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SPWLA 53rd Annual Logging Symposium, June 16-20, 2012

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IDENTIFYING BYPASSED OIL IN CAO LIMON WITH THECARBON/OXYGEN LOG

Marta Elena Becerra, David W. Hampton, Diana Mancilla, Jules Diaz, Rafael Rolon, Helbert Mackualo, and

Alejandro Salgado, Occidental de Colombia; Xavier Goddyn and Juan Angel, Schlumberger; Cesar Patio Ecopetrol

Copyright 2012, held jointly by the Society of Petrophysicists and Well Log Analysts (SPWLA) and the submitting

authors.

This paper was prepared for presentation at the SPWLA 53rd Annual Logging Symposium held in Cartagena,

Colombia, June 16-20, 2012.

________________________________________________

ABSTRACT

Cao Limon is one of the giant fields discovered in the 1980s in Colombia. As part of the Llanos basin, the field

contains many prolific sands distributed between Eocene Mirador and Upper Cretaceous, containing 29 API low

GOR, with a strong fresh meteoric water edge-drive aquifer.

Maintaining the current production rate in a mature field is challenging, and requires dedication from the productionand geoscientist teams to locate undeveloped sands.

During 2010 and 2011 the Occidental Llanos Reservoir Management Team implemented an extensive workover

campaign to locate bypassed oil sands, monitor the current level of saturation, and understand current drainage and

imbibitions mechanisms for each area. This campaign included logging approximately 30 wells, with pulsed neutron

tools in Carbon/Oxygen mode in order to differentiate between hydrocarbons and low salinity connate water or fresh

water from the aquifer. Wells in the field typically have ESP pumps with Y-tools installed. The logging tool is

capable of passing through the Y-tool to acquire both yield and windows carbon-oxygen data in order to ensure an

accurately computed oil saturation (So).

The logging campaign has been effective in locating bypassed oil and nearly undrained sands. Based on these

interpretations, the reservoir team was able to propose new producer wells. The results demonstrate that these tools

are helpful in sustaining field development. The methodology is applicable in many brown fields where similar

conditions exist.One of the most important factors in managing reservoirs is an accurate determination of oil saturation. The

accuracy of this value is critical in tracking reservoir depletion, designing enhanced oil recovery, identifying

workover strategies and understanding water injection breakthrough, especially in Cao Limon, where small

variations in oil saturation can signify a large volume of hydrocarbons. This problem is complicated by variable

water salinities caused by the aquifer influx.

Accurate evaluation of the remaining oil saturation can

be conducted by combining open hole and

Carbon/Oxygen Logs. The integration of these data,

acquired during shut-in conditions, gives valuable

information that can be used to identify bypassed oil,

invaded water zones, undrained sands, and to confirm

the viability of low-resistivity sands which were not

previously considered as net pay.

INTRODUCTION

The Cao Limn field is located in the eastern plains of

Colombia, South America (Fig. 1). The different

reservoirs are formed by a series of sand shale

sequences. The formations of interest are the Upper

Carbonera, the Lower Carbonera (LC-Mirador), and theFig. 1 Location of Cao Limn Field in Colombia
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Cretaceous K1, K2, and K3 (Fig. 2). The LC-Mirador is the most important formation in terms of oil-in-place, rock

quality, and oil production.

Average porosity for the field is about 25 percent, but it exceeds 30 percent in numerous places. Permeabilities can

be extremely high, on the order of several Darcies.

Good quality oil (29 API) with low gas-to-oil-ratios (GOR) and low bubble point pressures (Pb) is characteristic of

all formations. Strong aquifer support is found in the La Yuca and Cao Limn areas. The aquifer is recharged via

meteoric waters which are very fresh and quite different than the initial formation water salinities. The aquifer has

also been very efficient in sweeping the oil; however there are pockets of oil trapped by structural and stratigraphic

changes whose development is the focus of the reservoir development team.

Cased-hole saturation measurements provide

one tool for identifying these opportunities in

existing wellbores. In reservoirs around the

world, the conventional method for

determining oil and water saturation in cased

wells uses a measurement of the thermal

neutron capture cross section (), which is

strongly influenced by the presence of salt

water. For this method to work well, the

product of salinity and porosity must besufficiently high to provide a good contrast of

between the oil and water. Further, the true

formation water salinity must be known to

correctly interpret the water saturation. For

the Cao Limon field, both of these conditions

are problematic.

