2nd spwla-india symposium, november 19-20, 2009 n an … · 2020. 4. 14. · of assam and assam...

An Integrated Approach To Solve Reverse Resistivity Contrast Problem In

Fresh Water Shaly Sand Reservoir Of Changmaigaon Field, Assam India

– A Case Study.

Pardeep Kumar1, R. Solomon1, T.R. Varun2, Asim Samanta1

Tipam reservoir TS-5A is a typical example of

Low Resistivity Low Contrast Fresh Water Shaly

Sand, where resistivity in oil producing zones is

even less than in water zones. Due to uncertainty

in identification of OWC and correlation of pay

sands, proper development of this main producer

sand could not be done. The causative factors of

low resistivity in pay zones are the presence of

grain coating authigenic clays, iron minerals, phyl-

litic metamorphic rock fragments and fine-grained

nature. Due to fresh connate water (3 gm/lt salin-

ity), major part of the electric current is concen-

trated through the clay coatings and limited cur-

rent passes through bulk pores rendering resistiv-

ity measurement less sensitive to the presence of

hydrocarbons. Rather, the insulating hydrocarbon

phase concentrates exchangeable cations of elec-

trical double layer in lesser pore volume resulting

into enhanced effective Qv, which may reverse the

resistivity contrast.

An integrated approach using Rxo/Rt vs. SP

overlay, innovative interpretation of CMR log, SP

and MSFL logs in high salinity KCl mud have

been evolved to identify OWC and responses vali-

dated against known hydrocarbon and water bear-

ing zones. The use of KCl-PHPA mud with high

salinity has not only resulted into drilling a good

borehole with minimal formation damage but also

helped in solving the low resistivity contrast prob-

lem of fresh water Tipam sands. SP log in high

salinity mud is membrane potential log which is

highly sensitive to the changes in effective Qv and

hence hydrocarbon presence. High salinity mud

filtrate in the flushed zone concentrates electrical

current into the bulk pores and makes MSFL log

sensitive to residual hydrocarbon and hence de-

marcates oil water contact.

Fluid contacts and detailed pay-zone correla-

tion provided in the study will help in effective

field development. Additional hydrocarbon bear-

ing layers identified by the study will result into

immediate oil gain and productivity enhancement.

The extension of these innovative concepts and

methodologies to nearby areas with similar geo-

logical setting will open up new vistas in low-

resistivity fresh water environments.

Keywords: OWC, LRLC, Rxo/Rt, overlay, T2,

KCl-PHPA, membrane potential, SP, MSFL

INTRODUCTION

Changmaigaon field is located South of

Rudrasagar field, South West of Charali field and

North East of Amguri field in North Assam Shelf

of Assam and Assam Arakan basin (Fig.-1). The

field is broadly divided into two fault blocks.

Block-A lies on the Eastern side of a major fault

trending NE-SW with a down throw of about 200

m. In this field, 24 wells have been drilled so far,

of which, 5 have been drilled with high salinity

KCl mud. Block wise relative location of wells

along with log correlation profiles used in the

study are given as Fig.-2.

Tipam reservoir TS-5A of Miocene age, the main

oil producer in Block-A is a typical Low Resistivity

Low Contrast reservoir, where, resistivity in oil pro-

ducing zones is even lesser than that in water pro-

ducing zones. The first well A1, drilled in 1984, is

still producing clean oil from top 10 m. of TS-5A

sand having resistivity 6-7 ohm m, whereas the resis-

tivity in water section below is 7-8 ohm-m (Fig.-3).

To resolve this reverse resistivity contrast problem,

all hi-tech logs available at that time viz. Dielectric,

Natural & Induced Gamma ray Spectroscopy were

recorded in second nearby well A2 and the data was FIG.-1: Oil Fields in North Assam Shelf—A & AA Basin

1 Centre for Excellence in Well Logging Technology (CEWELL), ONGC, Baroda, India 2 Well Logging Services, ONGC, Mehsana, India

ABSTRACT

2nd SPWLA-India Symposium, November 19-20, 2009

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processed by the then state-of-art multi-mineral

software (Fig.-4) but no meaningful results for

identification and evaluation of hydrocarbon zones

could be achieved (Bansal C.R, 1990). Later on

CMR log was also recorded in two wells whose

conventional interpretation could also not solve the

problem of LRLC.

Due to uncertainty in identification of OWC

and correlation of pay sands, the field development

scheme for TS-5A in Block-A did not give the en-

visaged results and had to be staggered as three out

of six development locations went dry. In view of

the above, the present study was taken up to go

into details of the problem and develop methodolo-

gies for identification of OWC.

To identify OWC, three methods viz. Rxo/Rt vs.

