Well Log Interpretation

Earth & Environmental Science

University of Texas at Arlington

Interpretation

The primary goal of well log interpretation is to determine whether there is petroleum, and if so, how much can be recovered and how fast it will flow.

Well log interpretation is used to determine the economic viability of the well: How profitable it will be and how soon the drilling costs can be recovered.

Interpretation

Unless the stratigraphy and reservoirs are well known, the first step is to scan the well log for likely reservoirs.

The well site geologist will have information about the location of petroleum shows

Interpretation

Which logs would be used for each of these steps? What would you look for in those logs?

Quick-Look Methods:

Rxo/Rt

One of the reconnaissance methods is the relationship between the SP curve and the resistivity ratio Rxo/Rt

Quick-Look Methods:

Rxo/Rt

This technique works because the SP log is based on differences in salinity which in turn are related to differences in resistivity:

t

xo

w

mf

R

RK

R

RKSp log*log*

Quick-Look Methods:

Rxo/RtWhen the rock

contains only water, Rxo/Rt will differ from the SP only by a constant. If petroleum is present. Rt increases so the two curves deflect away from each other.

t

xo

w

mf

R

RK

R

RKSp log*log*

Quick-Look

Methods: Rwa

Apparent Water Resistivity (Rwa) compares the deep resistivity of various zones in the well bore. The lowest Rwa is assumed to be water, so high Rwa must contain petroleum.

Quick-Look

Methods: Rwa

This works because

a

RR

anda

RR

or

RFR

mt

wa

mo

w

wo

*

*

*

Need info about lithology and porosity

Quick-Look

Methods: Rwa

If the lowest Rwa reading reflects only water in the pores, then the apparent water saturation (Swa) can be estimated by:

zonewa

wawa R

RS min

Quick-Look

Methods: Rwa

This Swa assumes that the zones being compared have the same lithology and porosity.

zonewa

wawa R

RS min

Quick-Look Methods: Resistivity porosity

This method calculates a porosity from resistivity data using the Archie Equation, and assuming Sw = 1

In zones that are water filled, is high and equal to the true porosity.

In zones that have petroleum, Rt is high and is lower than the true value.

R is plotted with porosity logs and knowledge of the lithology is assumed.

m

t

wR

w

m

nwt

wR

n

mt

ww

R

Ra

SWhen

SR

Ra

or

R

RaS

1

1

1

*

,1

*

*

*

*

Quick-Look Methods: Wet Resistivity (Ro)

Ro is the actual resistivity of the formation and fluids. Rt is the measured value.

Ro can be estimated from the formation factor (a, m & ), and Rw.

Assuming a value for Rw and , then Ro is the estimate for the resistivity of a water saturated zone.

mwRa

R

*

0

Quick-Look Methods: Wet Resistivity (Ro)

When the calculated Ro is plotted with Rt, the deep measurement by the log, the two traces should overlay if there is no petroleum. Otherwise, the two curves will diverge.

mwRa

R

*

0

Detailed Log Analysis

Once prospective hydrocarbon zones have been identified, then calculations of the desired parameters for economic evaluation are made.

Detailed Log Analysis: Water Saturation

Water saturation in the flushed zone and the uninvaded zone are calculated using the Archie Equation.

n

mxo

mfxo

n

mt

ww

R

RaS

and

R

RaS

1

1

*

*

*

*

Detailed Log Analysis: Water Saturation

Instead of calculating Sw and Sxo separately, it is useful to calculate their ratio, because the lithology factors are eliminated.

w

mf

t

xon

xo

w

RR

RR

S

S

Detailed Log Analysis: Water Saturation

Sw/Sxo is the Moveable Hydrocarbon Index. If Sw/Sxo = 1, no hydrocarbons were moved. If it is less than 0.7 for ss, or less then 0.6 for carbs, then petroleum will move.

2

1

w

mf

t

xo

xo

w

RR

RR

S

S

Detailed Log Analysis: Water Saturation

Instead of calculating Sw using the Archie equation where lithology parameters must be known, water saturation can also be estimated using the ratio method without knowing the lithology parameters.

n

mt

ww R

RaS

1

*

*

Detailed Log Analysis: Water Saturation

2

1

w

mf

t

xo

xo

w

RR

RR

S

S

The saturation ratio can be determined using only resistivity data (above). If petroleum is present, then:

51

wxo SS

Detailed Log Analysis: Water Saturation

2

1

w

mf

t

xo

xo

w

RR

RR

S

S

Substituting Sxo gives Swr (water saturation ratio method).

51

wxo SS

8

5

w

mf

t

xo

wr

RR

RR

S

Detailed Log Analysis: Water Saturation

Swr can be used as a check on Sw computed using the Archie equation

8

5

w

mf

t

xo

wr

RR

RR

Sn

mt

ww R

RaS

1

*

*

Detailed Log Analysis: Irreducible Water

SaturationWater saturation, Sw, includes water that is

bound to particle surfaces, and water that will not move because of capillary pressure. This is called irreducible water saturation, Swirr.

If Sw = Swirr, then no water will be produced, which is important to know in making an economic evaluation of the well.

Detailed Log Analysis: Bulk Volume Water

Bulk water volume (BVW) = Sw * .

Table 7.1 shows estimates of BWV at irreducible water saturations, so calculation of BVW can show whether the reservoir will produce water along with petroleum

Detailed Log Analysis: Bulk Volume Water

Buckles plots are a way of determining whether the reservoir is at Swirr. (The ordinate should be Sw, not Swirr).

Detailed Log Analysis: Bulk Volume Water

Plots of against Sw will follow the hyperbolic curves of BVW if the reservoir is at Swirr (left). Otherwise, both petroleum & water production are likely.

