Fundamental Research On Extraction Natural Gas Hydrate In Porous Media

As an extensive reserves of gas hydrates in deep-sea sediments hydrate reservoir and land tundra, gas hydrate has caused widespread concern in the scientific community with its huge reserves. The study of Hydrate technology research applications is ascendant, in which the synthesis, decomposition of gas hydrates, In-situ mining theory and technique has been the focus of research in the field of hydrate. The essential reason is partly because its mild conditions and relatively high gas storage capacity is getting more and more people’s attention who hope to achieve a wider range of applications through research, in the other hand, is due to the increasing demand for energy of current social processes, and gradually reducing non-renewable resources which make people had a strong desire for future breeding of new energy, and so on. However, restricting of the preparation process, the growth rate and final amount made gas hydrate synthesis techniques have been not developed smoothly. In addition, because of complex metallogenic conditions, uncertain factors in mining process, complex hydrate decomposition and migration mechanism, and the physical parameters of reservoir which have been unable to accurately explore, the resources utilization has stagnanted.’The improved preparation technology of gas hydrate, decomposition mining theory" is the topic of study, on the basis of comprehensive analysis of the predecessors research achievements, the research ingeniously designs and sets up the first small gas hydrates spray reinforced synthetic experiment platform suitable for industrial production condition. By using this platform, exploratory research has been carried on the improved synthesis process of gas hydrate, and deep analysis study has been conducted on the technology and mechanism of reinforcement. On this basis, the synthesis and decomposition of gas hydrate in porous media of computed tomography (CT) scanning experiment research has been further carried out, the detailed study has carried on the covering of free gas hydrate step-down mining, submarine gas hydrates, hidden note in-situ in the hot water for mining theory and technology. The main research contents and results are as follows:(l)We carry on the improved synthetic process gas hydrate study. The atomization synthesis equipment Suitable for industrial production was developed, through a sample loading range with booster pump it can achieve constant pressure test, or closed sample loading constant volume range compression test, and through complement or exhaust gas or water, it also can realize the constant pressure capacity test way. With this set of equipment we can study industrial production under the condition of macroscopic controllable factors, such as gas composition, gas water mixed method, temperature and pressure condition, the composition of water, the specific impact of hydrate formation process and its mechanism of action. We also can explore the thermodynamics and kinetics principle in the process of gas hydrate formation.(2) The research Explore the strengthening mechanism of atomization gas hydrate synthesis of lans and key factors. Studies show that atomization system induction phenomenon is not obvious, the growth speed is extremely fast, air content is too high; The whole growth process is dominanted by mass transfer conditions, the non-ideal heat transfer conditions will seriously affect the growth rate, but non-ideal mass transfer condition will be the first to slow reaction process, then finally bring about air content great reduction;In the condition of ensuring good heat transfer, the growth speed depend on pressure obviously, also showed distinct increment relation:the higher the pressure, the faster the speed, super-cooling and overpressure can effectively accelerate the growth process, In addition, surfactant, seed and repeat generation can significantly speed up the reaction process, improve the air content in the end(3)Study has been conducted on porous media hydrate synthesis experiment, gas hydrate formation conditions in porous media is similar in aqueous solution, the gas consumption changing with time can be divided into three section called the initial synthesis section, rapid synthesis section and complete section. Initial synthetic segments of the pressure and temperature fluctuations are small, gas consumption is small too. In the rapid synthesis segment gas consumption rate, temperature and pressure change very frequently, which are caused by gas transformation from gas to amorphous significantly, in the synthesis completion segment gas consumption rate is extremely low, and continue at a very low rate until reaction stoping. To generate hydrate in porous media is not only on the surface of the specimen, but all in the specimen within the pore formation and occurrence, and it is evenly distributed, this is an important feature of gas hydrate in porous media.(4)The hydrate decomposition of CT scan was carried on in the experimental study of porous medium. When porous medium hydrates decompose, along with the increase of particle size its hydrate decomposition vertical movement is gradually increasing, the corresponding deformation on average0.953%, this is the medium of sand porous media hydrate decomposition of vertical deformation. The vertical deformation and sand grain diameter exponential change rule exponentially, Uz=0.0032exp(0.4318d).(5)We studied the step-down mining theory and technology. Based on the theory of multiphase seepage and hydrate decomposition dynamics, we set up mathematical model of step-down the mining process which are suitable for the dissociation of gas hydrate with the mining, in view of the strongly nonlinear problems, fully implicit method is firstly introduced to the discrete control equations, the detailed difference method is gived. For the method, the implicit degree is high, the solving process is more stable, its broader scope of application is more common than IMPES method, and a corresponding calculating program was compiled with FORTRAN language, with which the accuracy of the mathematical model and discrete method was verified. The program can accurately obtain mining in the process of spatial distribution of the field variables in each time, it also can obtained the mining moving front position and the output and migration regularity of gas, water.(6)The research explores the essence of the law of the hydrate decomposition and gas-water move in the process of step-down mining. By the numerical simulation of depressurization mining results show that in the step-down production process the reservoir pressure drop is relatively flat and the stability is better, by reducing the bottom hole flowing pressure it can effectively improve the hydrate production rate, besides by adjusting the bottom hole flowing pressure it is also able to juggle mining balance between speed and stability of the reservoir. From the point of gas-water output of the migration process, whether mining efficiency is high have a great relationship with reservoir absolute seepage rate:the greater its value, the faster the migration, the production efficiency is higher; under the condition of absolute same permeability, migration process affected by the values of permeability decline index is relatively small.(7)We studied the solid, flow, heat, decomposition of many field coupling theory in the process of thermal displacement mining. Aimed at hydrate mining problem in deep sea, many field coupling mathematical model of thermal production process is established which can accurately describe the gas hydrate in porous media. The equation is discreted by finite element finite and difference method, then its numerical solution is given. By computing program, the submarine water hydrates situ thermial mining process are simulated, which verified the accuracy of the mathematical model. Through simulation, it can accurately obtain field variables spatial distribution such as fluid pressure, medium temperature, solid displacement and deformation.(8)We studied the thermal displacement coupling effect laws in the process of mining. From the point of the results of numerical simulation, the typical injection-production process generally can be divided into three stages:early decompression guide, medium-term thermal displacement and the later slow decomposition. In the middle of the thermal displacement process hydrate decompose fast, gas water production is very big, early, later decomposition process is relatively slow. During the injection-production process an extremely obvious, zonal distribution decomposition front existed, there phase change drastically, hydrate decompose extremely fast, which is the key to dorminant the injection-production process stage, most of the hydrate decomposition accomplish in the complete process of the decomposition zone. In the injection hot mining progress pressure increase rapidly, water displacement effect is obvious. By improving the injection temperature, it can effectively improve the mining speed and final gas production. Comprehensive analysis shows that the pure thermal production will produce larger heat loss in the reservoir, it is an key factor affect the injection-production process economy efficiency, which can be solve through optimization of well pattern arrangement, and rational allocation of injection dosage, way combiningstep-down with thermal.