Experimental Verification of a New Approach to Long-Range EM Imaging

摘要

Proximity sensing was recently proposed as a way to simultaneously increase both range and resolution in cross-well electromagnetic(EM)tomography.The approach is applicable to reservoirs with resistive seals.Earlier reports were based on finite element models of layered structures,with dielectric and conductivity contrasts matching those of known reservoirs.Experimental work,now reported,is consistent with expectations based on finite element model simulations.Synthetic layered structures have been investigated using a 1.3 GHz ground penetrating radar(GPR)system.A scaled reservoir model was constructed in a 1 m tank comprising sand filled with fluids of variable dielectric constant and conductivity.In this system,dry sand,brine saturated sand,and a polymer foam,provide a useful mimic for the electrical properties expected for a carbonate reservoir sealed by anhydrite.Data was recorded in the time domain using EM transients.Observed trends in velocities and amplitude shifts were consistent with finite element models.Interestingly,polarization dependent signal transport first indicated by finite element modeling was supported by these experimental results.Results to date indicate that greatly increased EM propagation can be achieved through resistive geologic layers,rather than directly through relatively conductive reservoir media.We confirm that these layers act as planar transmission lines and not as waveguides-meaning that there is no hard lower cutoff frequency,and longer wavelengths can be used to sense and characterize reservoir fluids proximal to the dielectric channel.The results also confirm that variations in bounding layers modulate the amplitude and velocity of the signal in the dielectric channel,and thereby demonstrate the concept of proximity sensing.These results support a new technical direction for EM characterization of reservoirs,especially in conjunction with magnetic contrast agents,enabling efficient localization of bypassed oil,and mapping the remaining oil columns in mature reservoirs.