SP overlay, T2 spectrum in CMR log, SP and

MSFL logs in high salinity KCl mud, have been

evolved and validated against known hydrocarbon

and water bearing zones. Though these techniques

are applicable to all fresh water Tipam sands, em-

phasis of this study has been on the most challeng-

ing sand TS-5A.

CAUSES OF LOW RESISTIVITY CON-

TRAST IN TS-5A SAND

Authigenic smectite clays with honey comb

morphology coating the sand grains (Fig.-5) pro-

vide additional parallel conductive paths to electric

current and hence reduce the bulk resistivity as

reported in laboratory core studies (Roy Moulik,

2008). Fresh connate water further complicates the

problem as major part of the electric current is con-

FIG.-2 Well Location Map of Changmaigaon Field along with

Correlation Profiles

FIG.-3: Well A1, Formation TS-5A, flowing on self from

interval 2627-37 m since 1990

FIG.-4: Log data and processing results of well A2 with then

state-of-art multi-mineral software


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centrated through the clay coatings and limits cur-

rent through bulk pores, where hydrocarbons are

normally present. Hence, resistivity measurement

becomes less sensitive to the presence of hydrocar-

bons (Winsauer, 1953 & Sen, 1988). Many wells

of the area under study have produced fresh water

with salinity 3-4 gm/lt. In hydrocarbon zones, ex-

changeable cations from clay minerals coating the

sand grains are concentrated in lesser pore volume

resulting into enhanced effective Qv (Cation Ex-

change Capacity per unit pore volume), which,

further lowers the resistivity of oil bearing zones.

Iron minerals present in the reservoir rock as re-

ported in core studies add to the rock conductivity

through electronic conduction. The presence of

lithic rock fragments of phyllites, mica schist and

chloritized biotite in significant amounts within

fine grained sand matrix has been also been re-

ported in core studies. These squashed rock frag-

ments depict behaviour similar to shales on logs

while adding to rock conductivity (Boyd,1995).

In extreme case such as TS-5A sand of Chang-

maigaon field, resistivity reduction by effective Qv

and electronic conduction by iron minerals in oil

zones is more than the expected increase of resis-

tivity by oil, had the reservoir been clean Archie

type. This results into reverse resistivity contrast.

TECHNIQUES FOR IDENTIFICATION OF

OIL WATER CONTACT

Following three techniques have been used for

identification of OWC in such low resistivity –

low contrast complex reservoirs.

SP & MSFL in High Salinity KCl-PHPA Mud

The salinity of any mud formulation is gener-

ally more than 50 Kppm and as a result mud fil-

trate resistivity (Rmf) is about 20-40 times lesser

than conventional NaCl based muds used in area

under study. Not only ‗Rmf‘, but the zeta potential

‗’ and the dielectric constant ‗D‘ also decrease

with increase in salinity and as a result electro-

kinetic potential Ek, given by equation (1), de-

creases many fold when high salinity mud is used

in place of conventional low salinity NaCl mud.

----- (1)

The polymer PHPA (Partially Hydrated Poly

Acryl Amide) is generally used in KCl based mud

formulations, which increases its viscosity to

further reduce Ek.

Therefore, the contribution due to electro ki-

netic potential, if at all it develops, is negligible in

wells drilled with KCl-PHPA muds.

The electrical transport number for K+ ion is

0.496 and for Cl- ion it is 0.504. This similarity is

due to the fact that both the ions acquire the same

electronic configuration that of Argon atom and

both being monovalent. In a solution, cations and

anions will have same mobility under electrical

field and concentration gradients. Thus when two

solutions of different salinities make an interface,

no liquid junction potential will be developed.

Since formation water is very fresh and it always

contain some potassium ions in addition to nor-

mally present sodium ions and since diffusion of

ions takes place from higher to lower salinity re-

gion, hence, even if the formation water is NaCl

and mud filtrate is KCl, no liquid junction potential

is expected due to nearly equal ionic mobilities of

K & Cl ions. Liquid junction potentials for various

electrolytes were studied theoretically (Dakhanov,

1962). The value of ‗K‘ in equation (2) below for

liquid junction potential, Elj for solution is 0.1 mV

as compared to 12.7 mV for NaCl, which is 127

times less.

----(2)

From the above discussion, it can be concluded

that in high salinity mud SP log is devoid of liquid

junction and electro kinetic potentials and is

mainly due to membrane potential.

In 1968, LJM Smits developed an integral

equation for membrane potential developed across

shaly sands separating two electrolytes of different

salinities based upon the concept of transport num-

bers and thermodynamical reasoning when ions

move under concentration gradient.

