煤层气热力开采的气水两相流动机理研究

发布时间：2018-03-30 09:15

本文选题：煤层气　切入点：气水两相　出处：《太原理工大学》2015年博士论文

【摘要】：煤层气作为与煤伴生、共生的气体资源,以其清洁、热值高、开发利用前景广阔的特点,已经成为非常规天然气勘探开发的重点。我国煤层气形成的地质条件和赋存环境条件复杂,其开发受到地应力、含压地下水及温度等多场耦合作用。本文以煤层气热力开采为背景,采用实验与模拟相结合的方法,对煤体常温及高温应力作用下气水两相渗流规律做了研究,本文的主要研究内容及结果如下:1)从煤层气产出机理出发,分析了煤层气的开采是气体“解吸-扩散-渗流”的运动过程。根据汽化和凝聚的动力学平衡原理、Fick定律、Darcy定律,得到了煤层气气水两相渗流模拟的数值模型。在一般的求解过程中,根据不同情况对毛细管力及重力进行分析,得到了简化的渗流微分方程。2)采用自行研制的气水两相高精度渗透系统,进行了大量非稳态气水两相渗流实验,结果表明:非稳态全过程包含三个阶段,即水渗流阶段、气水混合阶段、含束缚水的气体渗流阶段,并得到了各阶段的渗流特性。煤体含水饱和度降低到0.5以下会发生“突变”,随后含水饱和度急剧减小,只存在气体渗流区域,该饱和度的变化可对产气进行预测。3)进行了不同应力条件下的单相渗流及非稳态两相渗流实验,结果表明:在同一轴压σ1、围压σ2(σ1σ2)条件下,增加孔隙压P1,气、水渗透率均随之非线性增加,孔隙压持续增大会有渗透率突增情况,且在相对低应力下,孔隙压对渗透率的作用更明显。对比注气前后水相渗透率可以看出:气相的存在不同程度增加了水的渗流阻力,且气体驱替压力越高,阻力越大,渗透率的差值越大。煤体相对渗透率的曲线形态特征为:随含气饱和度增大,气相相对渗透率曲线会急剧上升,液相相对渗透率则在较低值范围内缓慢减小,在气体突破后很快下降到零,煤样有较高的束缚水饱和度。4)煤试样在17种压力组合实验过程中,水测渗透率与气测渗透率均随有效应力的增加而减小,在相对较低有效应力范围内(小于4~5mpa),渗透率减小幅度较大。随着有效应力增加,后期渗透率变化幅度则较小。无论是选择水相作为流体还是气相作为流体,对试样进行的渗透率测量经归一化处理后,结果趋势一致。对实验数据数学拟合表明,煤体归一化渗透率与有效应力之间具有良好的幂函数关系。相比于常温下有效应力与渗透率关系曲线,加热后的曲线下降更加平缓,经过热作用后的煤应力敏感性下降,在煤层气热力开采过程中,煤应力敏感性下降会使排水产气过程更加平缓,避免了传统降压开采中遇到的气体突然大量产出的情况。5)采用气水两相渗透装置及控温试验台,进行了30℃~180℃范围内等温度间隔变化的煤体单相渗透实验及非稳态气水两相渗流实验。研究了在温度的影响下,两相流动过程的变化规律及流体渗流的特性。随温度的增加,各压力下产液量均为增加的趋势。产量增加在高温阶段(120℃以后),增幅更为明显。说明温度120℃~180℃范围内,温度越高,煤体孔隙裂隙连通性更好,同时,水分子在高温下所获的热能越大,越有利于煤体中液体产出,束缚水饱和度变小。相反,在相同温度条件下,增加围压轴压使有效应力改变,煤体内部的孔隙裂隙被压缩,部分液体无法被排出,束缚水增多,产水量减少。6)在温度作用下非稳态两相渗流全过程的三个阶段变化为:第一阶段以产水为主的液体线性渗流阶段,随温度升高,液体产量占总产液量百分比缓慢减小,温度高于120℃后变化幅度加大;第二阶段气水混合流动阶段,液体流速急速线性增加,气体流速平稳加快。随温度升高,该阶段液体产量占总产液量百分比逐渐减小,温度高于90℃后减小幅度更大。第三阶段束缚水下的气体渗流,该阶段受温度影响比较明显,液体产量占总产液量百分比提升较大。温度低于60℃该阶段液体产量仅占总产液量10%左右,产液量以第一、二阶段为主。当温度达到150℃左右,该阶段百分比达到50%左右,比例均超过了一、二阶段。7)温度在30℃到180℃的变化范围内,渗透率变化分为两个阶段:相对低温段(30℃至120℃),该阶段温度对流体影响占主导,对比气、水相渗透率,在该温度变化阶段,液体粘度变化显著,减小幅度较大,而气体粘度略有增加,渗透率测量结果与粘度的变化趋势一致。高温段(120℃至180℃),温度对多孔介质结构的改变成为主要影响因素,在此温度变化范围,液体、气体粘度变化均较平缓,而渗透率的变化却有大幅度的提高,说明受高温影响,煤样的孔隙裂隙发生了较大变化,渗流通道比低温段连通性更好,有利于流体的流动。同时,随着温度的升高,束缚水饱和度减小,气水两相渗流区变宽,水相相对渗透率随含水饱和度的降低而减小趋势变缓,温度越高,对气水两相相对渗透率曲线的影响逐渐减小。为客观地反映由于温度变化而引起的压降变化,提出了温度敏感性系数,该系数与温度较好的服从对数函数关系。8)采用欧拉观点和拉格朗日观点,对气水流动界面进行分析,将拟压力函数p=2∫PP0p/μdgZdp引入两相渗流方程。对压力函数分为三种情况讨论:①pμgZ为常数时,pg=pg;②μgZ为常数时, pg=pg2;③μgZ为压力p的一次线性函数时,p=2∫PP0p/ap+bdp。由此得到气水两相流动界面的不考虑压缩性驱替模型、考虑压缩性驱替模型及拟压力线性函数方程驱替模型。对三种模型的渗流方程及边界条件进行了详细推导,获得流体流量与位置函数的关系式。用Matlab软件分别对三种模型进行了求解计算,选取相同应力条件下的实验参数,得到了累计产出水量随时间变化曲线及不同时间下的气水流动界面位置。对比了三种模型的实验结果和误差,得到了三种模型的适用性。为实际工程中排水预测产气提供了较为可靠的模型选择依据。
[Abstract]:Coal seam gas is associated with coal, gas resources symbiosis, with its clean, high calorific value, wide utilization foreground features, has become the focus of natural gas exploration and development of unconventional. The formation of coalbed gas geological conditions and geological environment condition is complicated, and its development is affected by stress, pressure and temperature of groundwater containing multi-physics. Based on coalbed gas thermal recovery as the background, using the method of combination of experiment and simulation, the coal body at room temperature and high temperature should be the seepage law of gas water two-phase force to do the research, the main research contents and results of this paper are as follows: 1) starting from the production of coalbed gas mechanism analysis of coal seam gas mining is a process of gas desorption, diffusion and percolation ". According to the dynamic balance principle, vaporization and condensation of Fick law, Darcy law, the numerical simulation of coalbed methane gas water two-phase flow in the general solution. In the process, according to the different situation to analyze the capillary force and gravity, the seepage differential equation.2 simplified) using high precision gas water two-phase infiltration system developed by ourselves, a lot of unsteady gas water two-phase flow experiments, the results