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) Natural radioactivity 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

General Geology

-

Lithology -- general Unusual lithology: Volcanics Evaporites Mineral identification

-

Correlation: stratigraphy Facies, dep. environment Reservoir Geology

-

Fracture identification Over-pressure identification

-

Geochemistry Source rock identification Maturity

+

Petrophysics Porosity

-

Permeability

+

Shale volume

C

Formation water salinity Hydrocarbon saturation Gas identification Seismic

Interval velocity

-

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

Image logs

Dipmeter

Neutron

Photoelectric

Density

Sonic

Spectral GR

Gamma Ray

Resistivity

SP

Uses

Caliper

Log

Temperature

Table 5-2. Principal uses of wireline logs (modified after Rider, 1996)

-

+

+

+

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

5.1.1 Log Presentation The 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 tracks 2 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 log and 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 From caliper 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 but not 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 (Gen-3, Schlumberger Charts) shows invasion by mud filtrate of a permeable bed in a borehole. Also shown are the nomenclature of the corresponding resistivities and saturations in each zone.

5.1.3 Process of Interpretation • • • • • • •

Identify potential reservoir intervals; distinguish non-permeable, nonreservoir intervals from porous potential intervals. Estimate thickness of the potential reservoirs. Determine lithology (rock type) of the potential reservoirs. Calculate porosity (Φ). Determine resistivity of formation water (Rw). Calculate water saturations (Sw, Sxo) using resistivity (Rt, Rxo). Estimate in-place and movable hydrocarbons.

Figure here (Flow chart for log interpretation, Asquith, p.104-5) 5.2

Resistivity Logs

Resistance is the opposition offered by a substance to the passage of electric current. Resistivity is the resistance measured between opposite faces of a unit cube of the substance at specified temperature. Resistivity is measured in ohm-meter2/meter, more commonly shortened to just ohm-meter. Resistivity logs do not always measure resistivity directly. Some resistivity logs (actually induction logs) measures conductivity instead which is the reciprocal of resistivity. resistivity (ohms m2/m) =

1 × 1000 (millimhos/m) conductivity

Induction logs are used in wells drilled with a relatively fresh-water mud (low salinity) to obtain more accurate value of true resistivity. Table 5-3. Principal uses of the resistivity and induction logs Used for Knowing Formation water resistivity (Rw) Quantitative Fluid saturation: Mud filtrate resistivity (Rmf) Formation Porosity (φ) [and F] Invaded zone Temperature (detect hydrocarbons) SemiTexture Calibration with cores quantitative Lithology Mineral resistivities and Correlation qualitative Facies, bedding Gross lithologies characteristics Compaction, overpressure Normal pressure trends and shale porosity Source rock identification Sonic and density log values Source rock maturation Formation temperature

Figure 5-2. Idealized resistivity log. 5.3

Spontaneous Potential and Gamma Ray

The SP and GR logs measures naturally occurring physical phenomena in insitu rocks. 5.3.1 Spontaneous Potential The SP log is a measurement of the natural potential difference or self potential between an electrode in the borehole and a reference electrode at the surface (problem with offshore wells, no ground). No artificial currents are applied. Three factors are necessary to produce an SP current: 1. a conductive fluid in the borehole, 2. a porous and permeable bed surrounded by an impermeable formation, and 3. a difference in salinity (or pressure) between the borehole fluid and the formation fluid.

Figure 5-3. Idealized SP log. Table 5-3. Principal uses of the SP log Used for Quantitative Formation-water resistivity Qualitative

Shale volume Permeability indicator Facies (shaliness) Correlation

Knowing Mud filtrate resistivity and formation temperature SSP (static SP) and shale line Shale line Clay/Grain size relationships

Bed Boundary Definition and Bed Resolution Sharpness of a bed boundary depends on the shape and extent of the SPO current patterns. When there is considerable difference between mud and formation water resistivity, currents will spread widely and the SP will deflect slowly: definition is poor. When the resistivities are similar, boundaries are sharper. In general, SP should not be used to determine bed boundaries. If it has to be used, place the bed boundary at the point of maximum curve slope. (GR defines bed boundaries better.) Shale Baseline and SSP SP has no absolute values and thus treated quantitatively and qualitatively in terms of deflection, which is the amount the curve moves to the left or to the right of a defined zero. The definition of the SP zero, called shale baseline, is made on thick shale intervals where the SP curve does not move. All values are related to the shale baseline.

