煤层具有非均值性较强、压裂裂缝展布不均匀、非对称分布等特点,目前缺少煤层非对称裂缝产能评价模型。针对上述问题,运用Langmuir等温吸附方程及Fick扩散定律描述煤层气藏中的吸附、解吸及扩散现象,引入特殊的拟时间函数解决考虑吸附解吸现象的物质平衡方程中非线性较强的问题,利用新定义的参数表征压裂过程中水力裂缝的非对称性,得到裂缝不同对称程度时的产能评价模型及其解析解,通过时间叠加原理,实现了实际生产数据与理论数据的拟合。研究表明:裂缝的非对称性对气藏产能评价及预测结果影响较大;对称裂缝地层能量利用率最高;裂缝的非对称性对产能评价的影响主要发生在生产前期,随着裂缝导流能力的增加,裂缝的非对称性对气井产能的影响越来越小。研究成果对准确评价和预测煤层气藏压裂直井产能和高效开发具有重要指导意义。
The coalbed methane reservoirs are characterized by strong heterogeneity, uneven distribution of fracturing fractures, and usually asymmetric distribution. At present, there is no evaluation model for productivity of coalbed methane reservoirs with asymmetric fractures. In view of the above problem, Langmuir isotherm adsorption equation and Fick diffusion law were used to explain the adsorption, desorption and diffusion in coalbed methane reservoirs, and a special pseudo-time function was introduced to solve the strong nonlinearity in the material balance equation considering adsorption and desorption. The new defined parameters were used to characterize the asymmetry of hydraulic fractures in the fracturing process, and the productivity evaluation model and its analytical solutions under different symmetry of fractures were obtained. With the principle of time superposition, the actual production data was fitted with the theoretical data. It was found in the study that the asymmetry of fractures greatly affected the results of productivity evaluation and prediction of gas reservoirs; the energy utilization rate of symmetric fractures was the highest; the asymmetry of fractures mainly affected the productivity evaluation in the early production; with the increase of fracture conductivity, the effect of the fracture asymmetry on gas well productivity became lower. The study results are of great important guidance for the accurate evaluation and productivity prediction of fractured vertical wells and the efficient development of coalbed methane reservoirs.
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