电性源瞬变电磁法油气藏动态监测模拟分析

油气藏开采过程中,储层剩余油气被流体驱替前、后声阻抗变化较小,会造成时移地震监测失效,但驱替前、后储层电阻率变化较大,因而时移电磁法在油气藏动态监测中具有物性方面的优势。通过三维数值模拟,分析了电性源瞬变电磁法对剩余油气藏动态监测的能力。为提高数值模拟精度,基于非结构化网格矢量有限元法,采用二阶后退欧拉法(BDF2)变步长差分格式,实现了电性源瞬变电磁三维正演。对比均匀半空间电磁场解析解及三维复杂模型的数值结果,验证了该方法满足正演精度要求。基于此模拟方法,计算并分析了复杂地质背景油气藏模型的电场相对异常动态监测响应特征。基于涪陵页岩气实际地质资料进行建模,对比压裂前、后电场相对异常。研究结果表明,电性源瞬变电磁法对油气藏动态监测响应效果明显,能满足复杂三维油气藏动态监测的地质要求,应用前景广阔。

Simulation and analysis of dynamic monitoring of oil and gas reservoir based on grounded electric source TEM

The change of acoustic impedance is small before and after the remaining oil and gas in a reservoir is displaced by fluid during oil and gas reservoir recovery, which can cause the failure of time-lapse seismic monitoring. However, the resistivity changes greatly before and after the displacement, and thus the time-lapse electromagnetic method has inherent advantages in the dynamic monitoring of oil and gas reservoirs. In light of this, we explore the capability of the grounded electric source TEM (transient electromagnetic) method for dynamic monitoring of remaining oil and gas reservoirs through three-dimensional (3D) numerical simulation. More importantly, to improve the numerical simulation accuracy, we use the difference scheme with variable step size from the second-order backward Eulerian method (BDF2) on the basis of the unstructured-grid vector finite element method. In this way, we realize the 3D forward modeling of grounded electric source TEM. In addition, the comparison between the analytical solution of a uniform half-space electromagnetic field and the numerical result of the 3D complex model verifies that the method meets the accuracy requirements of forward modeling. Utilizing this simulation method, we calculate and analyze the dynamic monitoring response characteristics of the relative anomalies of an oil and gas reservoir model under complex geological background. Subsequently, numerical modeling is performed for the actual geological data of Fuling shale gas, and the relative anomalies of electric field before and after fracturing are investigated. The results show that the grounded electric source TEM method has a good response for dynamic monitoring of oil and gas reservoirs and can meet the geological requirements of dynamic monitoring of complex 3D oil and gas reservoirs, which thus has broad application prospects.