Log Analysis part 2

Question 1

How do acoustic/sonic logs work?

Answer 1

• Sound waves generated by a source on the sonde are recorded by a receiver a fixed distance away and the computed transit time is displayed as the sonic log
• Sensitive to borehole conditions

Question 2

How does the sonic log determine porosity from density?

Answer 2

• Harder, low porosity dense rocks have faster transit times than softer porous rocks
• Porosity can also be calculated using assumed velocities for the matrix

Question 3

What does a resistivity log measure and how?

Answer 3

Measures resistivity of a rock to an electric current through formation via electrodes measuring the voltage received in another set of electrodes

Question 4

How does a normal resistivity log work?

Answer 4

• Current of a constant intensity is passed between two electrodes on the sonde and measured as a potential difference between two others
• The spacing between the electrodes effects the depth of investigation
• Largely superseded

Question 5

How does a lateral resistivity log work?

Answer 5

• Current focussed and directed into formation by means of a guard, electrodes placed above and below main electrode
• Potential of the guard and the main electrode is then recorded, changing the angle of electrodes changes the depth of investigation

Question 6

How does a induction resistivity log work?

When is it used?

Answer 6

• Induction log has transmitter and receiver coil placed either end of the sonde which transmits high frequency alternating currents
• Induced magnetic field generates a current which fluctuates according to the formation resistivity and is measured in receiver coil
• Used where fresh water or oil based muds are employed

Question 7

When would you use induction and lateral logs? Rule of thumb?

Answer 7

Induction: Preferred where formation is very conductive and mud system relatively non-conductive

Lateral: more accurate in higher resistivity formations but require conductive mud system

Common rule:
RT 100 ohm = lateral

Question 8

In what ways can drilling mud effect the formation?

Answer 8

• Invasion of mud fluids into a porous formation: mud particles are filtered by porous rock and a mudcake develops, degree of mud invasion indicates poroperm characteristics
• Pore fluid at the borehole margin is displaced by the mud and replaced by the filtrate, the depth of displacement is related to the poroperm of the rock and of pore fluid types

Question 9

How does the resistivity logging measure shallow and deep resistivity? How is this shown on the log? How will permeable formations appear?

Answer 9

• Deep log: measures Rt, the uninvaded resistivities, shown by dashed curve
• Shallow log: measures flushed zone, shown as solid line
• In permeable formations they will read the same

Question 10

To calculate water saturation you need formation water resistivity. How can you acquire this?

Answer 10

• Analyses of recovered water samples
• Regional knowledge
• Log data

Question 11

What corrections may need to be made to compensate for effects of the borehole?

Answer 11

• Hole size: enlarged hole results in tool sensors being further from borehole wall, increasing contribution of borehole fluids to measurements recorded
• Mud properties: specific properties of mud (e.g. density, salinity)
• Invasion: Displacement of formation fluid by mud based fluids, change the bulk formation properties close to the borehole