability of the porous medium and the pressure of the injected gas.     

Injection of liquid carbon dioxide into a reservoir partially saturated with methane hydrate

A mathematical model is proposed to describe methane–carbon dioxide replacement in gas hydrate by injecting liquid carbon dioxide into a porous medium initially saturated with methane and its hydrate. Self-similar solutions of the axisymmetric problems are constructed that describe the distribution of the main parameters of the reservoir. It is shown that there exist solutions according to which the process can occur both with and without boiling of carbon dioxide. Diagrams of the existence of each type of solution are constructed.     

Modeling of Gas Hydrate Formation Upon Injection of Carbon Dioxide Gas into A Natural Reservoir

This paper presents the results of numerical modeling of gas hydrate formation upon injection of carbon dioxide into a finite-length reservoir saturated with methane and water. It is shown that at different stages, hydrate formation can occur on both the frontal surface and in a reservoir region of finite length. The effects of pressure at the reservoir boundaries and the effects of the permeability and initial water saturation of the reservoir on the hydrate formation process were studied. The dependences of the time of the complete conversion of water into gas hydrate in the entire reservoir on the injection pressure and reservoir permeability were obtained.     

Transport of Pseudomonas putida in a 3-D Bench Scale Experimental Aquifer

This study is focused on the transport of Pseudomonas (P.) putida bacterial cells in a 3-D model aquifer. The pilot-scale aquifer consisted of a rectangular glass tank with internal dimensions: 120?cm length, 48?cm width, and 50?cm height, carefully packed with well-characterized quartz sand. The P. putida decay was adequately represented by a first-order model. Transport experiments with a conservative tracer and P. putida were conducted to characterize the aquifer and to investigate the bacterial behavior during transport in water saturated porous media. A 3-D, finite-difference numerical model for bacterial transport in saturated, homogeneous porous media was developed and was used to successfully fit the experimental data. Furthermore, theoretical interaction energy calculations suggested that the extended-DLVO theory seems to predict bacteria attachment onto the aquifer sand better than the classical DLVO theory.     

Mathematical model of the dissociation of gas hydrates coexisting with ice in natural reservoirs

The dissociation of gas hydrate coexisting with ice in a low-temperature natural reservoir is investigated. A mathematical model of the process consisting of a generalization of the Stefan problem and containing two unknown moving phase transition boundaries — the hydrate dissociation and ice melting fronts — is constructed. It is shown that in high-permeability reservoirs the velocity of the dissociation surface is higher than that of the ice melting surface. As the permeability decreases, the fronts change places. The problem is solved in the self-similar approximation.Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No.2, pp. 84–92, March–April, 1993.     

Injection of carbon dioxide into a reservoir saturated with methane and water

This paper presents a mathematical model for the injection of carbon dioxide into a natural gas reservoir saturated with methane and water accompanied by the formation of carbon dioxide hydrate in an extended region. The dependence of the coordinates of the boundaries of the region of phase transitions on the pressure of the injected gas and the initial parameters of the reservoir are investigated. It is established that the velocity of the near boundary of the region of hydrate formation decreases with increasing water saturation and initial temperature of the reservoir and the velocity of the far boundary of the region of phase transitions increases with increasing pressure of the injected gas and reservoir permeability. It is shown that at high initial temperatures of the reservoir, a regime is possible in which replacement of methane by carbon dioxide without hydrate formation occurs at the far interface, and at the near interface, water is completely incorporated into gas hydrate.     

Fluid-solid coupling model for studying wellbore instability in drilling of gas hydrate bearing sediments

As the oil or gas exploration and development activities in deep and ultra- deep waters become more and more, encountering gas hydrate bearing sediments （HBS） is almost inevitable. The variation in temperature and pressure can destabilize gas hydrate in nearby formation around the borehole, which may reduce the strength of the formation and result in wellbore instability. A non-isothermal, transient, two-phase, and fluid-solid coupling mathematical model is proposed to simulate the complex stability performance of a wellbore drilled in HBS. In the model, the phase transition of hydrate dissociation, the heat exchange between drilling fluid and formation, the change of mechanical and petrophysical properties, the gas-water two-phase seepage, and its interaction with rock deformation are considered. A finite element simulator is developed, and the impact of drilling mud on wellbore instability in HBS is simulated. Results indicate that the re- duction in pressure and the increase in temperature of the drilling fluid can accelerate hydrate decomposition and lead to mechanical properties getting worse tremendously. The cohesion decreases by 25% when the hydrate totally dissociates in HBS. This easily causes the wellbore instability accordingly. In the first two hours after the formation is drilled, the regions of hydrate dissociation and wellbore instability extend quickly. Then, with the soaking time of drilling fluid increasing, the regions enlarge little. Choosing the low temperature drilling fluid and increasing the drilling mud pressure appropriately can benefit the wellbore stability of HBS. The established model turns out to be an efficient tool in numerical studies of the hydrate dissociation behavior and wellbore stability of HBS.     

Numerical investigation of the decomposition of gas hydrates coexisting with gas in natural reservoirs

The problem of methane hydrate decomposition in a reservoir saturated with a gas and hydrate mixture is investigated numerically. The results of the numerical simulation and an analytic solution obtained in the linear approximation are compared. It is shown that for high-permeability rocks the convective heat transfer in the near-well space of the reservoir predominates over the conductive transfer. This makes the use of intra-well heaters ineffective. It is found that an increase in the reservoir and well pressures and a decrease in the permeability suppress the formation of an extended hydrate dissociation region. Critical diagrams of existence of the frontal decomposition regime are constructed.     

A new simulation framework for predicting the onset and effects of fines mobilization

Fines release and migration is a universal problem in the production of oil from poorly consolidated sandstone reservoirs. This problem can result in the changes of porosity and permeability. It may not only damage a production facility, but it can also have a profound effect on oil recovery, resulting from the change in heterogeneity of the oil formation. Based on the macroscopic continuous porous media, continuity equations for multiphase flow in oil formations, and the theories of fines release and migration, a three-dimensional (3D) field scale mathematical model describing migration of fines in porous media is developed. The model is solved by a finite-difference method and the line successive over relaxation (LSOR) technique. A numerical simulator is written in Fortran 90 and it can be used to predict (1) the ratio of fines to production liquid volume, (2) the permeability change caused by colloidal and hydrodynamic forces resulting from fines release and migration, and (3) production performance. The numerical results of the one-dimensional model were verified by the data obtained by core displacement experiments. The sensitivity of numerical results with grid block size was studied by coarse grids, moderate grids, and fine grids. In addition, an oil field example with five-spot patterns was made on the numerical simulator. The results show that fines migration in an oil formation can accelerate the development of heterogeneity of the reservoir rock, and has an obvious influence on production performance, i.e., water drive front, water-cut trends, and oil recovery.     

多孔介质在压力梯度作用下的热质耦合数值模拟

多孔介质干燥导致热质耦合传输过程。本文基于连续介质力学的宏观尺度,对多孔介质的热、湿和气三者耦合迁移进行数值模拟,研究压力梯度对热质传输的影响。多孔介质传质机理主要为水汽和空气的对流和扩散传输、吸附水在含湿量梯度作用下的自由扩散和其在温度梯度即Soret效应驱动下的流动。采用Galerkin加权余量的有限元方法,提出了...