4

... mf

K

RPDE

mfe

welj a

aKE 10log

FIG.-5: SEM micrograph of cores showing authigenic

smectite coating grains with honeycomb morphology


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-(3)

SP amplitude is maximum against clean water

bearing sand, where the whole contribution is due

to membrane potential of overlying/underlying

shale beds. Against shaly sands, SP amplitude is

reduced by an amount equal to the membrane po-

tential of shaly sand given by equation (3), which

is proportional to Qv of the shaly sand. Further,

presence of hydrocarbons enhances Qv due to clay

minerals, phyllitic rock fragments present in the

reservoir rock and also due to fine grained nature

of rock and this enhanced effective Qv further de-

creases overall SP development. The presence of

hydrocarbons enhances Qv due to clay minerals,

phyllitic rock fragments present in the reservoir

rock and also due to fine grained nature of rock

matrix. LJM Smits related effective Qv to water

saturation in partially saturated rocks by the ex-

pression;

---- (4)

The effect of hydrocarbons on membrane po-

tential and hence on SP log has been studied by

other scientists also, both theoretically as well as

experimentally, (Ortiz, 1972, McCall, 1971). As

discussed above, in case of KCl mud, the mem-

brane potential contributes predominantly towards

development of SP and the large salinity contrast

between KCl mud filtrate and fresh connate water

enhances the overall amplitude of SP log and thus

making it more sensitive for presence of hydrocar-

bons in the reservoir rock.

When a fresh water formation is drilled with

high salinity KCl-PHPA mud, DLL and MSFL

investigate two radial zones of formation around

the bore hole having same mineralogy and poros-

ity but saturated with different salinity brines; one

very saline and one very fresh. The radial section

sensed by MSFL (flushed zone) is saturated with

KCl brine of low resistivity, whereas the un-

invaded virgin zone sensed by LLD contains fresh

connate water. Grain coating authigenic clays will

have minimal effect on MSFL measurement as

major part of electrical current in the flushed zone

will be concentrated in bulk pores and will sense

residual hydrocarbons. Therefore, MSFL will be

able to demarcate OWC. It has been observed that

even change in OWC during the course of produc-

tion can be predicted from the changes in MSFL

as discussed later.

Rxo/Rt vs SP overlay

Identification of oil water contact in wells

drilled with low salinity NaCl based muds and

logged with induction resistivity tools has been

attempted with the help of an overlay of ratio of

Shallow (Near Rxo viz. MSFL, DFL, LL3 etc) and

Deep Resistivity (Near Rt viz. HDRS, AT90, ILD

etc.) with SP log. The technique was applied to

wells drilled with high salinity KCl-PHPA mud

also and it has been observed that it works better

due to larger contrast in mud filtrate and formation

water resistivities and selective effect of clay min-

erals on MSFL and LLD. This technique is par-

ticularly helpful in formations with complex

lithologies and uncertain formation water salinity

as it does not require the knowledge of porosity,

formation factor and formation water resistivity.

It has been observed by many petrophysicist of

yesteryears that Pseudo-static SP (PSP) of water

bearing shaly sands is related to Rxo/Rt ratio in a

similar way as Static SP (SSP) of clean water

sands is related to Rmf/Rw ratio. Therefore PSP of

a water bearing shaly sand can be expressed in the

following form:

---- (5)

For oil bearing shaly sands with dispersed

clays, PSP is given by equation (6) below

(Bassouni, 1994):

---- (6)

For water bearing zones, Szo = Sz and the equa-

tion (6) will reduce to equation (5)

For hydrocarbon zones, Szo > Sz , the second

term in equation (6) is having some negative value

depending upon water saturation of virgin and

filtrate saturation in flushed zone.

If PSP and Rxo/Rt curves are plotted in the

same track using suitable scales so that the curves

track in water bearing zones, the curves will show

separation in hydrocarbon zones. Consequently,

OWC can be picked on the overlay. As absolute

value of SP in hydro-carbon bearing shaly sands

tends towards shale SP values, similar separation

is expected against shale and highly shaly water

bearing zones also. The technique is useful when

applied with the support of other lithology indica-

tors to distinguish between shales and hydrocar-

bon bearing reservoir.

To check the efficacy of the technique, wells

outside the established oil limit (A3, A5, A7, B2 &

mdBQC

BQtC

F

RTE

m

m vw

v

hf

wm ln

2 2

1

w

veffv S

QQ

t

xo

R

RKPSP log

Z

ZO

t

XO

S

SK

R

RKPSP log2log


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B8) i.e. known water bearing have been considered

and it is found that Rxo/Rt tracks with the SP log in

these wells. As an example, TS-5A sand section in

well A3 is presented in Fig-13.

