oa Results of Feasibility Study of Surface-to-Borehole Time-Domain CSEM for Water-Oil Fluid Substitution in Ghawar Field, Saudi Arabia

Monitoring the advancement of flood from water injection in carbonate reservoirs is a major challenge<br>for geophysical methods. 4D seismic has limited applicability to Middle East reservoirs with low gas-oilratio<br>in carbonate rocks. On the other hand, electromagnetic (EM) methods hold the largest potential in<br>such reservoirs due to the large resistivity contrast (over one order of magnitude) between oilsaturated<br>and water-saturated reservoir rocks. Electromagnetic measurements, however, are noise<br>sensitive thus special configurations need to be implemented to enable the detection of the extremely<br>small variations of the electromagnetic field that are induced by oil being replaced by injection water.<br>Controlled source EM transmitters on ground surface and borehole receivers represent the most<br>effective layout configuration to improve the signal-to-noise ratio and to augment the aperture of<br>investigation while addressing the signal-to-noise challenge through long recording times. Transient<br>time-domain controlled-source EM techniques also provide broadband EM measurements and adapt to<br>most geologic scenarios and to the conditions characterizing the Ghawar field.<br>An advanced 3D modeling study was carried out by considering real reservoir geometry from 3D<br>seismic interpretation, anisotropic resistivity distribution from tri-axial resistivity logs (acquired from<br>surface to reservoir depth in the monitoring well) and time snapshots of fluid saturations modeled in<br>reservoir simulators. The study allows the determination of EM field sensitivity to fluid saturation<br>changes in in-situ reservoir conditions. Results indicate the vertical component of the electric field (Ez)<br>is the most sensitive parameter to fluid replacement for a survey layout consisting of surface galvanic<br>transmitters radially distributed around the well and a single, multi-level, borehole receiver. Repeated<br>EM modeling over different time snapshots evidence the possibility to effectively monitor in three<br>dimensions the resistivity changes occurring in the reservoir as the water flood front advances.<br>Estimates of the EM field strength allow quantitative evaluations of the noise floor required to detect<br>the variations of the electromagnetic field. These estimates will be used in a successive phase of the<br>study where actual noise measurements and noise cancellation techniques will be tested in the field.