Invasion: Flushed and Virgin Zones

Part 2, Chapter 2: The Logging Environment and Invasion

Learning objectives

Describe how mud filtrate creates flushed and virgin zones
Define Rxo and Rt and the fluids that control each
Explain why the flushed zone reads lower in oil and higher in water
Recognize that the deep, virgin Rt is the value needed for the true saturation

The Filtrate Pushes In

Put the last two sections together. The overbalanced mud (the borehole) pushes its filtrate (with resistivity Rmf) radially into any permeable bed, sweeping the movable fluids ahead of it. The result is a near-wellbore flushed zone, where filtrate has displaced almost everything the rock will give up, fading through a transition into the deep virgin zone that no mud has reached. Invasion is the single biggest reason a log reading near the wall is not the reading we actually want.

The Radial Profile: Rxo and Rt

Each zone has its own resistivity. The flushed zone reads Rxo, controlled by the mud filtrate and the residual saturation

R_{xo} = \frac{a\,R_{mf}}{\phi^m S_{xo}^n}

where $S_{xo}$ is the (high) water saturation left after flushing. The virgin zone reads the true Rt, controlled by the formation water

R_t = \frac{a\,R_w}{\phi^m S_w^n}

Same Archie form, different water and different saturation. The radial resistivity profile steps from Rxo near the wall to Rt far away.

The Direction Tells the Fluid

Which way the profile steps depends on the fluid. In an oil zone the filtrate brings water into rock that held little of it, so the flushed zone reads lower than virgin (Rxo below Rt). In a water zone the only change is fresher water replacing saltier, so the flushed zone reads higher (Rxo above Rt). Either way, the value we need for the true saturation is the deep, uncontaminated Rt, which means we must read past the invaded zone. How tools reach different depths is the next section.

References

Asquith, G. and Krygowski, D. (2004). Basic Well Log Analysis, 2nd ed. AAPG Methods in Exploration 16.
Ellis, D. and Singer, J. (2007). Well Logging for Earth Scientists, 2nd ed. Springer.