NMR T2 SPECTRUM AND QUADRATURE

COMPONENT OF CONDUCTIVITY

The TS-5A sand as described above contains

grain coating paramagnetic clays and magnetic

heavy minerals giving rise to internal magnetic

field gradients. In clean water wet sands and sand-

stones containing light to medium viscosity crude,

oil signal in T2 spectrum is normally a distinct

peak at higher T2 value as oil protons undergo

bulk relaxation only, which is much slower than

surface relaxation. On the contrary in shaly sands

with dispersed paramagnetic clays and other mag-

netic minerals and even saturated with light oils

(high GOR) T2 spectrum is flattened and shifts

towards lower T2 end due to enhanced diffusion

under internal gradients and rock exhibiting mixed

wettability and hence surface relaxation for oil

protons (Zhang, 1998; Chanh Cao Minh, 2003).

This effect in TS-5A sand is observed due to clay

flake ends touching oil in bulk pores due to pres-

ence of asphaltenes, which are polar molecules

thereby imparting surface relaxation and enhanced

diffusion under internal gradients. Diffusional

coupling between micro and macro pores, under

the effect of internal gradients at pore scale level

can also not be ruled out. All these effects depend

upon the value of internal gradients, which in turn

depends upon susceptibility contrast between

grain surfaces and pore fluid. Oil being diamag-

netic is influenced more by diffusional effects

under internal gradients than formation brines

which are generally paramagnetic.

Hence, hydrocarbon layers in low resistivity

contrast sands like TS-5A can be identified on

NMR logs as flattened and shifted towards lower

T2 end Spectrum as compared to the water zone

where distinct high amplitude peak is observed at

around 100-200 msec. As per the conventional

interpretation, the oil zone would appear as poor

reservoir facies with high capillary bound water

and hence low permeability.

Another in-situ test for diffusion under internal

gradients is carried out by changing echo spacing

of NMR measurement. Decrease in total porosity

with increase in echo spacing indicates enhanced

diffusional effects.

The presence of iron minerals and paramag-

netic clays responsible for internal magnetic field

gradients is inferred by variations in X-component

of complex conductivity (e.g. HDX & HMX in

HRI tool) also known as quadrature component.

The X component is essentially recorded in new

generation induction tools to use for applying skin

effect correction. The variation in X component is

observed when the receiver coil or coil array

crosses an interface of magnetic and non-

magnetic layers (Barber Thomas, 1995). Varia-

tions in HDX are more in oil bearing section than

in water bearing. The X component has brought

out that some paramagnetic minerals, which are

present in this sand, have their predominance in

oil bearing sections. One more important feature

seen on X component curve is that, against shales

the variation are smoothened. This is due to the

fact that in sands these minerals are present in the

form of thin laminae due to high energy deposi-

tional environment but in the shales, they are inti-

mately mixed.

LOG CORRELATION OF SAND UNITS

Correlation of depositional units and fluid con-

tact mapping is a challenging task due to low re-

sistivity contrast, lithological complexity, varia-

tion in hole deviation, mud type and type of log-

ging tools & technologies used. Correlation of

various sand units and fluid contacts has been car-

ried out using maximum available log data. Six

profiles, 4 in Block-A (2 in strike direction and 2

in dip direction) & 2 profiles in Block-B, one each

in dip and strike direction were drawn as per Fig.-

2.

A new sand layer within TS-5A having maxi-

mum thickness up to 12 m, has been identified

(discussed later) and correlated across the field in

both the blocks. So far reservoir potential of this

layer was not recognised. Isopach map of this sand

in block-B is presented in Fig.-14 with areal extent

of 2-3 sq. km. This detailed correlation has further

helped to fix OWC in those wells, where either the

discussed techniques are inconclusive or data

availability is a constraint.

DISCUSSION OF RESULTS

Application of above techniques is showcased

with field examples in following paragraphs with

a special mention, where different techniques cor-

roborate each other. These examples demonstrate

the efficacy of developed methodology in known

hydrocarbon and water bearing zones and hence

their applicability to identify additional prospec-

tive layers. Well wise discussion of results is

given below :

Well A13: This well was drilled in 2007 with

KCl-PHPA mud and DLL-MSFL-SP log was re-


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corded. Deep resistivity in the top oil producing

portion of TS-5A is 5-6 ohm m., whereas below

OWC it varies from 6-8 ohm m. On logs, there is a

reduction in SP and increase in MSFL readings

from 2666 m up to TS-5A top, which indicates

presence of hydrocarbons and demarcates the

OWC at 2666 m. (Fig.-6).

