The main purpose of sonic logging is to determine the mechanical properties of the formation surrounding the borehole:
- Poisson’s ratio is the ratio of transverse contraction strain to longitudinal extension strain in the direction of stretching force. Tensile deformation is considered positive and compressive deformation is considered negative.
- Young’s or elasticity modulus measures opposition of a substance to external stress.
- Bulk modulus is the coefficient of incompressibility and measures opposition of a substance to compressional stress.
- Shear modulus, also called rigidity modulus, measures the opposition of a substance to shear stresses.
- The uni axial compressive strength (UCS) can be calculated from dynamic Young’s Modulus.
- The indentation hardness index (IHI) can be predicted from UCS.
Density is required in order to calculate all mechanical properties, except Poisson’s ratio.
A. Mechanical properties are extremely powerful tools for hard rocks, to predict how they will behave under “excess” pressure:
- Drilling:
Drilling cost increases with formation hardness (drilling bits)
Will the formation fracture and the drilling mud disappear?
- Hydraulic fracturing:
How much pressure will fracture the formation and how far will the fracture extend?
Experimental models are used to compute parameters such as tensile strength. Simulations are used to predict the pressures that will “crack” the rock and lengths of fractures.
B. Main uses of mechanical properties in soft formations are:
- Hole bore stability
The prediction of formation failure / collapse while drilling.
This is especially relevant in deviated wells when drilling at high angles through soft rock can be problematic.
The physical mechanism is similar to that of sand stability evaluation.
- Sand stability evaluation
The prediction of the formation collapse under producing conditions.
Using theoretical failure criteria it is possible to predict if the perforation will produce sand.
The full wave sonic tool measures the time it takes for a pulse of “sound” (elastic wave) to travel from a transmitter to a receiver, which are both mounted on the tool. The transmitted pulse is very short and of high amplitude. This travels through the rock in various different forms while undergoing dispersion (spreading of the wave energy in time and space) and attenuation (loss of energy through absorption of energy by the formations).
When the sound energy arrives at the receiver, having passed through the rock, it does so at different times in the form of different types of wave. This is because the different types of wave travel with different velocities in the rock or take different pathways to the receiver.
FWS interpretation could provide a record of “seismic” P velocity and travel time throughout a borehole. This information can be used to calibrate a seismic data set / synthetic seismograms (i.e., tie it in to measured values of seismic velocity).
The travel time of the different elastic waves as well as their amplitude are measured:
- Compressional wave – P
- Shear wave – S
- Rayleigh wave – R
- Mud wave – M
- Stoneley wave (boundary) – SW
- Fluid compressional wave – F
Permeability may be estimated qualitatively from sonic logging.
There is a correlation between permeability and the Stoneley wave data recorded by sonic logging tools. The low-frequency Stoneley wave is sensitive to formation permeability.
The sonic log can calculate the formation porosity.
Sonic logging it is useful to calculate secondary porosity (the porosity resulting from diagenesis).
The sonic log is sensitive only to the primary intergranular porosity. By contrast, the density and neutron logs record the total porosity.
The neutron and density logs are responses to pores of all sizes. However, field observation over many years has shown that the sonic log is a measure of interparticle (intergranular and intercrystalline) porosity but is largely insensitivity to either fractures or vugs. This discrimination can be explained largely by the way that the sonic tool measures transit time by recording the first arrival waveform which often corresponds to a route in the borehole wall free of fractures or vugs.
When sonic porosities are compared with neutron and density porosities, the total porosity can be subdivided between “primary porosity” (interparticle porosity) recorded by the sonic log and “secondary porosity” (vugs or cavities and/or fractures) computed as the difference between the neutron and// or density porosity and the sonic porosity. Typically, moderate values in secondary porosity are caused by vugs, because fracture porosity does not usually exceed 1 to 2% by volume.
Cement Bond Logging
