基于分形理论的致密砂岩储层微观孔隙结构表征r——以冀中坳陷致密砂岩储层为例

摘要

为了评价致密砂岩储层类型,为致密油气的勘探与开发提供理论依据,利用分形理论和高压压汞方法,结合储层物性资料,通过对11个致密砂岩样品的压汞实验,研究了冀中坳陷致密砂岩储层微观孔隙结构.结果表明:根据进汞曲线拐点,将致密砂岩储层孔隙系统按直径大小划分为裂隙(>10μm)、大孔(1~10μm)、中孔(0.1~1.0μm)和微孔(<0.1μm).依据分形理论,分别求取各尺度孔隙分形维数,验证了孔隙系统划分的正确性.根据不同尺度孔隙的分布频率,结合样品孔渗、排驱压力和退汞效率等参数将致密砂岩储层分为3类:Ⅰ类储层微孔分布频率高,但几乎无连通孔隙,具有较低的渗透率;Ⅱ类储层连通孔隙发育,但微孔较少;Ⅲ类储层不仅有大量微孔,同时有丰富的连通孔隙,渗透率也较高.通过分析得出,微孔分布频率越高,退汞效率越高,孔隙结构越简单,均质性越好;裂隙和大孔均决定了储层的渗流能力.因此,Ⅲ类致密砂岩储层为最优质的储层,可作为致密油气勘探与开采的首选目标.%To evaluate the type of tight sandstone reservoir and provide theoretical basis for the exploration and development of tight oil and gas,the fractal theory,mercury intrusion capillary pressure analyses,as well as reser-voir physical properties,were used to discuss the pore structure characteristics of tight sandstone reservoir in Ji-zhong Depression,by analyzing 11 tight sandstone samples. The results show that an evaluation method for frac-ture(>10μm,in diameter),macropores(1-10μm),mesopores(0.1-1.0μm)and micropores(<0.1μm)within tight sandstone reservoir was established by mercury intrusion curves. This method was verified using fractal geo-metry theory. Based on the above classification,the volume contents of pores and fractures were estimated by mercury intrusion curves. Morever,three types(Ⅰ,ⅡandⅢ)of tight sandstone reservoir in Jizhong Depres-sion were analyzed based on the distribution of pore volume,porosity,permeability,displacement pressure and mercury withdrawal efficiency. Type I tight sandstone reservoir is characterized by abundant micropores and rarely open fractures,which leads to a low permeability. TypeⅡtight sandstone reservoir has high open fractures but a few micropores. TypeⅢtight sandstone reservoir not only has abundant micropores but also has high open frac-tures. Two main parameters obtained from the mercury porosimetry analysis, the mercury withdrawal efficiency and permeability,reflect the totally minable potential and production rate of tight oil and gas,respectively. The more the micropores develop,the higher the mercury withdrawl efficiency is,and the simpler and more homoge-neous the pore structure is. The flowing capacity is controlled by fractures and macropores. Therefore,the typeⅢtight sandstone reservoir in Jizhong Depression is the most appropriate to exploit.