Well Logging Techniques Questions and Answers

Answer: C

The acoustic impedance log is used to generate synthetic seismic logs (synthetic seismograms).

Acoustic impedance is density multiplied by velocity, and velocity comes from the sonic log. Changes in impedance between layers give reflection coefficients, and convolving these with a seismic wavelet produces a synthetic seismogram. This is compared with seismic sections to tie well depths to reflections.

Electrical, neutron and density logs are measured directly by their own tools; they are not built from the impedance log.

Answer: D

The three resistivity methods are the normal, lateral and induction logs.

The normal and lateral logs are conventional electrode tools that send current into the formation and measure potential at set spacings. The induction log uses coils to induce currents in the formation, and works even in boreholes filled with oil-based mud. All three measure formation resistivity, from which water and hydrocarbon saturation are estimated.

S.P., spontaneous potential and self potential are three names for the same log, which records natural voltages, not resistivity. Sonic and acoustic logs measure sound velocity, and the gamma ray logs measure natural radioactivity.

Answer: B

The 'football' gas effect appears on a Neutron-density cross-plot.

Gas lowers both logs in opposite senses. Its low hydrogen index makes the neutron log read too low a porosity, while its low density makes the density log read too high a porosity. On the combined display the two curves separate widely, and the shaded crossover between them has the rounded shape called the football or balloon effect, the classic sign of free gas.

Sonic logs respond to gas less diagnostically, and NMR-neutron is not the standard gas cross-plot, so the other pairs are wrong.

Answer: A

The log used for depth matching and correlation is the Natural gamma log.

The gamma ray log records the natural radioactivity of the formations, mostly from potassium, thorium and uranium in shales. It works in open and cased holes and in any borehole fluid, so it is run with almost every logging tool and used to tie all logs to the same depth and to correlate shale and sand beds from well to well.

The gamma-gamma (density) and neutron logs measure porosity using an artificial source, and the SP log needs a conductive mud in an uncased hole, so none of them serves as the universal correlation log.

Answer: B

The sonic log gives primary porosity.

The sonic log measures the travel time of a compressional wave through the rock. The fastest path goes through the rock matrix and the evenly distributed intergranular pores, so the porosity calculated from it with the Wyllie time-average equation reflects primary (intergranular) porosity. Vugs and fractures are largely bypassed by the first-arriving wave.

That is why sonic porosity is compared with total porosity from the density or neutron log: the difference estimates secondary porosity. Sonic porosity therefore is not total or secondary porosity, and it does not depend on whether the pores hold water.