Surface to Borehole Electromagnetics for 3D Waterflood Monitoring: Results from First Field Deployment

Abstract

Monitoring the waterflood oil recovery process is a difficult task for seismic-based methods in hard carbonate reservoirs. The changes in velocity/density due to water-oil substitution are too small when compared to the errors involved in repeating the measurements. We detail the development of a novel technique based on surface-to-borehole controlled-source electromagnetics (CSEM), which exploits the large contrast in resistivity between injected water and oil to derive 3D resistivity distributions (proportional to saturations) in the reservoir. Surface-to-borehole CSEM responses using surface electric transmitters and borehole EM receivers were modeled using a black-oil simulator for a large oil field in the Middle East. Results indicate that the vertical electric field can be used to detect waterfront changes in 2 and 5-year time-lapse scenarios. A surface-to-borehole acquisition system was engineered comprising the development of a powerful, custom-made electric transmitter (2000V/500A) and borehole electric and magnetic field sensors. The surface-to-borehole 3D CSEM technology was deployed for the first field trial in the same well used for the modeling study. The field demonstration was carried out in 2017 comprising 144 permanent surface electrodes drilled in a radial configuration around a vertical observation well. The current electrodes were designed in an L pattern to form 48 inline (radial) and 48 cross-line (tangential) dipoles at a nominal range of 600m to 3,500m from the vertical observation well. A wireline sensor array comprising two vertical electric and two vertical magnetic field sensors recorded the EM signal transmitted from the surface in regularly spaced positions in the reservoir section. Surface measurements of transient EM (TEM), CSEM and magnetotellurics (MT) were recorded together with the borehole acquisition to characterize the overburden and the shallow subsurface. The acquired dataset was processed to increase the signal/noise content of the data as well as to correct for casing effects and surface distortions. A marked asymmetry of the EM responses is recorded in the vicinity of the observation well which translates, after 3D inversion, to resistivity distributions consistent with the saturation/production logs acquired in the nearby wells. The sensitivity of the inversion extends up to 1.8 km away from the observation well. Analysis of the measurement repetition errors compared to predicted EM responses after 2 and 5 years indicates that time-lapse surveys would provide detailed mapping and an independent estimation of saturation variations related to waterflooding. The encouraging results obtained from the first surface-to-borehole CSEM survey in a producing oil field suggests that the technology may become an important tool for analyzing the waterfront evolution in the interwell space. By doing this, the technology is expected to enhance reservoir management and history matching.