原位开采状态下油页岩渗透特性演化规律研究

Study on evolution of permeability characteristics of oil shale under in-situ exploitation

为研究油页岩在原位热解状态下的渗透特性演化规律,以抚顺西露天矿油页岩为研究对象,利用高温三轴渗透实验系统进行渗透实验,并借助压汞方法对孔隙微观结构特征进行表征,在此基础上对渗透特性演化机制进行分析讨论。研究结果表明:(1)油页岩孔隙率和渗透性演化具有很好的相关性,随着温度的升高,油页岩由原始致密状态演变为孔隙发育的多孔介质,孔隙率和渗透率增大;(2)孔隙压流体对孔隙结构变化产生多重物理作用影响,尤其在有机质热解产出油气阶段,经过高温渗透实验后油页岩的孔隙特征参数值较马弗炉加热有显著提升,300℃,400℃,500℃,600℃对应孔隙率分别增大0.6%,8.62%,1.82%和7.83%,600℃时大孔(孔径>1 000 nm)和中孔(100 nm≤孔径<1 000 nm)体积分别增大约1.3和2.4倍。(3) 300℃之前,油页岩渗透性主要受矿物颗粒不均匀热膨胀产生热应力和水分蒸发析出的影响;300℃~400℃时,油页岩内部的反应复杂,产生以不具有渗透性的孔隙为主,导致孔隙率和渗透性的变化出现不协同性;400℃~600℃为油气的主要热解生成阶段,油页岩内部热解孔隙不断生成并得到连通,构成了油气产物的流动通道,孔隙率增加25.77%,孔隙压力为1.0 MPa时,渗透率增大约9.06倍。

In order to study the evolution of the permeability characteristics of oil shale under in-situ exploitation,permeability tests on the oil shale from Fushun west open pit mine were performed by high temperature triaxial permeability testing machine, and the pore microstructure of the oil shale was characterized by the mercury injection apparatus. The evolution mechanism of the permeability was discussed. The results show that the porosity and the permeability of the oil shale have a good correlation with each other. With increasing the temperature,the oil shale specimens evolve from the dense state to the porous medium,and both the porosity and the permeability increase. The pore pressure fluid has multiple physical effects on the variation of the pore structure, especially at the stage of oil and gas production due to pyrolysis of the organic matter. The pore characteristic parameters of the oil shale after high temperature permeability tests are significantly improved compared with those heated in muffle furnace. The porosities of the oil shale samples at the temperature of300 ℃,400 ℃,500 ℃ and 600 ℃ increase by 0.6%,8.62%,1.82% and 7.83%,respectively. At 600 ℃,the macropore and mesopore volumes increase by about 1.3 and 2.4 times, respectively. Before 300 ℃, the permeability of the oil shale samples is mainly affected by the thermal stress caused by uneven thermal expansion of mineral particles and water precipitation. At 300 ℃-400 ℃,the complex reaction inside the oil shale occurs and many impermeable pores form,resulting in an uncoordinated change between the porosity and the permeability.400 ℃-600 ℃ is the main pyrolysis stage of the oil shale. In this stage,the pyrolysis pores in the oil shale are continuously generated and connected, forming the flow channel of oil and gas products and increasing the porosity by 25.77%,and the permeability increases by about 9.06 times when the pore pressure is 1.0 MPa.