show that the unsteady process includes three stages, namely water seepage stage. The gas water mixing stage, gas seepage stage of bound water, and the seepage characteristics of each stage. The coal water saturation is reduced to less than 0.5 will happen "mutations", then the water saturation decreases sharply, there is only the gas seepage area, the change of saturation can be predicted for.3 gas production) were different under the condition of single-phase seepage force and unsteady two-phase flow experiment, the results showed that in the same axial pressure and confining pressure of 2 sigma 1, sigma (sigma 1 sigma 2) under the condition that the increase of pore pressure P1 gas, increase the water permeability were also nonlinear, continuous pore pressure There will be a sudden increase of permeability increases, and the relatively low stress, pore pressure on permeability effect is more obvious. Compared before and after gas injection water permeability can be seen: the gas phase there are different degrees of increased flow resistance of water, gas and the displacement pressure is higher, the greater the resistance, the greater the difference in permeability morphological characteristics of relative permeability curve. Coal is: with gas saturation increases, the gas phase relative permeability curve will be a sharp rise in liquid phase relative permeability is slow in the lower range decreases in gas after the break quickly drops to zero, the coal samples have higher irreducible water saturation) of coal samples in 17.4 a combination of pressure in the experimental process, increase the permeability with effective stress of water and gas permeability decreases, at a relatively low effective stress range (less than 4~5mpa), the permeability decreases greatly. With the increase of effective stress, permeability of late Change range is smaller. Whether it is the choice of water as the fluid or gas phase as fluid permeability measurement of samples after normalization, the results showed the same trend. The experimental data fitting, a good power function relationship between normalized coal permeability and effective stress. Compared to the normal temperature effective relation curves between stress and permeability, after heating curve decreased more gently, after the action of coal after heat stress sensitivity decreased in the thermal recovery process of coalbed methane in coal, the stress sensitivity will decrease the drainage gas production process more gentle, to avoid the traditional gas encountered depressurization in sudden and large output situation of gas and water by.5) two-phase osmosis device and temperature control test bench, the 30 DEG ~180 DEG range and temperature interval changes of coal single-phase permeability test and unsteady gas water two-phase flow was studied in the experiment. Under the influence of temperature variation and fluid flow characteristics of two-phase flow process. With the increase of temperature, the pressure of liquid production was increased. Yield increased at high temperature (120 C), rate of increase is more obvious. The temperature of 120 DEG C within the range of ~180 DEG C, the higher the temperature, crack coal pore connectivity is better, at the same time, the energy of water molecules at high temperature is larger, more conducive to the production of liquid in the coal body, irreducible water saturation decreases. On the contrary, at the same temperature, increasing the axial pressure and confining pressure so that the effective stress, pore fracture of coal body inside is compressed, part liquid can not be discharged, bound water increased, the water yield reduced.6) under the action of temperature changes in the whole process of three stages of non steady two-phase