An alternative tool whose measurements can

be used to determine water saturation in low

salinity and unknown salinity formations is

the carbon/oxygen (C/O) tool.

The primary focus of this paper is to

demonstrate the application of the C/O tool to

measure formation oil saturation in Cao

Limon where the formation water salinity is

very fresh or unknown, usually because of mixing different salinity waters, and where the borehole fluid is also

unknown.

GEOLOGIC SETTING

The Cao Limon field was the initial discovery of a series of fields along the same fault trend. These field share

similar productive intervals and depositional environments.

The LC-Mirador sands, Eocene in age, are primarily deltaic distributary channels. The LC-Mirador contains varying

amounts of dispersed kaolinite. Formation resistivity values (Rt) and spontaneous potential (SP) development are

not appreciably reduced due to the presence of fresh waters (low contrast resistivity zones). For this reason, it is

difficult at present to distinguish between shalier zones due to kaolinite and zones encroached by fresh aquifer

water, as both conditions exhibit good porosities and high resistivities.

The Upper Carbonera is formed by relatively discontinuous fluvial or distributary channels. Sands in the Upper

Carbonera are shalier than in the Lower Carbonera-Mirador. Medium to coarse grained with fair to well sorted

sands are encountered in the Upper Carbonera and LC-Mirador.

The Cretaceous K1 formation is shalier and thinner, with generally poorer quality reservoir rocks. The rock is

bioturbated in places and reservoir quality has been reduced significantly over these intervals. Some tight limestone

stringers and minor glauconitic zones exist in this formation, consistent with its marine origin. Grain size and sorting

Fig. 2 Stratigraphic column, Cao Limn

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deteriorate in the deeper section of the Cretaceous K1. The top of the K2 formation is characterized by fine grained

intervals.

TOOL PHYSICS AND MEASUREMENT THEORY

The formation C/O ratio measurement is used to determine the formation s oil saturation in a cased well when the

formation water salinity is very low or unknown. Carbon-oxygen logging provides a direct measure of formation

carbon, and therefore oil saturation, independent of water salinity. The C/O measurement is based on PulsedNeutron Spectroscopy (PNS).

Fast neutrons (14 MeV) produced by the C/O pulsed neutron source collide inelastically with nuclei of the formation

material, losing some of their energy to the nuclei. The nucleus can release the energy it has gained by emitting a

gamma ray with an energy characteristic of the nucleus. This kind of interaction takes place during and very soon

after the neutron burst, before the neutrons lose too much energy. The gamma rays emitted during this inelastic

scattering process are analyzed during inelastic mode logging with the C/O tool. Figure 3 shows the technical

specifications for the Reservoir Saturation Tool (RST) (courtesy Schlumberger). Two different diameter RSTs are

available, one with 1-11/16 OD where the neutron generator and the detectors are centered aligned and the other

with 2-1/2 OD with eccentered detectors to optimize acquisition in flowing wells and avoid spectrum d istortion.

Figure 4, courtesy Schlumberger) shows the nuclear iterations that occur during the burst of neutrons.

Fig. 3 RST Hardware and Technical Specifications Fig. 4 MeasurementNuclear Interactions

C/O MODEIn C/O logging a single neutron burst is followed within micro seconds by 3 distinct time intervals of measurement.

A net spectrum is composed from these time dependent results and divided into 6 distinct and traceable

contributions: Oxygen, Carbon, Silicon, Calcium, Iron and Tool background. The carbon and oxygen peeks are so

distinct, windows are placed along these intervals (See Figure 5, courtesy Schlumberger). The windows ratio gives

a repeatable and precise answer, however, it is only with the assistance of the Spectral processing can the ratio be

made accurate. The process in combining these two is known as Alpha Processing.

The first processing step decomposes the acquired net

inelastic spectrum, using elemental standard spectra, into

inelastic yields, including carbon and oxygen. The yields

and the C/O yield ratios are computed for the near and far

detectors individually. In parallel, the carbon and oxygen

windows and the resulting C/O window ratios arecomputed for each detector. Windows are simple net

inelastic count rates within an energy window that is

dominated by the given element.

The yields-based and windows-based C/O ratios are

individually transformed into saturation using a database of

measured tool responses in known environmental

conditions.Fig. 5 C/O Energy Spectrum

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The spectral processing has been made more robust by evaluating several parameters as simple functions of

counting rate rather than as additional free parameters in the weighted-least-square spectral analysis. These

parameters are the capture background subtraction factor, and the gain, offset and resolution of the background

spectrum relative to the foreground spectrum. Analytically determining the background subtraction factor allows an

accurate measurement of sulfur, previously difficult to obtain because of the prominent sulfur gamma ray at about

the same energy as the 2.2-MeV hydrogen line that is always present in the capture background.

