petrophysics 14 resistivity logs

Shell Nigeria Graduate Training Programme Petrophysics

Martey, A.O Univation 14.1

14 RESISTIVITY LOGS

PRINCIPLE

Various devices are available for measuring the resistivity of the virgin (uninvaded)

formation. There are two main types of resistivity measuring devices, Dual

Laterologs (DLL) and the Dual Induction logs (DIL). The true formation resistivity

must be measured to be able to evaluate the hydrocarbon saturation in the reservoir.

When the resistivity measurement is carried out by a sonde in the borehole, the

measurement will be influenced by the mud filtrate that has invaded the formation.

Three independent resistivity measurements are required to eliminate the effect of

the invaded zone, and determine the true formation resistivity. These measurements

are the Deep, the Shallow and the MSFL, each having a different depth of

investigation. Figure 14.1 shows the zones and measurements for a DLL.

v LLD looks deep into the reservoir and measures the true formation resistivity.

The LLD is hardly influenced by the borehole, mudcake and invaded zone. It will

usually read the resistivity of the uninvaded reservoir rock (Ro or Rt).

v LLS looks shallow into the reservoir and measures the invaded zone resistivity.

The LLS is significantly influenced by the borehole, mudcake and invaded lone. It

can be used to correct the LLD when necessary.

v MSFL reads the resistivity of the invaded reservoir rock (Rxo) close to the

wellbore.

In any resistivity measurement, in fact in any log measurement, there will be the

unwanted influence of the borehole. Logging tools are designed in such a way that

these borehole tnfluences are minimized under normal conditions. Correction

procedures, which take into account the borehole size, mud resistivity, etc., are

available to remove the remaining influences of the borehole. These procedures are

described in a series of correction charts that are available from each logging

contractor.



Non Reservoir

Because of the lack of permeability in non reservoir rock, for instance Shale,

Anhydrite, Salt, there is no invasion of mud filtrate in the formation. All three

resistivity devices will therefore read the same resistivity.

Reservoir

If the reservoir is porous, mud filtrate (resistivity = Rmf) will invade the zone close

to the wellbore, replacing all the formation water (resistivity = Rw) and part of the

hydrocarbons, if present.

Figure 14.1



THE LATEROLOG

The laterolog emits a "measuring" current into the formation from one electrode, and

"focussing" currents from a series of auxiliary electrodes positioned symmetrically

about the measuring current electrode. This focuses the measuring current into a

sheet to obtain the best tool resolution (Figure 14.2).

The focussing currents can be adjusted so that the tool simultaneously measures the

"deep" resistivity (Figure 14.2 left), and the "shallow" resistivity (Figure 14.2 right).

The shallow resistivity is better described as an intermediate resistivity. This gives

two of the three independent resistivity measurements. As current flows from the tool

into the formation, the laterolog is particularly suited for use with conductive borehole

fluids, for example salty, water based muds. The tool will not work in oil based mud.

THE INDUCTION LOG

The induction log is based on an entirely different principle. A current is induced in

the formation around the borehole by electromagnetic coupling with an alternating

current (about 20 kHz) flowing in a coil inside the tool. The induced current flowing in

the formation induces a response in a receiver coil in the tool (Figure 14.3).

Figure 14.2



The response can be analysed in terms of formation conductivity, the reciprocal of

resistivity. By adjusting the arrangement of the receiver coil, the formation resistivity

can be measured at a longer or shorter distance from the borehole. This gives two of

the three independent resistivity measurements, a deep reading and an intermediate

reading.

As there is no direct flow of current from the induction tool to the formation, this tool

can be used with low conductivity borehole fluids, such as fresh water muds or oil

base muds. The resolution of the tool is not as good as that of the laterolog, because

the arrangement of the coils does not allow sharp focussing of the measurements.

Figure 14.3



THE MICRORESISTIVITY LOG

Both the Laterolog and the Induction logs give two of the three independent

resistivity measurements, a deep and an intermediate reading. A shallow resistivity

reading can be provided by a microresistivity device. The most widely used of such

devices is the micro-spherically focussed log (MSFL).

The measuring device is a rubber pad with rectangular electrodes on it, which is

pressed against the borehole wall (Figure 14.4). Current emitted from the central

electrode is focussed by means of currents from the other electrodes. The resistivity

of essentially only the invaded zone is obtained. The tool has a very good resolution

as a result of the pad geometry .The tool will not work in oil based mud, because

current has to flow from the pad into the formation.

EVALUATION OBJECTIVE

v Differentiate between hydrocarbon and water-bearing intervals.

v Quantify the Rw in water bearing intervals.

v Quantify the water saturation in hydrocarbon bearing intervals.

Figure 14.4



INTERPRETATION OF RESISTIVITY LOGS

Charts are available to determine the true formation resistivity from the three

resistivity measurements at different depths of investigation (e.g. in Schlumberger

Chart books).

The log shown in Figure 14.5 is an example of the three readings, recorded over a

sand interval. Above 833 m there is a clear separation between the curves. Close to

the borehole the resistivity is low, due to the presence of mud filtrate. Going deeper

into the formation the resistivity increases. This strongly suggests the presence of

hydrocarbons. The true formation resistivity will be even higher than the deep

resistivity device reading. Below 833 m no separation is observed between the

curves. They all read low resistivities. This indicates that even deep in the formation,

where the mud filtrate did not penetrate, the resistivity is still low. We conclude that

the formation is water bearing. The hydrocarbon water contact is at 833 m.

