致密气藏压后关井压裂液静态渗吸实验研究——以大庆油田徐家围子区块为例

影响致密储层渗吸的因素较多,但系统的因素划分和敏感性分析依然具有很大挑战。为了高效开采致密气藏,为致密气藏压后焖井时间即开井制度优化提供依据,选取大庆油田徐家围子区块致密砂岩天然岩心,结合压汞、电镜扫描和X射线衍射等实验,在室内开展了静态渗吸实验,研究了致密储层静态渗吸的规律及其影响因素,包括渗吸时间、致密储层的渗透率、孔隙类型、润湿性和压裂液的离子浓度。实验结果表明:致密砂岩岩心静态渗吸气水置换主要作用在静态渗吸过程的早期阶段;在致密气藏的储层渗透率范围内,静态渗吸采出程度随渗透率增大而增大;原生孔为储气空间,与原生粒间孔相连的次生溶蚀孔、粒间溶蚀缝和颗粒边缘溶蚀缝为渗吸提供了畅通的通道;静态渗吸速度和最终采出程度随岩心亲水性的增强而增加;压裂时所用的压裂液浓度越低,储层的渗吸效果越好。研究结果可为致密气井压裂后排采制度设计提供基础。

Experimental study on fracturing fluid static imbibition in tight gas reservoirs in the shut-in stage after fracturing: An example of the block Xujiaweizi in Daqing oilfield

Many factors can influence fluid imbibition into tight formations. The comprehensive classification of such influencing factors and sensitivity analyses remain challenging so far. Further studies on this issue will help efficiently develop tight gas reservoirs and provide a basis to optimize the shut-in time and working schedules for tight gas wells after hydraulic fracturing. Combined with mercury tests, electron microscopy, and X-ray diffraction experiments, natural cores of tight sandstone in the Daqing Oilfield are selected to conduct an indoor static imbibition experiment to examine the effects of different factors on the static imbibition of tight gas reservoirs, including imbibition time, permeability, pore types, wettability, and ion concentration of fracturing fluid. The experimental results show that static imbibition of liquid and gas-liquid exchange in tight sandstone cores mainly play a role in the early stage of static imbibition process. The degree of static imbibition increases with increase of the permeability within the range of tight gas reservoirs. Linked with the primary intergranular pores which are the main spaces for gas storage, the secondary generated dissolution pores, intergranular and marginal dissolution cracks provide smooth channels for imbibition. The stronger the wettability of core is, the higher the imbibition rate and recovery factor are; The lower concentration of fracturing fluid used during fracturing yields the better imbibition effect. The research results can be used as a basis to design the fracturing fluid flowback for tight gas wells after fracturing.