2.1.4 Well Logging / Wireline Logging

One of the most common methods for evaluating geological formations is the use of well logs. Logs are conducted by lowering an instrument into the well and taking a profile of one or more physical properties along the length of the well. A variety of well logs is available that can measure several parameters from the condition of the well to the composition of pore fluids and the mineralogy of the formation. Permanent downhole measurements of pressure and temperature are standard for oil and gas production and likewise, e.g. in the Snøhvit CO2 storage project, pressure and temperature are continuously measured every second in the CO2 injection well.

Well logging in CO2 storage projects will also be very useful for inspecting the condition of the well and ensuring that the well itself does not provide a leakage pathway for CO2. Several logs are routinely used for this purpose, including temperature, noise, casing integrity, and radioactive tracer logs (Benson et al., 2002). However, the resolution of well logs may not be sufficient to detect very small rates of seepage through microcracks (Benson and Myer, 2002).

A sonic log is a porosity log that measures interval transit time (Δt) of a compression sound wave travelling through one foot of formation. Sonic logging is essential to calibrate surface seismic methods. The sonic log device consists of one or more sound transmitters and two or more receivers. However, in sonic logging only sound travel time is measured and additional data manipulation is required, e.g. compensation for borehole size variations as well as for errors due to tilt of the sonic tool, to arrive at porosity. No standard protocol is available for conversion from travel time to porosity; there are many variations of the travel time/porosity relationship.

Sonic logging is used usually for the determination of porosity and permeability in porous rock, the detection of fractures, and even for lithology characterisation (Paillet and White, 1982). The sonic velocity contrast between water and CO2 is strong, so that this log type can be used to assess changes in fluid as the CO2 plume moves past the wellbore.