流体异常高压对深层储集层物理性质的作用机理——以准噶尔盆地南缘侏罗系头屯河组为例

Action mechanisms of abnormal fluid pressure on physical properties of deep reservoirs:A case study on Jurassic Toutunhe Formation in the southern margin of Junggar Basin,NW China

针对超压对深层储集层骨架颗粒物理变化的作用机制以及超压与静水压力条件下骨架颗粒物理变化的差异性问题,以准噶尔盆地南缘侏罗系头屯河组砂岩为例,开展超压对深层储集层物理性质作用机理的物理模拟实验。研究结果表明:①在模拟埋深6000~8000 m的条件下,超压环境下机械压实作用使剩余原生孔的减小值约是静水压力下的二分之一,超压可有效抑制机械压实作用,使粒间原生孔隙得以保存。②超压环境下的颗粒线状接触比例始终小于相同深度静水压力下的颗粒线状接触比例,但二者差值向深层呈现减小趋势,揭示流体异常高压抵抗有效应力增大的作用减弱,超压下机械压实程度向深层有逐渐接近静水压力下机械压实程度的趋势。③由于异常高压流体的存在,深层储集层在超压下易形成拉张裂缝,骨架颗粒内微裂缝呈细长特征,而静水压力下骨架颗粒内微裂缝呈短宽特征。④超压下骨架颗粒内微裂缝发育的主要时期晚于静水压力下,微裂缝发育程度及长度均有向更深层延伸的特点。⑤超压下骨架颗粒内微裂缝的发育期次主要受控于流体异常高压的发育期次以及作用于骨架颗粒上的有效应力大小,深层储集层超压下骨架颗粒内微裂缝发育期次多于静水压力下的发育期次,流体异常高压的多次发育对改善深层储集层的物理性质具有重要作用。

Considering the action mechanisms of overpressure on physical changes in skeletal grains of deep reservoir rocks and the differences in physical changes of skeletal grains under overpressure and hydrostatic pressure,the sandstone of the Jurassic Toutunhe Formation in the southern margin of Junggar Basin was taken as an example for physical modeling experiment to analyze the action mechanisms of overpressure on the physical properties of deep reservoirs.(1)In the simulated ultra-deep layer with a burial depth of 6000–8000 m,the mechanical compaction under overpressure reduces the remaining primary pores by about a half that under hydrostatic pressure.Overpressure can effectively suppress the mechanical compaction to allow the preservation of intergranular primary pores.(2)The linear contact length ratio under overpressure is always smaller than the linear contact length ratio under hydrostatic pressure at the same depth.In deep reservoirs,the difference between the mechanical compaction degree under overpressure and hydrostatic pressure shows a decreasing trend,the effect of abnormally high pressure to resist the increase of effective stress is weakened,and the degree of mechanical compaction is gradually close to that under hydrostatic pressure.(3)The microfractures in skeletal grains of deep reservoirs under overpressure are thin and long,while the microfractures in skeletal grains of deep reservoirs under hydrostatic pressure are short and wide.This difference is attributed to the probable presence of tension fractures in the rocks containing abnormally high pressure fluid.(4)The microfractures in skeletal grains under overpressure were mainly formed later than that under hydrostatic pressure,and the development degree and length of microfractures both extend deeper.(5)The development stages of microfractures under overpressure are mainly controlled by the development stages of abnormally high pressure and the magnitude of effective stress acting on the skeletal grains.Moreover,the development stages of microfractures in skeletal grains are more than those under hydrostatic pressure in deep reservoir.The multi-stage abnormally high pressure plays an important role in improving the physical properties of deep reservoirs.