flow for liquid phase to produce linear seepage water of the first stage, with the increasing of temperature, the liquid yield of the total liquid yield percentage slow Reduced temperature higher than 120 DEG C after the amplitude of variation increases; second stage gas water mixture flow stage, the rapid increase of liquid velocity linear gas velocity smooth speed. With the increase of temperature, the liquid yield the total liquid yield percentage decreases, temperature higher than 90 DEG C after the reduction of slightly greater. Third stage bound gas seepage water next, the stage is affected by temperature obviously, liquid yield the total liquid yield percentage improved greatly. A temperature below 60 DEG C in the liquid phase output accounted for only about 10% of total liquid production, liquid production in first, second phases. When the temperature reaches 150 degrees Celsius, the percentage reached 50%, the proportion of more than the two stage,.7) at a temperature of 30 DEG C to 180 DEG C within the range, the change of permeability is divided into two stages: the period of relatively low temperature (30 DEG to 120 DEG), the temperature influence on fluid dominated, contrast gas, water permeability The change of temperature, phase, liquid viscosity changes significantly, decreased greatly, while the gas viscosity increased slightly, the same trend of permeability and viscosity measurements. The high temperature (120 DEG C to 180 DEG C), the change of temperature on the structure of porous media has become the main influencing factors, changes in the scope of the temperature of liquid, gas viscosity change are relatively flat, and the variation of permeability has greatly improved, that affected by high temperature, great changes have taken place in pore and fracture of coal samples, seepage channel connectivity is better than the low temperature section, facilitates fluid flow. At the same time, with the increase of temperature, the irreducible water saturation decreases, gas water two-phase flow zone broadening the water relative permeability, water saturation decreases with decreasing trend slowed down, the higher the temperature, the effect of gas water two-phase relative permeability curve decreased gradually. To objectively reflect due to temperature change The pressure drop, the temperature sensitivity coefficient, the temperature coefficient and good logarithmic function relationship with Euler and Lagrange.8) point of view, the flow of gas and water interface were analyzed, the pseudo pressure function p=2 PP0p/ dgZdp formula introduced the two-phase flow equations. The stress function is divided into three types: the P discussion gZ is a constant, pg=pg; the gZ is constant, pg=pg2; the gZ is a linear function of pressure P, p=2 formula PP0p/ap+bdp. obtained from the gas water two-phase flow interface without considering the compression displacement model, considering the compression displacement model and quasi linear function equation of pressure displacement model. The seepage equation of the three models and boundary conditions are deduced in detail, relationship between fluid flow and position function through the Matlab software. The three models are calculated, the experimental parameters should select the same stress condition, get The time curves of cumulative output and the location of gas and water flow at different time intervals are compared. The experimental results and errors of the three models are compared, and the applicability of the three models is obtained. It provides a reliable model selection basis for drainage prediction in practical engineering.

【学位授予单位】：太原理工大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：TD841

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