Further at 2652 m. there is sharp increase in

MSFL in spite of decrease in LLD indicating the

zone above this level to be a clean oil zone and

interval 2666-2652 to be swept out zone but still

having residual hydrocarbons. It is interesting to

note that in clean oil zone, LLD decreases and

MSFL increase to make all the three curves almost

track. The interval 2633-42m perforated in the top

part of TS-5A produced oil @ 33 m3/d initially on

self and the well is still producing. The strong cut

& very good GYF indicated in the entire conven-

tional core cut against 2648-57m supports the con-

cept regarding swept out zone. This observation

makes the SP & MSFL in KCl mud useful for

monitoring the movement of fluid contacts during

the course of production from the field also. Sharp

decrease in MSFL at few places observed above

OWC are due to bad bore hole as seen on caliper

in Track-1. Separation between Rxo/Rt and SP in

the interval 2666-2633 m. on overlay presented in

track-4 of Fig.-6 (shaded green), indicates the

zone to be hydrocarbon bearing. Good tracking of

the curves in water bearing section below OWC is

clearly seen. Further, the decrease in separation

from 2652 m indicates the zone 2666-2652 m. to

be swept out zone and the zone above 2652 to be

clean oil zone supporting the conclusions from SP

& MSFL logs. Large separation against shale sec-

tion above top of TS-5A is also observed as de-

scribed earlier.

Well A10: This well was also drilled with

KCl-PHPA mud and logged by DLL-MSFL-SP

run on TLC. Deep resistivity in top clean oil zone

is 6-7 ohm-m, whereas in water leg it is 8-10 ohm

m. Both SP and MSFL logs (Fig.-7) are helpful in

identification of OWC at 2743 m. as described

earlier. Shift in SP at 2734 m. is due to TLC pipe

change. Again in this well of same block as well

A13, two OWC are observed at 2743 m. and

2729.5 m and there is a tendency for all the resis-

tivity curves to track in clean oil zone. The inter-

val 2729.5–2743 m. appears to be swept out zone,

which indicates that the original OWC has moved

up during the course of production from the field.

This well provides very good calibration of the

technique as the testing results corroborate the

interpretation of OWC‘s. During testing, three

intervals ie.2723-2728 m, 2732-2737 m & 2752-

2757 m in TS-5A were perforated and flow of oil

was observed with water. Subsequently during

FIG.-6: Logs and Rxo/Rt vs SP overlay of well A13

showing original and present OWC‘s. A clean water

bearing section about 20m below is also shown for cali-

bration

FIG.-7: Logs of well A10 showing original and present

OWC‘s. Notice the shift in SP at 2734.5m due to pipe

change during TLC logging


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work-over, the bottom interval 2752-2757m was

tested separately through packer and produced

only water. After isolating by bridge plug at

2748m, flow of oil and water was observed and

BHS confirmed presence of 500m of oil column.

The second interval also lies below present OWC

and needs isolation to control water production. It

has been recommended to close the interval 2732-

2737m and test & complete the well in 2721-

2728m. A meaningful Rxo/Rt vs SP overlay could

not be generated due to shift in SP described

above.

Well B7: This was the first well drilled

with KCl-PHPA in block-B. The section from

2888–2913.5m depicts decrease in SP, increase in

MSFL and positive separation on Rxo/Rt vs SP

overlay (Fig.-8) and is interpreted as hydrocarbon

bearing. OWC is placed at 2913.5m. A moderate

separation below OWC is due to higher shaliness

observed on GR and N-D logs.

Well B3: This example demonstrates ap-

plicability of Rxo/Rt vs SP technique in wells

drilled with low salinity NaCl mud and interpreta-

tion of NMR T2 spectrum for identification of

OWC in TS-5A sand and identification of a new

sand layer. TS-5A sand is very difficult to inter-

pret as upper oil bearing portion of this sand is

having lesser resistivity than water bearing one.

This well presents a very good case for analyzing

NMR response in oil and water bearing low resis-

tivity shaly sands. Conventional open hole logs

along with NMR T2 spectrum and overlay is pre-

sented in Fig.-9. The section from 2857.5– 2889m

depicts good separation on Rxo/Rt ratio and SP

overlay and is interpreted as hydrocarbon bearing

with OWC at 2889m.

CMR response against water bearing zone in

the interval 2889-2905 m, with T2 distribution

having a consistently high amplitude peak at the

right edge of the spectrum corresponds to bulk

macro-pores saturated with formation brine. The

low amplitude peaks at left end of the spectrum

indicate clay and capillary bound water. Above

the oil water contact and below the overlying

shale bed, two changes in T2 distribution are no-

ticed: firstly, there is an overall reduction in am-

plitude, and secondly, T2 spectrum shifts towards

left end i.e. lower transverse relaxation time or

fast relaxation. The conventional interpretation

holds that in low viscosity oil zones the T2 spec-

trum shifts towards right i.e. higher T2 time indi-

cating slower relaxation of protons. Fig.-10 illus-

trates the diffusional effects under internal gradi-

ent, where T2 spectrum obtained with two differ-

ent echo spacings of 0.2 msec and 2 msec. are

compared against the oil zone in well B3. A dras-

tic decrease in total porosity demonstrates the role

of internal gradients in T2 spectrum and also indi-

cates that protons in oil zone also undergo diffu-

sional effects.