Lithology is a key input in the transformation of the inelastic C/O ratio into So, because it drives the compensationfor the carbon in the rock matrix. In addition, the clay content of the rock can be better determined from the capture

yields than from other measurements such as gamma ray, sigma or neutron-density (Herron and Herron, 1996)

As mentioned earlier, the yields-based and windows- based C/O ratios are individually transformed into volumes of

oil using a database of measured tool responses in known environmental conditions. Each database measurement

characterizes the tools response in terms of a specific combination of formation lithology, porosity, borehole size,

casing size and casing weight. Actual well environments, particularly porosity and lithology, are rarely identical to

the characterized environments. An interpolation/extrapolation method must be used to interpret tool response in a

generalized logging environment.

The processing is based on an algorithm developed for the RST sigma/porosity measurement (Plasek et al. 1995),

called weighted multiple linear regression. The basic idea is to represent the variation of the tool response

throughout the database as a linear combination of specially chosen, fundamental parameters defining the

environment. The tool response is defined in terms of near- and far-detector carbon and oxygen yields at the endpoints of 100% oil and 0% oil (or 0% and 100% water) in the formation and in the borehole (Figure 6 -

Characterization Quadrilaterals). The environmental parameters are chosen or constructed in such a way that they

fully describe the environmental sensitivity of the tool, while remaining maximally independent of each other.

Fig. 6 RST C/O Characterization Quadrilaterals

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CARBON/OXYGEN LOG EXAMPLES

C/O Example - Well 28

Cao Limon Upper Carbonera channels are closed systems and are characterized by uncertain pressure support, oil

saturation, water resistivity and connectivity between channels. These uncertainties make it difficult to identify

undrained, swept or wet zones with conventional tools.

Well 28 was drilled and completed in the Lower Carbonera sands in 1989, with good oil production and high

resistivity values around 1800 ohm-m through 2010. High water cut production motivated running a saturation log

to confirm undrained intervals or new opportunities because the original petrophysical interpretation showed two C5

channels (Upper Carbonera) with good net sand and moderate original oil saturation compare to the main reservoir .

On December 2010, an RST log was run and results showed potential in one of those two C5 channels The

lowermost channel appear to be drained showed residual oil saturation, while the uppermost channel showed good

oil saturations, which had not been identified as a good potential in the past. The upper channel was opened and

started production with 2% water cut, and continues producing with 15% water cut. Without the RST log

information, large amount of reserves would have been lost, due to the low prospectivity and high risk of opening a

potential water sand. See Fig. 7.

Fig. 7 C/O Results Well 28 Upper Carbonera Channels

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Well 172 was drilled and completed as an oil producer in the Cretaceous sands in 2007 with good results until 2010.

Final production in the Cretaceous sands showed a 98% water cut and low oil rate. Due to an ESP pump failure, a

decision needed to be made whether to restart the well in the same zone or open another zone based on open-hole

log evaluation. Two possible opportunities were identified but the risk to open those sands and find wet zones was

considered very high. Testing each of those sands implied an expensive workover, thus a saturation log was run to

reevaluate the potential of the identified sands.

The first sand was in the Upper Carbonera C2-C3 sands, which are located above the main reservoirs and could have

implied a significant volume of reserves to develop. Unfortunately, the oil saturation was very low, despite the high

deep resistivity reading (very fresh formation waters). The second zone, a Cretaceous sand (K1B2) located above

the initially completed zone showed good oil saturations and potential for a new completion. This zone was opened

and confirmed oil accumulations in that part of the field, but produced at higher initial water cuts than expected

(94%). However the recompletion avoided abandoning the well, and allowed recovery of oil reserves that otherwise

would have been lost. See Fig. 8a and 8b.

Fig. 8a C/O Results C2-C3 Sands Well 172

Fig. 8b C/O Results K1 Cretaceous Sands Well 172

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Well 12 was drilled and completed in the Cretaceous K2 sands in 1991, with good oil production through 2011.