Detailed Log Analysis: Assignment

On your spreadsheet from the previous resistivity assignment, add columns to calculate water saturation using the ratio method (Swr), Moveable Hydrocarbon Index (MHI), and Bulk Volume Water (BVW).

Make a Buckles plot of Sw and to determine whether the zones are at Swirr.

For each of the zones you have analyzed, describe and explain the potential to recover hydrocarbons economically.

Detailed Log Analysis: Saturation Crossplots

With the advent of computers, graphical solutions to the Archie equation aren’t so necessary any more. However, there are two that are sometimes used to get a visual picture of the productive zone saturation.

Detailed Log Analysis: Pickett Crossplot

The logarithmic form of the Archie equation can be written in a couple of ways:

tm

w

wt

w

wwt

tm

wnw

Rm

Ra

or

mRaR

Sifand

SnmRaR

R

RaS

log1

*loglog

log*loglog

,1,

loglog*loglog

*

*

1

Detailed Log Analysis: Pickett Crossplot

The form below is the one traditionally used for the Pickett crossplot. (Note equation 7.26 in text and the description in Fig. 7.4 is wrong).

tm

w Rm

Ra log1

*loglog1

Detailed Log Analysis: Pickett Crossplot

tm

w Rm

Ra log1

*loglog1

When is plotted with Rt on log-log graph paper, the slope of the line is -1/m and the intercept, when Rt=1, is (a*Rw)1/m.

Detailed Log Analysis: Pickett Crossplot

tm

w Rm

Ra log1

*loglog1

Note that this plot requires Sw =1.0.If enough points can be plotted, a value of m can be determined. “a” can be calculated if Rw is known (or vice versa).

Detailed Log Analysis: Pickett Crossplot

tm

w Rm

Ra log1

*loglog1

This plot also requires that the lithology (“a”) and Rw be the same in all zones plotted.

Detailed Log Analysis: Pickett Crossplot

tm

w Rm

Ra log1

*loglog1

Lines for Sw < 1 can be drawn parallel to the Sw=1 line using the factors in table 7.2. Find Rt for Sw=1 at any arbitrary , and multiply that Rt by 1.56 to get the Rt at Sw=0.8 for that . Draw the line parallel to Sw=1.

Detailed Log Analysis: Hingle Crossplot

m

w

nw

m

t RS

Ra

11

Hingle crossplots are strange and based on this form of the Archie equation:

Detailed Log Analysis: Hingle Crossplot

m

w

nw

m

t RS

Ra

11

, or any proxy such as the density or sonic logs is plotted on a linear scale at the bottom. The ordinate is 1/Rt (or conductivity) and has to be scaled for particular values of “a” and “m”.

Detailed Log Analysis: Hingle Crossplot

m

w

nw

m

t RS

Ra

11

The scaling of the ordinate must be designed so that values of Rt and plot as a straight line for constant Sw.

Detailed Log Analysis: Hingle Crossplot

m

w

nw

m

t RS

Ra

11

While “a” and “m” must be assumed to design a Hingle plot to get a straight line, the data plotted on the Sw=1 line can be used to calculate Rw. Sw<1 can also be estimated once the Sw=1 line is established.

Detailed Log Analysis: Permeability

Permeability can be estimated from porosity, resistivity, Sw and hydrocarbon density data. However, Sw must equal Swirr, the irreducible water saturation.

Bulk Volume Water (BVW) must be calculated and plotted in advance to made sure the zone of interest is at Swirr.

Detailed Log Analysis: Permeability

23

250

wirrSK

There are two simple formulas for medium gravity oil and dry gas (i.e. hydrocarbon density is assumed.

For medium gravity oil:

For dry gas:23

79

wirrSK

Detailed Log Analysis: Permeability23

250

wirrSK

The equations can be solved graphically. Each hydrocarbon density requires a separate graph.

Detailed Log Analysis: Permeability

2

4

2

2

2

2.2log2

175.3

18846523

irrt

w

W

irrt

w

hh

RRW

CK

RRW

C

A more complicated formula that includes variables for hydrocarbon density is:

Detailed Log Analysis: Permeability

The most reliable permeability comes from well testing and direct measurements of discharge and hydrocarbon density. If cores are available, permeability can be measured in the lab.

Detailed Log Analysis: Shale/Clay Analysis

Shale and clay in in rock directly affects resistivity and porosity measurements and all of the parameters derived from them, especially Sw.

Phyllosilicates do not all affect resistivity the same way. It is the cation exchange capacity of the layer silicate that is critical; Kaolinite, chlorite muscovite and biotite with low capacities do not affect the resistivities as much as the smectites. Logging tools can not make those distinctions so clay content in rocks is a significant problem.

Detailed Log Analysis: Shale/Clay Analysis

Some knowledge of the resistivity of the phyllosilicate component is important, so the usual assumption is that Rt of nearby shale zones is the same as Rsh in the reservoir. This is often a bad assumption leading to erroneous Sw.

Detailed Log Analysis: Shale/Clay Analysis

The usual procedure is:1. Calculate a volume of shale (Vsh) using the

gamma ray log the SP log, or a lithology crossplot.

2. Use the Vsh to correct porosities calculated by the sonic, density and/or neutron logs.

3. Measure a bound water resistivity (Rwb) from zones with 100% shale and a free water resistivity Rt from a shale free zone.

Detailed Log Analysis: Shale/Clay Analysis

4. Calculate a water-bound saturation Swb for the 100% shale zone.

5. Find the apparent resistivity Rwa of the reservoir using the weighted average of Rt and Rwb knowing Vsh.

6. Calculate a total, shale corrected, water saturation (Swt) for the reservoir.

Detailed Log Analysis: Shale/Clay Analysis

7. Then the effective water saturation for the reservoir (Swe) is

wb

wbwtwe S

SSS

1