Variations in X component of conductivity

logs, viz. HDX (HRI log) in track-6 of Fig.-9

indicates the presence of magnetic minerals re-

sponsible for internal magnetic field gradient and

it is further observed that these minerals are

dominant in oil zone. The top part up to 2858 m.

is shale as evident from CMR as well as conven-

tional open hole logs as indicated by T2 peak at 3

ms, flat HDX response, marked change in SP

and high GR. Therefore, interpretation of CMR

clubbed with conventional logs helped to deline-

ate this highly challenging low resistivity pay.

During testing, the interval 2872.5-2875m pro-

duced oil @ 35 m3/d with 5% water-cut validat-

ing the said techniques.

Zone in the interval 2905.5–2916 m is identi-

fied as reservoir facies from N-D, moderate GR,

HDX /HMX and resistivity as distinct from shale

section above top of TS-5A. Good separation on

Rxo/Rt vs SP overlay and flattened and shifted T2

distribution is observed against this interval. By

analogy with hydrocarbon producing layers, the

interval 2905.5–2916 is interpreted as hydrocar-

FIG.-8: Logs and Rxo/Rt vs SP overlay of well B7

showing the OWC in TS-5A


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bon bearing. By correlation this layer is equivalent

to New Sand described in wells of block-A above.

As an extension of the applicability of the tech-

niques and validation, TS-5B sand is included in

the study, though OWC is visible on deep resistiv-

ity log also (Fig.-11). The interval 2935.5–

2951.5m depicts separation on Rxo/Rt vs SP over-

lay and is interpreted hydrocarbon bearing by over-

lay technique, therefore OWC is placed at

2951.5m. During testing, the interval 2936-2942m

produced oil @ 70 m3/d & gas @1,514 m3/d with

14% water-cut.

FIG.-9: Composite log display of well B3. Formation : TS-5A , Interval : 2872.5-75m. Produced oil @ 35 m3/d with 5% w/c during

initial testing. It is very difficult to identify OWC from resistivity log but CMR is clearly demarcating OWC as per the new inter-

pretation. Presence of magnetic minerals is evident from HDX curve in track-6. Rxo/Rt vs SP overlay also confirms the OWC in

TS-5A. New Sand and its hydrocarbon potential in the interval 2905.5-2916 m is clearly identifiable from overlay, CMR & HDX.

FIG.-10: Well: B3: Drastic reduction in NMR total porosity

on changing echo spacing from 0.2 ms to 2 ms, indicating

enhanced diffusion under internal magnetic field gradients in

oil bearing zone.

FIG.-11: Logs and Rxo/Rt vs SP overlay of well B3 show-

ing the OWC in TS-5B which is also seen clearly on resis-

tivity logs


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Well B5: This well is drilled with low sa-

linity NaCl mud and HRI-DFL-SP log is re-

corded. Separation on Rxo/Rt vs SP overlay up to

3014 m indicates presence of hydrocarbons with

OWC at 3014 m. (Fig.-12). The interval 2981-

2986 m in top portion of TS-5A produced oil @

20 m3/d. This zone selection was initially based

on SP & MSFL in KCl mud in well B7 and test-

ing results of well B3 and is corroborated by

overlay technique as well.

Interval 3026–3037.5m within TS-5A corre-

sponds to New Sand. The intervals 3027.5–

3029.5m & 3031.5–3037.5m depict good reser-

voir character from GR, N-D and X component

of Conductivity HDX & HMX. These appears to

be promising from hydro-carbon bearing point of

view as they depict good separation on Rxo/Rt vs

SP overlay. The lower part of TS–5A is inter-

preted to be water bearing by Rxo/Rt vs SP over-

lay technique.

Well A3 : In this well the top of TS-5A

sand is much below OWC and the well being far

FIG.-12 : Composite log display of well B5, Formation TS-5A, Interval 2981-86 m produced oil @ 20m3/D. There is hardly any

resistivity contrast between oil and water legs. Rxo/Rt vs SP overlay indicates OWC at 3014m. New Sand in the interval 3027-38

m. appears interesting from H.C. point of view.

FIG.-13: Logs of well A3 showing good tracking on Rxo/Rt

vs SP overlay in water producer


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away from the producers. This outlier is included

in the study to provide calibration and validation of

the Rxo/Rt vs SP overlay technique. TS–5A is inter-

preted to be water bearing by Rxo/Rt vs SP overlay

technique (Fig.-13). During testing, interval 2635-

2639 m produced water of salinity 3.6~3.9 gpl.

CONCLUSIONS

The main reason for Reverse Resistivity Con-

trast between oil and water bearing zones in TS-

5A is presence of grain coating authigenic clays,

metamorphic rock fragments, chloritized biotite

and conducting iron minerals coupled with fresh

water environment and fine grained nature of the

reservoir. KCl based high salinity mud and DLL-

MSFL have been successful in identification of

OWC and monitoring of movement of OWC.

