Cross-Well Electromagnetic Surveys for Fluid-Front Monitoring

Oil or gas production from reservoirs implies a systematic depletion of these reserves, optimizing the overall recovery rate. Secondary recovery often uses nearby wells in the reservoir, operating one well as water injector to push remaining hydrocarbons to the other, producing well. Such a process requires careful monitoring of the moving fluid front and identification-localization of possibly unmoved, bypassed reserves. Electromagnetic surveys are best suited to monitor such fluid-front movements. Conductive water replaces the high-resistivity hydrocarbons; this change in conductivity distribution is monitored between two nearby wells with electromagnetic tomography. In one well an array of receiver coils is clamped in place; in the other well, a transmitter coil is moved over an extended depth interval around the zone of interest. The receiver array is then placed over the next interval and the transmission process is repeated, until the entire zone of interest between the two wells has been illuminated. The data set is then inverted for the spatial conductivity distribution between the two wells.