评价煤层吸附气解吸能力的生产数据系统分析新方法

主要考虑了煤层气开发过程中吸附气非稳态解吸的关键因素作用，建立了考虑吸附气非稳态解吸的煤层气直井的线性渗流模型，并通过全隐式有限差分方法求得了内边界定流量、外边界封闭渗流模型的数值解。通过数值结果分析主要表明了非稳态解吸系数会使得无因次瞬时井底压力与压力导数的双对数特征曲线在封闭边界控制的拟稳态流动段均出现下落趋势，两曲线间距离变宽，不再急剧上翘收敛为一条直线；这是由于吸附气解吸能及时补充到气藏，减缓了地层压力的降落；而井筒储集系数不影响由封闭边界控制、受吸附气解吸作用影响的拟稳态流动段；表皮因子影响拟稳态流动段无因次井底压力的变化，但不影响拟稳态流动段无因次井底压力导数的变化。该线性渗流模型仍满足Duhamel原理，生产数据分析时可利用反褶积技术手段，首先将按照拟压力定义线性化的变流量生产数据转化为单位流量下瞬时井底拟压力数据，然后基于所建渗流模型的数值解以及主要参数的敏感性影响分析结果，通过瞬时井底压力特征曲线拟合方法对反褶积处理后的归一化生产数据进行分析，可解释出表征煤层吸附气解吸能力的非稳态解吸系数、煤层渗透率、表皮因子、外边界半径等重要参数。由此建立了煤层气直井生产数据的特征曲线系统分析方法。最后，通过一口煤层气直井的实际生产数据分析验证了所提出的煤层气直井生产数据特征曲线分析方法的有效性和实用性；并采用解释参数数据进行模型计算，分析了吸附气解吸作用对煤层气井流量贡献的瞬时变化规律；还通过公式推导表明了当煤层中渗流到达稳态时，整个煤层吸附气解吸速度将等于煤层气井生产的流量。

A new method of systematic analysis of production data for evaluating the desorption ability of adsorbed gas in coal beds

In consideration of the key influential factor of the unsteady desorption of the absorbed gas in the development process of coalbed methane,a linear radial seepage flow model for a vertical production well of coalbed methane is constructed through the definition of a pseudo pressure of gas.The numerical solution for the seepage flow model with constant production rate at the inner boundary and no flow at the outer boundary is obtained by a fully implicit finite difference method.Through the analysis of numerical results,it mainly indicates that the unsteady desorption coefficient can make the log-log type curve of the dimensionless transient wellbore pressure and the dimensionless wellbore pressure derivative at the pseudo-steady flow stage,which is dominated by the outer boundary at the late period,and have a falling trend.The two curves become distant and do not both upturn steeply to converge to one straight line yet.The reason is that the desorption of the adsorbed gas can supply the gas sources for the gas reservoir in time,which can slow down the formation pressure drop.However,the coefficient of wellbore storage does not affect the type curve at the pseudo-steady stage,which is dominated by the closed outer boundary and is also affected by the desorption function of absorbed gas.The skin factor affects the transient change of the dimensionless wellbore pressure at the pseudo-steady flow stage,but has little effect on the dimensionless wellbore pressure derivative at the pseudo-steady flow stage.The linear seepage flow model still satisfies the Duhamel principle,and thus the deconvolution technique as a tool can be used in the production data analysis,which can transform the linearized production data according to the definition of pseudo pressure under variable production rate into the transient pseudo wellbore pressure data under the constant unit production rate.Then based on both the aforementioned numerical solutions of the constructed seepage flow model and the analysis results for the sensitivity effects of the main parameters,the normalized production data through the deconvolution treatment can be analyzed through the type curve fitting method for the transient wellbore pressure.As a result,some significant parameters such as the unsteady desorption coefficient representing the desorption ability of the adsorbed gas in coal beds,the permeability of coal beds,the skin factor and the radius of the outer boundary can be interpreted.Therefore,a systematic method for the production data analysis for a vertical production well of coalbed methane is presented.Finally,the validity and practicability of the type curve analysis method for the production data for a vertical production well of coalbed methane are verified through the analysis for the actual production data from a vertical well of coalbed methane.Furthermore,the model computation is performed by using the data of the interpreted parameters,and then the transient change principle of the contribution of the desorption function of the adsorbed gas to the wellbore production rate of coalbed methane can be analyzed.It is also demonstrated through the formula deduction that when the seepage flow in the coalbed comes to the stable state,the desorption rate of the absorbed gas in the whole coalbed will be equal to the production rate of the coalbed methane well.