“Invasion effects on time-lapsed array induction logs,” SPWLA 48th Annual Logging Symposium, Austin, Texas, USA, June 3-6, 2007.
Invasion is one of the important factors considered by well logging analysts to obtain more accurate formation resistivities. In order to overcome the invasion effects, various generations of well logging tools have been developed. The array induction logging tool is one of them. Compared with others, array induction tools have many advantages, such as a deeper radius of investigation and sensitivity to formation resistivities close to the borehole. But like others, array induction tools cannot completely solve the invasion effects, especially for conductive invasion cases. So, studying the variations of array induction logs under invasion is very useful for well logging analysts in identifying oil reservoirs and obtaining more accurate formation resistivities. In this paper, a simulation model to study the invasion effects on resistivity time-lapsed logging is described. The model considers invasion of water based mud due to pressure differential into a formation containing oil and water, accounts for capillary pressure, salt transport due to convection and dispersion and mud cake permeability. The borehole is vertical and the simulation is carried out in the radial direction. The differential equations that simulate fluid flow and transport of salts are discretized and solved in the time domain. Salt concentration, water saturation and formation pressure as a function of time and position are used to calculate the time-lapsed formation. Using the computed formation resistivities, Schlumberger?s Array Induction Tool (AIT) logs are obtained. The simulated formation resistivity distribution has three invasion models: resistive invasion, low resistivity annulus invasion and conductive invasion; the low resistivity annulus invasion always appears when fresh mud filtrate invades oil reservoirs. For resistive invasion cases, array induction logs can reflect the formation resistivity distribution very well if invasion radius is not too large; for low resistivity annulus invasion cases, the logs sometimes show the annulus and sometimes not; for the conductive invasion cases, sometimes the deepest apparent resistivity could be less than one third of the true value even when the invasion radius is within the investigation region of the tool. Results were matched with field logs. AIT logs in a water reservoir in an oilfield of West China are very close to true formation resistivity. In the oilfield, all reservoirs showing low resistivity annulus wereinterpreted as oil reservoirs, matching oil testing.