Abstract:

In order to facilitate efficient modeling of SP-log, and investigate the influences factors quantitatively, investigation on the stimulating source for SP, the electrical double layer (EDL) was performed. Through analyzing of the physical origin of spontaneous potential and the contribution of EDL to SP, it is clear that only the EDL at the borehole wall of shale layer, sand-shale boundary with invaded zone and that between invaded zone and virgin formation would contribute to the value of SP-log. In case only the anomalies of sand layer are concerned, to calculate the contribution of the EDL between invaded zone and virgin formation is sufficient. By analog of direct current electric field and static electric field, expression for the surface current density of EDL is deduced. SP-logs for different depth of invasion were calculated for media with uniform resistivity. The results show that the form of SP-log becomes flatter with the increasing depth of invasion, the amplitude of anomaly become smaller at the same time.

Key words: spontaneous potential logging, electric double layer, electrochemical EMF, influence of invasion depth