Well Logging

Cap-17 Geophysical &Chemostratigraphic Correlation

Well Logging• One of the oldest and most used methods

that depends on the geophysical propertiesof rocks is subsurface well logging.

• These methods are particularly good whensurface outcrops are not available. Forexamples in central US rocks are coveredby meters of glacial deposits.

• Seismic can be used to determine thesubsurface structure but a direct sample ofthe rock is needed to be sure of thelithology.

Well Logging

• The most practical way is todrill a well and record all theuseful information possiblefrom a core.

Well Logging

• Continuous core recovery is fartoo expensive and is not doneexcept when great detail isneeded.

Modern drilling use a lubricatingmud that is pumped downthrough the drill pipe to cool andlubricate the drilling area. Thedrilling mud is usually a mixtureof bentonitic clay and oil orwater, plus barite to regulatedensity. It is forced up the well tothe surface so that mudconstantly flows toward the mudtanks, carrying the cuttings, orchips, away from the drilledformation to prevent the wellfrom clogging.

This chips are a primary sourceof information about thesubsurface unit. The sitegeologist usually keeps acontinuous log and sample of thechips as they come up and arescreened out of the drilling mud.The chips are the only record ofthe lithology. If the density ofone unit greatly differs from thenext the chips may rise to thesurface at different rates and givean erroneous impression of thesequence.

Well Logging• The second and more important source of

stratigraphic information comes after thewell has been drilled and the drill piperemoved. A caliper lowered down the holesmeasures the hole diameter. This will helpto indicate the presence of shales that tendto cave.

• A sonde is lowered down the hole and as israised it calculates the response of thelogging tool to various electrical; otherproperties of the hole are measured.

The electric logging sondeand the configuration ofelectrodes for recording shortnormal, long normal, andlaterlog resistivity curves.The short normal curvemeasures the resistivitydifference between thecurrent electrode A and pointM1, about 40 cm away. Thelong normal curve measuresthe resistivity between A andM2, about 160 cm apart. Thelaterlog measures theresistivity difference betweenA and O.

Spontaneous Potential• SP log measures the difference in electrical

potential between two widely spacedelectrodes, a grounded electrode at the topof the well and an electrode on the sonde.

• Drilling mud invades the pore space of therock adjacent to the well. When drillingmud and the natural pore fluid come intocontact, they set up an electrical potential.

Spontaneous Potential• The movement of ions

from the drilled formationto the borehole accountsfor 85% of the measuredvoltage difference, andinvasion of drilling mudfrom the borehole into theformation accounts for15%.

• For this reason, the SP logis a measure ofpermeability.

Spontaneous Potential• Limestones are low in

permeability unless they areporous or fractured. Sandstoneusually show a large deflectiontoward the negative pole becauseof their permeability.

• If the sonde encounters a fluidthat is a better conductor than thedrilling mud (such as salt water),the curve will deflect to the left;if the fluid is a poor conductor(such as fresh water or oil), itwill deflect to the right.

Resistivity ( R ) log

• R-log measures the resisitivity of the fluidscontained in the surrounding rock to an appliedelectrical current.

• The R indicates the amount of fluid in the rockand therefore the pore space.

• Because the drilling mud invades the pores rockfrom the borehole, this log actually measures thedifference in resistivity between the invaded zone(mud has a high resistivity) and the uninvadedzone (natural pore fluids have a lower resitsivity).

Resistivity ( R ) log

• Resistivity increases with decreasing pore space;10% porosity is about 10 times more resistive that30% porosity.

• Dense rock with no pore space have very highresistivity, so they usually deflect the record tothe right, event off the scale.

• Sandstone filled with a nonconductive fluid suchas oil or fresh water also deflect to the right.

• Shales have low resistivity and deflect to the left.