Wire Well Logging Techniques

A. Electric, Radioactivity and Acoustic (Sonic) Logging

Subsurface geological information can be obtained by wireline well-logging techniques. Measurements are made of the electrical, radioactive and acoustic properties of rocks and their contained fluids encountered in the wellbore. Several types of measurements produce information on formation rock acoustic velocity, density, radioactivity, porosity, conductivity, resistivity, fluid saturation and permeability.

Rock lithology, formation depth and thickness and fluid type can also be determined. Caliper logs measure borehole diameter. Geologic maps and cross-sections are readily constructed from a variety of well-log data and assist in understanding facies and geometric relationships and the locations of wildcat and development drilling sites.

Logs are obtained by lowering a sonde or tool attached to a cable or wire to the bottom of a wellbore filled with drilling mud. Electrical, nuclear or acoustic energy is sent into the rock and returns to the sonde or is obtained from the rock and measured as the sonde is continuously raised from the wellbore bottom at a specific rate.

The well is logged when the sonde arrives at the top of the interval to be investigated. Formation water saturation, permeability, porosity, radioactivity and resistivity are rock properties that affect logging and the types of logs to be obtained.

As a wellbore is drilled the rock formations and their contained fluids are penetrated by the bit and affected by the drilling process. Drilling mud invades the rock surrounding the wellbore, affects the logging of the hole and must be accounted for. A permeable, porous formation which has been penetrated and affected by drilling and invasion by drilling mud, develops parameters important to logging. Significant of these parameters from the center of the wellbore outward into the formation are hole diameter, drilling mud, mudcake, mud filtrate, flushed zone, invaded zone and uninvaded zone.

B.Spontaneous (Self) Potential Logs (SP Logs)are used to detect
permeable formations and their upper and lower contacts, volume of shale, where present, in permeable formations, and to determine the resistivity of water in permeable formations.

C.Resistivity logsillustrate permeable formations, formation fluid (water
versus petroleum) content, and the porosity characteristics of formation resistivity. Resistivityrepresents the tendency of rock materials and their contained fluids to resist the flow of electrical current. Salt water contains dissolved salt and, because it conducts electricity very easily, has low resistivity. Fresh water contains no salt and demonstrates low conductivity and high resistivity. Rock materials that contain salty or fresh water offer differing degrees of resistivity and response on resistivity logs.Formation
resistivity is measured by induction electrical logs.

D. Radioactivity logsare gamma-ray, neutron and density logs,which are
often obtained together.

Gamma-ray logs measure formation radioactivity and are useful in identification and correlation of formation rock types. Gamma-ray logs are useful in estimating shale volume in potential or actual reservoir sandstone or carbonate.

Neutron logs illustrate formation porosity by measuring hydrogen ions. Water/oil-filled, shale-free, clean formations will be logged as liquid filled porosity. Zones of low porosity on the neutron log correspond to zones of higher radioactivity on the gamma-ray log and are reflective as approximate mirror images of each other.

Density log evaluates formation porosity. It detects gas, evaluates hydrocarbon density and complex rock sequences, identifies evaporate minerals and shale-bearing sandstone units. It is often taken in the same log suite as gamma-ray logs.

E. Acoustic logsillustrate formation porosity. The acoustic log measures the
velocity of a sound wave through a rock medium. Sound wave velocity is
dependent upon lithology and porosity. The sonic logillustrates both the sound wave transit time, which indicates sound velocity and the related porosity of the rock.