长期注水对致密砂岩油藏岩石力学性质影响机制研究

Mechanism of the effect of long-term water injection on mechanical properties of tight sandstone

注水开发是致密砂岩油藏增产的重要手段,长时间注水会改变地层岩石物理力学性质,影响油井产量。为探究长时间注水对地层岩石物理力学性质影响的微观机制,选取同一储层已注水15a及尚未注水岩芯进行对比试验分析。通过试验得到了注水前后地层岩石在弹性力学参数、矿物成分、微观孔隙结构等方面的差异。长时间注水后,岩石矿物成分及内部结构发生变化,主要表现为黏土矿物及方解石含量降低,岩石颗粒间填充物及胶结物大量减少,中小孔隙发育为大孔隙,孔隙度增大,进而导致致密砂岩力学性质弱化,变形能力增大。

Water injection is an important means to increase the production of tight sandstone reservoirs, which is widely used in tight oil and gas exploration. However, long-term water injection changes the physical and mechanical properties of formation rocks and further affects the production of oil wells. Comparison experiments were carried out on the same formation cores with water injection for 15 years and without water injection to explore the microscopic mechanism of the effect. The differences of elastic mechanics parameters, mineral composition and microscopic structure of the formation rocks before and after water injection were obtained through experiments. After the long-time water injection, the mineral composition and internal structure of rock were changed. Especially the loss of clay minerals in the form of cement and fillers caused a decrease in the cohesion among the rock particles and an increase in the porosity. After long-time water injection, clay minerals and calcite content decreased, the fillers and cements between rocks particles reduced, the small pores developed into large pores. Thus, the mechanical properties of sandstone were weakened and the deformation capacity increased.