低渗致密气藏压裂返排数值模拟研究

Numerical simulation research on fracturing fluid flowback in tight gas reservoir

气井压裂返排情况直接影响着低渗致密气藏气井压后的产能和稳产时间。考虑侵入区渗透率的损失、不同区域相渗曲线的差异,同时考虑返排初期含水饱和度和地层压力的不均匀分布,应用局部网格加密、示踪剂追踪、饱和度分区等方法建立了压裂返排的气水两相黑油模型。以示踪剂产量递减率定义了压裂返排结束的标志,研究了低渗致密气藏压裂液的滞留机理,分析了裂缝导流能力和侵入区渗透率损失对气井稳产时间和初期阶段返排率的影响。研究表明:平面上气体沿井眼处指进突破、侵入区高毛管力形成的渗吸效应、裂缝内部纵向上气椎的形成是造成低渗致密气藏压裂液滞留的主要原因;裂缝导流能力在40 D·cm左右、侵入区渗透率损失系数在小于0.3时,既有利于压裂液的返排,同时又不会降低气井的稳产时间。为制定压裂施工设计和指导压后产能评价工作提供了理论依据。

Fracturing fluid flowback directly affects gas well production and stable production period. Considering permeability loss of invaded zone, the differences of relative permeability curve in different regions, uneven distribution of initial water saturation and formation pressure, the author establishes two phase black oil model about fracturing fluid flowback by using the technologies of local grid refinement, tracer tracking and saturation partition. Based on the criteria of tracer production decline rate, the author defines the mark of end of fracturing fluid flowback, studies the retention mechanism of fracturing fluid in tight gas reservoir, analyzes the affection of fractured conductivity and invaded zone permeability loss on stable production period and early-staged flowback rate. The result shows that gas fingering along the borehole, imbibition, gas coning in vertical direction are the major causes of fracturing fluid retention. It is advantageous to fracturing fluid flowback and stable production period when fractured conductivity is about 40 D·cm and permeability loss coefficient of invaded zone is less than 0.3. This study provides a references for fracturing design and productivity evaluation after fracturing.