The theoretical maximum deflection of the SP opposite permeable beds is called the static SP or SSP. It represents the SP value that would be measured in an ideal case with the permeable bed isolated electrically. It is the maximum possible SP opposite a permeable, water-bearing formation with no shale. The SSP is used to calculate formation-water resistivity (Rw). Formation-water Resistivity (Rw) (S)SP = − K log

( Rmf )e ( Rw)e

S(SP) = SP value: this should be the SSP (Rmf)e = equivalent mud filtrate resistivity: closely related to Rmf (Rw)e = equivalent formation water resistivity: closely related to Rw K = temperature-dependent coefficient K = 61+ (0.133 x T°F) K = 65 + (0.24 x T°C) Shale Volume

Vsh (%) = (1.0 −

PSP ) × 100 SSP

PSP (Pseudo-static SP) – the SP value in the water–bearing shaly sand zone read from the SP log. SSP (Static SP)– the maximum SP value in a clean sand zone. The formula simply assumes that the SP deflection between the shale base line (100% shale) and the static SP in a clean sand (0% shale) is proportional to the shale volume. This is qualitatively true but quantitatively there is no theoretical basis. Shale content from SP is subject to complications due to SP noise, Rw/Rmf contrast, HC content, and high salinity drilling fluids.

5.3.2 Gamma Ray

Figure 5-4. Idealized GR and SGR log. Volume of Shale from GR

Vsh = 0.33 [2(2 x IGR) - 1.0] Vsh = 0.083 [2(3.7 x IGR) - 1.0] I GR =

GRlog − GRmin GRmax − GRmin

5.4

Porosity Calculations – sonic, density, and neutron logs

5.4.1 Sonic

Figure 5-5. Idealized Sonic log. Wyllie’s Time Average Equation ∆t = Φ ∆tf + (1- Φ) ∆tma Φ = porosity ∆t = log reading in microseconds/foot (µs/ft.) ∆tf = transit time for the liquid filling the pore (usually 189 µs/ft.) ∆tma = transit time for the rock type (matrix) comprising the formation Φ=

∆t − ∆t ma ∆t f − ∆t ma

5.4.2 Density

Figure 5-6. Idealized Density log. ρb = Φ ρf + (1- Φ) ρma Φ = porosity ρ = log reading in microseconds/foot (µs/ft.) ρ f = transit time for the liquid filling the pore (usually 189 µs/ft.) ρ ma = transit time for the rock type (matrix) comprising the formation Φ=

ρ ma − ρ b ρ ma − ρ f

5.4.3 Neutron

Figure 5-7. Idealized Neutron log.

Read directly from logs May need matrix correction

Φ=

Φ=

φD + φN

φD + φN 2

2

if no light hydrocarbons

if light hydrocarbons as present

5.5

Water Saturation (Sw) Calculations

Archie’s Equation F = Ro/Rw F = formation resistivity factor or simply formation factor Ro = resistivity of rock when water saturation is 1 (100% saturated) Rw = resistivity of saturating water F=

a

φm

Φ =porosity a = cementation factor m = cementation exponent

Figure 5-8. Schematic illustration of three formations with same porosity but different values of F (formation factor). Formation factor equations have been approximated through the years by various workers and the following are the commonly used. F=

0.62

best average for sands (Humble)

φ 2.15 F= F=

1

φ2

0.81

φ2

simplified Humble compacted formations

Swn = Ro/Rt Sw = water saturation Rt = resistivity of rock when Sw < 1 Combining the above equations gives Archie’s equation, the most fundamental equation in well logging. Swn =

aRw Rw =F m φ Rt Rt

Practical average Archie’s Equation – general equation for finding water saturation. Sw = Symbol Φ

Character Porosity

0.62

F (formation factor)

φ

2.15

Rw

Formation water resistivity

Ro Rt

Rock resistivity saturated 100% with water True formation resistivity

Sw

Water saturation of pores

Sw Calculations Conventional Quick look Rwa “F” overlay SP Quick Look Clean Formation Shaly

0.62 × Rw φ 2.15 × Rt Derived from Porosity logs (sonic, neutron, density), cross-plots, etc. Calculated using empirical formulae (e.g. Humble formula) and porosity as above SP or laboratory measurements of resistivities of formation water samples Ro = F x Rw (can only be calculated, cannot be measured with logs) Induction Logs and Laterologs (deep resistivity) Sw hydrocarbons Ro = Sw 100% water Rt