The reading of the deep resistivity tool is usually taken as the true formation

resistivity for a quick look evaluation. This ignores the influence of the invaded zone,

which tends to make the reading of the deep resistivity tool lower than the true

Figure 14.5



formation resistivity. The resulting interpretation is somewhat pessimistic in terms of

hydrocarbon saturation.

EVALUATION TECHNIQUE

v Identify potential reservoir intervals by looking for separation of the resistivity

curves in combination with GR and porosity logs.

v Water bearing reservoir can usually be recognised by a relatively low deep

resistivity, while a hydrocarbon bearing reservoir can be recognised by a

relatively high deep resistivity

v Rw and Sw’s can be calculated as will be described as described in the previous

chapter.

v The water saturation in the invaded zone (Swxo) can be determined by inserting

Rxo and Rmf in the Archie equation. In most cases Swxo will be a lot higher

than the initial Sw, showing that part of the hydrocarbons were displaced by the

invading mud filtrate.

ESTIMATING Rw AND Sw USING RATIO METHOD

A quicker method is the so called Ratio Method. Comparing resistivities in two

zones with equal porosities, one can eliminate F from the Archie formula’s, as

shown below:

In water bearing reservoir, determine Rw:

Read Ro and Rxo from the logs. Determine Rmf at reservoir temperature using the

chart in Figure 14.6.

mf

w

xo

o

RR

RR =

In hydrocarbon bearing reservoir, determine Sw:

Read the Ro in a water and the Rt in a hydrocarbon-bearing interval with equal

porosities to determine Sw in the latter.



wo

t SRR =

The residual oil saturation in the invaded zone (Swxo) can be calculated using the

MSFL in both zones:

)( owxo

cxo xSwRhR −=

−−

Resistivities of Solutions

Actual resi,tisny mca,urements are always prefene<l, bin It necessary. the chart em

the opposite page stay be eased to eshma(e the resishvity of a water sample al a

given Icmpcrawre when the salinity (NnCI concenhafron) a known, or to estimate tile

stdfintv when resishuay and temperaWre are known It may also he used to concert

resislivUy fmm one temperature to another tempcralure. l vunplr Revslnaly of a water

saatplc is 0. 1 ohm-n) at 25 C:

what is the re,ulnaly .a RSW" Fnterlhe chart with 25°C and 0 3 on The, Inlcrscchon

Indncat s a,alinny of appmxnnatclv -O,U00 ppm. Aloulug vlong this connlanl ,alnnly

Ilne kind, a wnferv.unplc mslstrt it-, of 0.13 ohln-m al X5°C

The resists ity of x ,ate r,alnplc can be esWnaW d from ifs chcnlical analysis. An

equivalent NaCl concenlrnllon determmcd by use of the citarf blow n eluertxl Into

Cheu7 Ge,,-() to ,rin,mt, in, resistlcity of the sample.

Me chant a cntele<I In absci,ea with the total solids conccntrauon of the sample In

ppm (mg/kg) to find wciglvlng nmlllpliore for the a al loos Ion, present. The

concentration of each

Gen-8

ion a Inultl rll'd h it' 1 e / weipltllnglnultlpher.undtltcpleducl, to' I) Ions are,unlmed to

nbmln cyunalcnl NA I amccnuntiwt Conccntreloons a rIlk porssed in ppnl or no/log,

both by uclyJal These unite as, umon ,,,IJN equal Fnr more "If,) nnalinll see Kcfelcnct7.



lurmppk Atonnallon-watcl,alnplcanalysls,howslGOppm Ca. Id(10 ppln SO, and

19.000 Pill), N, pi" CI folel ,ohds « mccnmnmn n J60+ Id00+ 19,OI10= 20.410 Id....

]-,It the char bclom mltlt Ihl, total solid, conacu Ilalloln, me fend U R I

a, the (,I Intlhlplicl and 0 45 .n the SO _ nlultlpha Muhlplyin,• the coneenaauon M Ihu

colrcspolldlnc lnulliphcr,, [Ile cqmvalenl faCl aonc.:nnallon is luund n, appnotlntatcly

460 x0.81 e1 100 x0 45 +19.000x1-20,000 hpa1.

I~.nlcnngthe lfaCl Ld .... to on) .... )g,q)h(G111-9i with 20,000 ppm and 75

I-r_1 ('l. [hr 'I'll loll., n loulld to he U.l to ,'S"I



TUTORIAL

The attached logs are taken in an interval which consists almost entirely of

sandstone reservoir of uniform porosity. A small shale streak and a shale bed within

the sand show up clearly on the GR log.

A) Why do the MFSL, the LLS and the LLD overlay across the shale bed?

B) Part of the reservoir is oil bearing. At which depth would you put the OWC?

C) What can you conclude from the fact that the MSFL and the LLD read the same

resistivity in the water bearing interval?



D) From log header, Rmf = 0.12 ohmm at 23 °C and BHT = 60 °C. Use “Resistivity

of NaCl Solutions” chart on to determine the value of Rmf?

E) Use the Ratio Method to determine the average Sw in the oil bearing interval.

F) Use the Archie equations to determine the residual oil saturation in the invaded

zone.

Note: Shxo = 1 -Swxo

petrophysics 14 resistivity logs

Education

resistivity devices

mud resistivity

intermediate resistivity

formation water resistivity

true formation resistivity

reciprocal of resistivity

deep resistivity figure

shallow resistivity