超临界CO2在致密油藏中的扩散前缘预测

国内外大部分学者认为超临界CO2在致密油藏中的扩散是吞吐提高采收率的关键因素之一。通过设计基质-裂缝模型结合压降法搭建了测定超临界CO2在饱和油岩芯中扩散系数的实验装置,系统研究了压力、储层物性等油藏条件对超临界CO2扩散系数及浓度分布的影响规律,建立了超临界CO2浓度场及扩散前缘的预测方法。实验结果表明,超临界CO2在致密岩芯(0.06 mD)中的扩散系数为10-12 m2/s数量级,扩散系数随着初始注气压力的升高而增大,最终趋于平缓,但在临界压力点附近出现最大扩散系数;扩散系数随基质渗透率和孔隙度增大而增大,随岩芯迂曲度的增大而快速递减。经过900 d扩散,扩散前缘仅前进了0.095 m,因此,在致密油藏CO2吞吐现场作业周期内忽略扩散作用是合理的。在扩散后期,CO2浓度梯度越来越小,扩散速度逐渐降低。

碳酸水-原油体系中CO2分子的扩散行为

CO2分子在油水相间的扩散传质行为对于致密油藏注CO2提高采收率具有重要意义。通过融合实验测定数据和理论扩散模型,利用试错法同时确定了碳酸水-原油二元体系中CO2分子在油、水两相中的扩散系数。重点研究了油水混合体系的初始压力对扩散系数的影响规律,深入探讨了扩散过程中油水相密度、CO2浓度、油水界面移动以及CO2在油相中扩散前缘位置的变化过程。研究结果表明:(1)随着CO2从水相扩散进入油相,碳酸水-原油体系压力增大,60℃下,扩散10 h,初始压力分别为15.39 MPa、19.25 MPa和22.82 MPa(实验1—实验3)体系,压力增加24%~31%;(2)在两相界面附近,水相密度及水相中CO2浓度逐渐降低,而油相密度及油相中的CO2浓度逐渐升高,导致油相体积膨胀,水相体积收缩,两相界面向水相方向移动;(3)初始压力越高,扩散系数越大。实验1—实验3水相和油相中CO2扩散系数的增幅分别为52.0%和9.2%,表明水相中CO2扩散系数对初始压力更敏感;(4)初始压力越高,相同扩散时间内CO2在油相中的扩散前缘运移越远,扩散100 h后,实验1—实验3前缘位置运移距离分别为4.11 cm、4.32 cm和4.43 cm。

Unsteady Behavior of Leading-edge Vortex and Diffuser Stall in a Centrifugal Compressor with Vaned Diffuser

The characteristics of a rotating stall of an impeller and diffuser and the evolution of a vortex generated at the diffuser leading-edge(i.e., the leading-edge vortex(LEV)) in a centrifugal compressor were investigated by experiments and numerical analysis. The results of the experiments revealed that both the impeller and diffuser rotating stalls occurred at 55 and 25 Hz during off-design flow operation. For both, stall cells existed only on the shroud side of the flow passages, which is very close to the source location of the LEV. According to the CFD results, the LEV is made up of multiple vortices. The LEV is a combination of a separated vortex near the leading-edge and a longitudinal vortex generated by the extended tip-leakage flow from the impeller. Therefore, the LEV is generated by the accumulation of vorticity caused by the velocity gradient of the impeller discharge flow. In partial-flow operation, the spanwise extent and the position of the LEV origin are temporarily transmuted. The LEV develops with a drop in the velocity in the diffuser passage and forms a significant blockage within the diffuser passage. Therefore, the LEV may be regarded as being one of the causes of a diffuser stall in a centrifugal compressor.