Overlay technique of Rxo/Rt vs SP has enabled

identification of OWC in wells drilled with low

salinity NaCl mud and having induction resistivity

logs as well as in wells drilled with high salinity

KCl mud. Detailed structural and stratigraphic cor-

relation of depositional units and pay zones using

all available log data has been accomplished en-

compassing the variations of hole deviation, mud

type, logging tools & technology and missing data.

These correlations have helped to fix OWC in

wells where the techniques were inconclusive.

Possibility of TS-5A reservoir having mixed wet-

tability is inferred from shape of T2 spectrum in

CMR log and presence of residual hydrocarbons

below clean oil zones in wells A13 and A10 from SP

& MSFL logs.

A new sand layer within TS-5A having maxi-

mum thickness up to 12 m, identified from Rxo/Rt

vs SP overlay and other log features has been cor-

related across the field in both the blocks. The

areal extent of this sand layer is estimated to be 4-

5 sq. km. This sand has been mapped in block-B

and isopach map has been prepared (Fig.-14).

NOMENCLATURE

Eqn.1: ∆P is pressure differential from borehole

to formation. Other symbols are defined in the

text.

Eqn.2 K = - (t- - t+) RT/F, t- & t+ are the transport

numbers of the anions and cations respectively,

R=Gas Constant, F=Faraday constant, T absolute

temperature, awe and amfe are the equivalent activi-

ties of formation water and mud filtrate

Eqn.3 ± are the molal activities of cation & an-

ion, m1 & m2 are molality of electrolyte. t+hf is

Hittorf Transport Number for cation in the free

electrolyte

Eqn.5 & 6 K is the same temperature dependent

constant as for clean sands in SP equations.

Eqn.6 Szo & Sz are water saturations of flushed

and un-invaded zones. Subscript z & zo instead of

xo & w were used to indicate that the conducting

phase in the pore space is a mixture of formation

water and dispersed clay.

ACKNOWLEDGEMENTS

The authors express their sincere gratitude to

Shri D.K. Pande, Director (Exploration), ONGC

for permitting publication of the paper. The au-

thors express their grateful acknowledgements to

Shri Dinesh Chandra, ED-Chief Logging Ser-

vices, ONGC, for his encouragement and techni-

cal guidance. Shri D.R. Rao, GM(W), Shri Jai Pal

Singh GM(W), Shri Yogesh Chandra DGM(W),

Shri Sairam Prasad C.G.(Wells) and Sub-surface

and EPINET teams of ONGC, Nazira are sin-

cerely acknowledged for providing necessary

data and support. The views expressed in this pa-

per are of the authors only and not necessarily of

ONGC.

FIG.-14: Isopach map of New Sand in Block-B. Areal extent

estimated to be about 2.5 sq.km


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ABOUT THE AUTHORS

Pardeep Kumar, Chief Geophysi-

cist(Wells) passed M.Sc. Physics

with Electronics as Specialization

in 1981 and M.Phil Physics with

dissertation on optical communica-

tion theory in 1982 from Punjabi

University Patiala, India. He joined

ONGC in 1985 as Geophysicist

(W) at Ahmedabad, where he car-

ried out field operations in open hole, cased hole and

production logging for eight years. In 1993, he was

transferred to Agartala, where apart from field opera-

tions he was actively engaged in planning and provi-

sioning, Budget and inventory control, Explosive and

radiation safety. From 1996 to 2007, he was posted in

Petrophysical Research Division of KDMIPE, where he

carried out several integrated field studies of various

Indian basins, data processing and interpretation of

some foreign basins. He developed an innovative inter-

pretation technique for evaluation of Low Resistivity

Low Contrast Tipam Sandstones of Assam, which has

resulted into significant oil gain and ONGC has applied

patent for this technique. He was awarded CMD award

on 26th Jan., 2008 for this technique. Apart from this, he

got Director Exploration‘s second best paper award and

several other awards including two from Head ONGC

Academy for outstanding Faculty. He is presently

posted at Centre for Excellence in Well Logging Tech-

nology (CEWELL), ONGC, Baroda and his special

interests are LRLC and unconventional reservoir

evaluation. Sh. Kumar has published 14 research papers

and is a member of SPWLA, SEG and Indian chapter of

SPWLA.


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Roland Solomon, Chief Geophysi-

cist(Wells), ONGC completed his

Bachelor Degree in Science with

Physics Honours and Masters De-

gree in Science with Electronics

Specialization from Delhi Univer-

sity in 1980 and 1982 respectively.

