Electromagnetics: Technologies for Reservoir Surveillance and Monitoring

The formation electrical resistivity is a basic petrophysical parameter used to evaluate subsurface reservoirs, particularly in petroleum fields. Data are normally collected in open holes immediately after drilling, applying galvanic or inductive physics in wireline deployed tools. The tools themselves are very well engineered devices in continuous development since the late 30’s. When used in combination with gamma ray and neutron porosity logs, resistivity logging data are frequently used to estimate lithology and fluid saturations and thereby evaluate the quality of oil bearing strata.

As reservoirs become more mature it has become imperative to extend this knowledge deeper into the reservoir to better characterize the rocks and fluids distribution away from the wells, and thereby improve field management. This can be accomplished by applying tools that are sensitive to the reservoir scale. One of these tools is inductive crosswell electromagnetics (EM).

Cross-well EM, initially developed in the 1990’s, involves applying inductive physics and 2D/3D inversion to interrogate the interwall resistivity distribution. The method has developed into a mature technology in the oil and gas community, especially for EOR and time lapse studies. A cross-well EM system consists of a transmitter in one well that broadcasts a time varying magnetic field in the 3D region surrounding the boreholes, and multiple receivers that detect the magnetic field in another well some distance away. The collected data are used to image the interwall conductivity structures providing insights to fluid distribution and saturation mapping.

With the same physics, one can place the transmitters on surface and keep the receivers in a well, then it is a surface to borehole EM measurements. Alternatively, the receivers can be placed on the surface and the transmitter is placed in a well, then we have borehole to surface EM measurements. Both source-receivers configurations can image subsurface EM structures, but with much larger investigation range than the crosswell EM.

This presentation will outline the theory of cross-well EM system, survey design and modelling, data acquisition configuration and workflow. Two field examples will be presented to demonstrate the usefulness of the method for fluid monitoring, reservoir characterization and locate by-passed hydrocarbon. The first example is time lapse survey in the Middle East to monitor water flood. The project consists of five surveys in two and half years. The data and interpretations clearly demonstrate the water flood pattern and effectiveness of using cross-well EM for monitoring. The second example is from a cross-well EM survey on two horizontal wells, the first ever in the world. The purpose of the survey is to understand the water breakthrough from a peripheral injector to a producer and map water flood path through a system of fractures. The data interpretation requires 3D modelling/inversion, a very challenge task considering the limited data coverage. The final resistivity model is used to compute saturation map and define by-passed hydrocarbon which resulted in placement of new wells.