Numerical modeling of surface-to-borehole electromagnetic surveys for monitoring thermal enhanced oil recovery

Electrical conductivity is an important petrophysical property used to predict lithology and fluid content in petroleum reservoirs. Conductivity distribution between wells can, in principle, be mapped with electrical or electromagnetic (EM) techniques when sources or receivers (or both) are located in the wells. Unfortunately, the resolution of these methods is relatively poor. Resolution is improved, however, in monitoring applications where the response of a dynamic reservoir process is recorded at different times and compared. Examples of such dynamic processes are fluid replacement during primary production, secondary recovery, and enhanced oil recovery (EOR) techniques as fireflooding, steamflooding, and CO₂ flooding. Such measurements can be made with technology currently available within the geophysical industry and at relatively low expense.

A numerical model study of the Holt Sand in situ combustion EOR experiment was conducted to test the feasibility of electromagnetically monitoring the progress of the advancing fire flood. The resistivity and geometry of the burn zone was obtained from pre-burn and post-burn well log data. The study shows that the surface-to-borehole electromagnetic method detects a clear signature from changes in resistivity of the burned reservoir horizon from distances as great as 100 m. Similar conclusions hold for steam flood processes. An important phenomenon which complicates interpretation of EM monitoring of thermal EOR processes is a zone of decreased resistivity adjacent to the reservoir horizon caused by conduction of heat into the bounding shales.