He completed his Master of Tech-

nology from Indian Institute of

Technology, Delhi, in 1984. He

joined ONGC in 1984 as Geophysi-

cist(Wells) at Regional Interpretation Centre at Baroda

and has since worked in Log interpretation, field ex-

ploration & development. With special interest in log-

ging software, he has actively contributed to the devel-

opment of in-house softwares. Thereafter he worked in

logging field operations including open-hole and cased

-hole. He is also experienced in handling procurement

procedures and logging contract. He has experience as

a Solaris systems administrator on Sun workstations

along with Oracle and GeoFrame having been trained

at Schlumberger training centre, Muscat, Oman. He

has also worked as ONGC team member for conceptu-

aliation and implementation of Ultra-Short Radius

Drain-hole Drilling (USRDH) project along with other

hi-tech drilling projects in Eastern Region, Assam. He

also has experience in the use of data archiving soft-

ware viz. LogDb and Finder. He has been awarded

several times for exemplary zeal and commitment.

Currently, he is posted at CEWELL, Centre for Excel-

lence in Well Logging Technology, Baroda. He has co-

authored several technical papers presented at various

technical fora. He currently designs and maintains the

intranet website of CEWELL. His special interests

apart from well logging technologies is computer sys-

tems—both software and hardware. He is a member of

Indian chapter of SPWLA and CSI (Computer Society

of India).

Dr. T R Varun, GM(Wells),

passed M. Sc (Physics) with

Electronics as specialization,

SSV College, Hapur, India. He

has also acquired Diploma in

Industrial Relations & Personnel

Management from Bhartiya

Vidhya Bhavan (1993), MBA

(HRM) from IGNOU (1998),

Post Graduate Diploma in Finan-

cial Management from IGNOU (2000) and in 2008 he

has been conferred Ph.D in Management & Commerce

from H. P. University Simla, India. He joined ONGC

as Graduate Trainee in 1978 at Dehradun and thereaf-

ter he was posted at Mumbai where he worked as Log

Analyst. He was associated with effective water injec-

tion programme for Bombay High Field as injection

profiling expert. Next 5 yrs. (1985-90) at Nazira

(Assam) he worked as Logging party chief. In 1990 he

moved to Basin Studies Division, KDMIPE where he

carried out various integrated field studies of carbon-

ate, clastic and fractured basement reservoirs of on-

shore and offshore basins and headed Basin Modelling

group. From 2001 to 2004 he worked in Western On-

shore Basin Baroda for prospect generation, and from

2004 to 2007 as team leader he headed JV CB-OS-1

Block. Before joining CEWELL in June 2008, he

headed the Regional Data Base division at Baroda. At

CEWELL he headed and co-ordinated the Field Stud-

ies and Reservoir Characterization group. Presently he

is posted at Mehsana as Head of Well Logging Ser-

vices group. Dr. Varun has got eight merit awards in-

cluding the appreciation letter from Director

(Exploration) and second best paper award from CMD,

ONGC. He is member of SPWLA, SPG and APG, and

SPWLA India Chapter. He is also a life member of

NIPM and AEG.

Asim Samanta, GM Geophys-

ics (Wells), Hons. Graduate in

Mathematics, post graduate in

Exploration Geophysics from

IIT, Kharagpur, joined ONGC

as Graduate Trainee in 1977. He

has worked at several work cen-

tres of ONGC including Bom-

bay Offshore Project, GEOPIC,

Nazira-Assam, Ahmedabad,

Western Onshore Basin, Baroda

and CEWELL. He heads CEWELL, ONGC, Baroda

since June 2008. At Mumbai he worked on exploration

and development of offshore fields leading to discov-

ery of new fields. He got recognition from Member

(Exploration) at Bombay for working on the installa-

tion of well logging software on Main frame system.

At Dehradun as head of log analysis group of INTEG,

GEOPIC and worked on integration of logs with seis-

mic. At INTEG he was responsible for developng in-

house log analysis software on mainframe system. He

also processed and interpreted data of Vietnam wells

and trained the Vietnamese in log analysis for which

he was awarded by Director (Exploration). He success-

fully set up a workstation at Nazira for log analysis for

which he got appreciation of Director (Exploration).

At Nazira he was also responsible for Planning, provi-

sioning and budgeting of the logging base. After the

stint at Nazira he moved to Ahmedabad, he spent some

time in monitoring safety of logging operations and

later he took up the assignment of Head Database to

implement the Phase-I EPINET project. He then

moved to Ahmedabad, Cambay -Tarapur block at

Western Onshore Basin, Baroda. At Baroda, after

working in the Block he was served as Head Database

and during his tenure the progress of Database group in

various front is innumerable, viz. Virtual Reality centre

came into existence, IIWS saw its hardware and soft-

ware upgraded, Phase-II of EPINET was implemented

and EPINET was rolled out etc. As Head, CEWELL he

has taken several initiatives which includes setting up

of state of art computer centre with latest hardware &

software, completion of more than 32 log based pro-

jects of different fields of ONGC etc.


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