【作者】 杨杰；

【导师】 王志荣；

【作者基本信息】 郑州大学 ， 岩土工程， 2020， 硕士

【摘要】 煤层气井的产能预测一直都是天然气工业领域致力于解决的关键技术难题,更是合理开发我国丰富煤层气资源的重要理论依据,而建立能够准确反映煤岩储层中煤层气流动机理的渗流产出模型是合理预测煤层气长期产能的关键。由于煤岩储层内部含有多重孔隙结构,开采过程中存在基质扩散、原生裂隙渗流、水力裂缝失效变形以及井底动态流压变化等多种复杂机理的综合作用,而现有产能预测模型主要基于常规流体渗流的基本理论,尚未全面考虑煤岩储层内的多种复杂因素的耦合效应,难以准确反映非线性流动条件下煤层气渗透机理及产出规律。鉴于此,本文以焦作典型“三软”矿区(软顶、软煤和软底)为研究对象,将含煤地层力学性质作为切入点,以求解压裂目标层渗流产出规律为关键,结合区内现场压裂抽采实验,展开了一系列理论与实践相结合的研究。首先,深入分析井筒周围源岩应力的影响,考虑煤岩储层中原生裂缝的几何特征与压裂液渗透作用,引入了含裂缝几何特征参数的煤岩体起裂应力模型;着重分析了煤岩储层裂缝延伸过程中应力损伤对煤岩本体变形的影响,建立了岩体损伤本构关系,并在起裂压力模型的基础上,推导出煤岩储层中水力裂缝内延伸应力计算公式;再结合经典的PKN模型假设以及水力裂缝内净压力的非线性压降规律,继而建立了改进的煤岩储层水力裂缝扩展模型,拟合出煤岩储层水力裂缝长度、宽度及高度等几何特征参数。然后,基于煤岩储层水力裂缝扩展模型,考虑煤体孔隙率随压裂抽采时间的变化规律,并结合渗透率与煤体孔隙率的动态变化关系,运用数学物理方程建立了煤岩储层动态渗透率模型。其次,基于动态渗透率模型,结合垂直井筒周围的实际受力情况,考虑非线性渗流条件下气体压力梯度的变化规律,利用流体流动的能量守恒方程建立了井底动态流压模型,并深入分析了井底动态流压的变化对煤层气渗流产出的影响,进而建立了煤层气井产能预测模型。在此基础上,应用数值分析软件将煤层气试验井实采数据与理论产出试验期70d的产气量进行对比,发现二者产能动态变化曲线吻合度较高,从而验证了煤层井产能预测模型的正确性。最后,结合含气量测定分析试验、等温吸附试验、岩石力学性质测定试验以及原始渗透率测试成果,运用煤层气井产能预测模型分别对焦作矿区四口煤层气压裂试验井进行了生产期2430d的长期产能预测。结果表明:随着井底动态流压下降,四口煤层气试验井产能曲线总体变化趋势均表现为先上升后下降。1号煤层气试验井在排采期为30天时左右时,井底动态流压降至0.82Mpa,产气量达到峰值,累计产气量为9.92×10~4m~3,平均日产气量为40.82m~3;2号煤层气试验井排采期为60天左右时,井底动态流压降至0.27Mpa,产气量达到峰值,累计产气量为1.14×10~5m~3,平均日产气量为46.98m~3;3号煤层气试验井排采期为55天左右时,井底动态流压降至0.44Mpa,产气量达到峰值,累计产气量为1.96×10~5m~3,平均日产气量为80.63m~3;4号煤层气试验井排采期为50天左右时,井底动态流压降至0.18Mpa,产气量达到峰值,累计产气量为3.97×10~5m~3,平均日产气量为163.31m~3,研究成果对同类压裂抽采的产能预测具有一定的借鉴意义。更多还原

【Abstract】 Production capacity prediction of CBM wells has always been a key technical problem in the field of natural gas industry,and it is also an important theoretical basis for the rational development of China’s abundant CBM resources.The key to predict long-term coalbed methane production capacity is to establish a seepage output model which can accurately reflect the coalbed methane flow mechanism in coal and rock reservoirs.Due to the multi-pore structure in coal-rock reservoir,matrix diffusion,fracture seepage,failure deformation of hydraulic fracture and dynamic change of bottom-hole flow pressure,the existing capacity prediction model is mainly based on the basic theory of conventional fluid seepage,and coal-rock has not been fully considered.The coupling effect of various complex factors in the reservoir makes it difficult to accurately reflect the permeability mechanism and output law of CBM under nonlinear flow pressure.In view of this,this paper takes the typical"three soft"mining area of Jiaozuo(soft top,soft coal and soft bottom)as the research object,takes the mechanical properties of coal-bearing strata as the entry point,and takes solving the seepage production law of fracturing target layer as the key point.First of all,the influence of source rock stress around wellbore was deeply analyzed,considering the geometric characteristics of primary fractures and fracturing fluid permeability,the fracture stress model of coal rock body with fracture control parameters was introduced.The influence of stress damage on deformation of coal rock body during fracture extension was analyzed,and the constitutive relation of rock mass damage was established.Based on the assumption of classical PKN model and the nonlinear pressure drop law of net pressure in hydraulic fracture,an improved model of hydraulic fracture propagation in coal-rock reservoir was established.Geometric parameters such as length,width and height of hydraulic fractures in coal and rock reservoirs.Then,based on the hydraulic fracture expansion model of coal and rock reservoir,considering the variation law of coal body porosity with fracturing extraction time,and combining the dynamic variation relationship between permeability and coal body porosity,the dynamic permeability model of coal and rock reservoir was established by mathematical and physical equations.Then,based on the dynamic infiltration model,considering the actual stress situation around the vertical well bore,considering the change law of the gas start pressure gradient under the non-linear seepage condition,using Energy Conservation Equation for Fluid Flow to establish the bottom hole dynamic flow pressure model,and the influence of the change of the bottom hole pressure on the seepage output of the coal bed gas was emphatically considered,And then a capacity prediction model was established.On this basis,using the numerical simulation software,the data of the coal bed gas test well and the gas production of the theoretical output test period of 70d were compared and analyzed,and the dynamic change curves of the two were found to be consistent,so that the correctness of the capacity prediction model was verified.In the end,combined with analysis test for gas content determination,isothermal adsorption test,rock mechanical properties test and original permeability test results,the production capacity prediction model was used to predict the long-term production capacity of 2430 d in four CBM fracturing test wells in Jiaozuo mining area.The results show that the production capacity of the four CBM test wells is basically the same.With the decrease of the dynamic flow pressure at the bottom of the well,it shows the trend of first rising and then decreasing.When the No.1 CBM test well is around 30 days,the bottom hole dynamic flow pressure is0.82Mpa,the gas production peaked,the cumulative gas production is 9.92×10~4m~3,and the average daily is production was 40.82m~3;When the No.2 CBM test well drainage period is about 60 days,the bottom hole dynamic flow pressure is 0.27Mpa,the gas production reaches the peak,the cumulative gas production is 1.14×10~5m~3,and the average daily gas production is 46.98m~3;the No.3 CBM test well drainage period When it is about 55 days,the bottom hole dynamic flow pressure is 0.44Mpa,the gas production reaches a peak,the cumulative gas production is 1.96×10~5m~3,and the average daily gas production is 80.63m~3;when the drainage period of No.4 CBM test well is about 50 days The bottom hole dynamic flow pressure is 0.18Mpa,the gas production reaches the peak,the cumulative gas production is 3.97×10~5m~3,and the average daily gas production is 163.31m~3.The research results have certain reference significance for the similar fracturing drainage production prediction.更多还原