Well logging basics

MODULE 5 5.0 WELL LOG ANALYSIS 5.1 Wireline Geophysical Well Log – continuous recording of a geophysical parameter along a borehole.

Table 5-1. Common wireline geophysical well measurements (Rider, 1996) Measurement Log Type Parameter Measured Mechanical Caliper Hole diameter Spontaneous Temperature Borehole temperature Self-Potential (SP) Spontaneous electrical currents Gamma Ray (GR) Naturalradioactivity Induced Resistivity Resistance to electric current Induction Conductivity of electric current Sonic Velocity of sound propagation Density Reaction to gamma ray bombardment Photoelectric Reaction to gamma ray bombardment Neutron Reaction to neutron bombardment Table 5-2. Principal uses of wireline logs (modified after Rider, 1996)
Photoelectric Temperature Gamma Ray Spectral GRSonic SP

Log Uses
General Geology Lithology -- general Unusual lithology: Volcanics Evaporites Mineral identification Correlation: stratigraphy Facies, dep. environment Reservoir Geology Fracture identification Over-pressure identification Maturity Petrophysics Porosity Permeability Shale volume Formation water salinity Hydrocarbon saturation Gas identification Seismic Interval velocity

+ + C -Geochemistry Source rock identification

dip Acoustic impedance Legend: (-) essentially qualitative; (+) qualitative and semi-quantitative; (C) strictly quantitative

- - + + + + - - - - + + - - - - + - + - - - - - - - + + + + + + + + + + + C C C + + C - C dip C C

-

Image logs

Resistivity

Dipmeter

Neutron

Density

Caliper

-

+

+

+

5.1.1 Log PresentationThe values of the parameter measured are plotted continuously against depth in the well. Hard copies of well logs are in standard API (American Petroleum Institute) log format. The overall log width is 8.25 in., with three tracks of 2.5 in. wide each. A column 0.75 in. wide separates tracks 1 and 2 where the depths are indicated. Track 1 is always linear, with ten division of 0.25 in. while tracks2 and 3 may have a linear scale similar to track1, a 4-cycle logarithmic scale, or a combination of logarithmic scale in track 2 and linear scale in track 3. For most well logs, the common vertical scales used are l:200 and 1:500 but for image logs (microresistivity) it is usually 1:20 and 1:40. Every log is preceded by a header. It shows pertinent information for proper interpretation of the logand in addition, some details of the well and the log run. 5.1.2 The Logging Environment Pressure Formation pressure – the pressure under which the subsurface formation fluids and gases are confined. Hydrostatic pressure – the pressure exerted by a column of fluid. In the borehole, it is due to the column of drilling mud and is: Ph (psi) = 0.052 x height of fluid column (ft.) x density (ppg)Overpressure – any pressure above the hydrostatic (or normal) pressure Temperature Geothermal gradient G = 100 (Tf − Ts ) / D Formation temperature Tf = Ts + G(D/100) G = geothermal gradient, °F/100 ft. Tf = formation temperature, °F Ts = surface temperature (80°F) D = depth of formation, ft. Graphical solution of formation temperature is provided by Schlumberger Gen-6 chart.

Borehole Geometry Fromcaliper Gauged hole – diameter of hole is about equal to the bit size Increased borehole diameter Washout – general drilling wear, esp. in shaly zones and dipping beds, both caliper larger than bit size, considerable vertical extent Keyseat – asymmetric oval holes, formed by wear against the drill string at points where the borehole inclination changes (doglegs) Breakout – similar to keyseat butnot due to doglegs, small brittle fractures (spalling) due to existing stress regime of the country rock Decreased borehole diameter - generally due to formation of mud cake Mud cake thickness = (bit size diameter – caliper diameter reading)/2 - mud cake formation indicates permeability and involves loss of mud filtrate into a permeable formation – invasion. Invasion Profile

Figure 5-1...