GEOL 115: The Earth’s Energy Resources

This lecture was about formation evaluation. Formation evaluation is that set of techniques that is used to determine whether a well can produce oil or gas at commercial rates. Formation evaluation includes mud logging, geophysical logging, and well testing. Mud logging is the process of examining and recording the material that is returned as the drilling mud circulates out of the hole. Mud logging geologists have three primary responsibilities. 1)They look for hydrocarbon “shows” by searching for evidence of oil in the drilling mud and the cuttings that are returned with the drilling mud. 2)They monitor the amount and composition of gases (methane, ethane, propane, iso-butane, and butane) that are returned with the mud. 3)They examine the lithology (type of rock) of the drill cuttings that are returned with the drilling mud. They then make a mud log that displays all this information. Geophysical logging examines the physical properties of the formation.  The most important physical property is the resistivity -a measure of the formation’s resistance to conducting electricity. The figure to the right is an example of what a resistivity log looks like. The numbers in the middle are the depth, the curve on the left does not measure resistivity but another parameter. That curve will deviate to the left when the tool is passing through a good reservoir and to the right when passing through seals. The curve on the right displays the resistivity of the formation. High resistivity (to the right) is good, low resistivity is bad. These data can be used to calculate the water saturation from equations similar to S_w = [(0.81*R_w)/(φ²*R_t)]^0.5 That’s simple enough. S_w is the water saturation, R_w is the resistivity of the water in the formation, and R_t is the resistivity of the formation at depth. As you might guess, measuring all those other parameters can be pretty tricky. I should note that ExxonMobil (among others) has developed a technique to measure formation resistivity remotely (from the surface). This is an interesting development and suggests that we’re getting close to a detailed understanding of the fluids in formations prior to drilling. Drive mechanisms are also important for forcing the oil out of the reservoir. Most reservoirs are initially solution gas drive (shake up a bottle of Pepsi and open it). Solution gas drive is pretty ineffective and will only force a small fraction of the oil in place out. Better mechanisms are gas cap expansion (the overflowing soft drink in the drive-through example), water drive, or gravity drive. Water will normally displace oil (because water oil floats on water) and almost all reservoirs have a natural water drive. The real question is how strong a water drive is likely to be. If there is reason for water to flow in a particular direction in the subsurface and if the reservoir is permeable and porous then a water drive can be quite strong and can systematically flush the reservoir. Remember that the East Texas Field was saved from reservoir damage by over drilling because it has a thick permeable reservoir and a strong water drive. Gravity drainage can be very effective but is typically observed only with very heavy oils.