油页岩不同温度原位热解物性变化核磁共振分析

NMR analysis of the physical change of oil shales during in situ pyrolysis at different temperatures

油页岩原位热解过程中产生的孔隙和裂缝的连通程度是制约转化后的油页岩油气能否原位可采的关键要素,而常规岩石物性测试手段无法全覆盖测定油页岩层内不同级别的孔隙及裂缝。利用核磁共振仅对岩石孔隙流体有响应可以识别刻画不同级别孔、缝的优势,根据核磁共振分析岩石物性的方法和相关参数模型,开展了模拟地下500 m原位加热到不同反应温度后的油页岩热解系列样品的核磁共振测试。结果表明,不同转化温度原位开采过程中,油页岩的孔隙度演变可以分为3个阶段,250-350℃时逐渐增大,350~400℃时略有减小,400℃之后大幅增大;渗透率在400℃之前变化不大,400-450℃渗透率提高了2个数量级,500℃时改善更为可观,提高了4个数量级。油页岩原位干馏开采需要400℃以上的高温,而实际地下开采大尺度的油页岩受热均一性较差,可能大部分区域温度达不到400℃,可以采取升到更高的温度并延长加热时间或加热前对油页岩层进行储层压裂改造,以改善油页岩层的物性,提高油页岩原位开采油气采收率。

The connectivity between pores and fissures during in situ oil shale pyrolysis is an important element which controls shale oil and gas recoverable amount. However,conventional petrophysical testing methods can not cover all levels of pores and fissures in oil shales. Nuclear magnetic resonance（ NMR） can show fluids in core pores and fissures,hence can be used to identify different levels of pores and fissures. We carried out NMR tests with oil shale samples by simulating the same conditions as 500 m underground and heating the samples to different temperatures. Results showed that the porosity of oil shales change according to temperature during in situ exploitation. Porosity increases from 250 to 350 ℃,decreases slightly from 350 to 400 ℃,and then increases again after 400 ℃. Permeability remains stable when temperature is lower than 400 ℃,increases by 102 times from 400 to 450 ℃,and increases by 104 times at 500 ℃. The in situ retorting of oil shales should be made at a temperature higher than 400 ℃; however,oil shales underground might not reach 400 ℃ in many areas. In this case,we should explore at a higher temperature and heat for longer time,or fracture oil shales before heating.