CROSS-BOREHOLE TEM FOR ENHANCED OIL-RECOVERY - A MODEL STUDY

A computer algorithm was developed to model time-domain electromagnetic (TEM) fields for conductive structures radially symmetric about a horizontal loop transmitter. This algorithm improves on previous finite-difference algorithms by calculating the product of the radius times the electric field, which is more accurate than solving for the electric field directly, by using the Crank-Nicholson method of stepping through time which allows for coarser time steps and by using simplified boundary conditions which require less computational effort. These improvements allow models to be calculated on an IBM PC instead of main frame computers. The finite-difference algorithm was used to calculate results for simplified hot-water flood and streamflood simulations at four different stages of the flood front advance, and for thin layers. All the simulations used small horizontal coil sources and receivers and assumed the oil reservoir to be a resistive, horizontal layer (50-OMEGA.m) in a conductive (5-OMEGA.m) background. The TEM responses were dominated by the resistive reservoir layer. In model results, beds as thin as 1/24 the distance between the transmitting and receiving wells are detectable. For both the steam flood and the hot-water flood the greatest change in TEM responses between the flooded and the unflooded reservoirs occurs at early times, from 10 to 100-mu-s. The early-time responses can be qualitatively described as being influenced by the resistivity changes along a relatively narrow signal path which is a straight line in homogeneous regions and refracts along the high velocity reservoir boundary. As a conductive water flood front moves outward from the source borehole, the TEM response is delayed and attenuated for signal paths which pass through the flood zone but little change is seen for signal paths that do not cross the flood zone.