Surface-to-Borehole Electromagnetics Hold Promise for 3D Waterflood Monitoring

Abstract

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 191544, “Surface-to-Borehole Electromagnetics for 3D Waterflood Monitoring: Results From First Field Deployment,” by Daniele Colombo and Gary McNeice, Saudi Aramco, and Nestor Cuevas and Mauro Pezzoli, Schlumberger, prepared for the 2018 SPE Annual Technical Conference and Exhibition, Dallas, 24–26 September. The paper has not been peer reviewed. Monitoring the waterflooding oil-recovery process is a difficult task for seismic-based methods in hard carbonate reservoirs. The changes in velocity and density caused by water/oil substitution are too small when compared with the errors involved in repeating the measurements. The authors detail the development of a 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. Introduction CSEM techniques for reservoir-fluid characterization and monitoring are applied on a commercial basis for cross-well configurations. The method is based on electromagnetic (EM) induction and, as such, uses magnetic sources and magnetic receivers. While it has been implemented successfully in many field trials, the method is limited to 2D geometries and its sensitivity is biased toward detecting conductors. Modeling has shown that the setup of magnetic sources and magnetic receivers in a 3D surface-to-borehole configuration does not provide a useful signal above the estimated noise floor. For such configurations, an electric source needs to be adopted. Realistic modeling of 3D surface-to-borehole CSEM suggested that the vertical electric field (Ez) is the only component of EM radiation showing a signal above the estimated noise floor for waterfront variation over a period of 2 years. The results of the study led to modifications of the cross-well EM acquisition system where electric field sensors were added to the existing magnetic field sensors. A powerful custom electric-current source was also developed. Survey Design Three-dimensional surface-to-borehole CSEM data were acquired in a deep (reservoir-level) research well drilled near the known waterfront position. Surface current dipoles (transmitting antennae) were prepared by drilling shallow boreholes and completing them with a steel pipe coupled to the rock formations using a slurry of carbon backfill (coke breeze) to achieve sufficient electrical contact. The electrode setup was completed by cementing the top. Electrodes forming the dipole antennas were installed at a 200-m separation forming a setup of 48 inline (radial-direction) and 48 cross-line (tangential-direction) dipoles generating Ez and a vertical magnetic field (Hz) at the observation well.