High water cut production motivated running a saturation log to identify by-passed oil or new opportunities because

the original petrophysical interpretation showed Cretaceous K1 sands, as having good quality sands but with high

uncertainty regarding the oil saturation, due to low resistivity contrast between oil and water and high Gamma Ray

responses, highly affected by a combination of fine grain, and presence of clays.On January 2010, an RST Log was run, and results showed potential in those Cretaceous K1 sands. One year later,

Cretaceous K2 sands were isolated and Cretaceous K1 sands were opened. Initial production showed a 2% water

cut, and continues producing a year later with only 5% water cut. Without the RST log information, considerable

amount of reserves would have been lost, because of the low prospectivity and high risk of opening potential low

productivity water sands. See Fig. 9.

Fig. 9 C/O Results Cretaceous Sands Well 12


Well 5 was drilled in January 2008 and completed in the M1 sand with poor production results (water cut of 99%).

Based on these results, the M1 sand was isolated with a bridge plug and the upper C5 sand tested. This too produced

at a 99% water cut. A production logging tool (PLT) was run to evaluate whether there was fluid communication

past the bridge plug isolating the M1 sand. The PLT did not show flow past the bridge plug and the well was

cataloged as an abandonment candidate.

In November 2010 an RST Log was run to evaluate lower C5 channels and the oil saturation of the upper C5

channel sand previously opened. The RST data confirmed that the upper C5 sand had unattractive oil saturations,

but showed good oil saturations in the middle C5 sand. Based on the RST data, the upper C5 sand was squeezed

and the middle C5 sand was perforated with good production results (water cut 1%). Running the RST log, at

substantial cost savings of a WO identified and confirmed the presence of an additional zone to develop in the area.

See Fig. 10.

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Fig. 10 C/O Results Upper Carbonera Channel Well 5

CONCLUSIONS

The C/O log can be effectively used to identify bypassed oil or undrained sands to identify new possible locations orfuture opportunities in existing wells, thereby avoiding potentially expensive and uncertain initial completions or

workovers in formations with low or unknown water salinities.

Few C/O tools have sufficiently small ODs that they can pass through the Y-Tool bypass around the electrical

submersible pump, thereby eliminating the need to remove the tubing and pump for logging.

In comparison to workovers and zonal tests, the C/O log has provided a cost effective approach to identify bypassed

reserves.

REFERENCES

Herron S.L., Herron, M.M., Quantitative Lithology: An Application for Open and Cased hole Spectroscopy,

SPWLA 37thAnnual Logging Symposium, June 16-19, 1996.

Plasek, R.E., Adolph, R.A., Stoller, C., Willis, D.J., Bordon, E.E. and Portal, M., Impro ved Pulsed Neutron

Capture Logging with Slim Carbon-Oxygen Tools: Methodology, SPE 30598, SPE Annual Technical Conference

and Exhibition, Dallas, October 22-25, 1995.

ABOUT THE AUTHORS

Marta Elena Becerra R is a Sr. Petrophysicist in the Llanos Norte Reservoir Management Team with Occidental de

Colombia. She received her BS degree in Petroleum Engineering in 1999 from the Universidad de America. She

gathered experience in open-hole and cased-hole log interpretation from different locations around the world

(Colombia, Ecuador, Peru Venezuela and USA) when she was working for Schlumberger. In 2010 she joined Oxy in

the Llanos and Cravo Norte area in Colombia as a Petrophysicist.

David W. Hampton, P.E. is the Chief Reservoir Engineer for Occidental de Colombia. He received a MS degree in

Petroleum Engineering from the University of Houston in 1987, and a BS degree in Petroleum Engineering in 1983

from the University of Texas at Austin. Upon graduation he was employed by a major service company and was

involved in log analysis and development of a variety of interpretation methods including PNL tool ( and C/O)

interpretations among others. In 2004 he joined Oxy US in the Permian Basin as a reservoir engineer managing

CO2 WAG floods.

Diana M. Mancilla is a Reservoir Engineer with Occidental de Colombia. She received a BS degree in Petroleum

Engineering in 2006 from the Universidad de America in Bogota. Upon graduation she was employed by Occidental

in operations as a well-planner, production with different artificial lift systems and reservoir engineering, involved

mainly in reservoir surveillance, PLT interpretation, waterflooding projects and reserves estimation.

U C5

M1

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Jules Daz received a BS degree in Petroleum Engineering in 2009 from the Universidad Industrial of Santander

(UIS) at Bucaramanga, Colombia. Upon graduation he was employed by Occidental de Colombia as a Trainee

Engineer working for areas such as Production and Operations Engineering. Nowadays he works as a reservoir

engineer for Cao Limon area.

J. Rafael Rolon H. is a Petroleum Engineer in the Llanos Norte Reservoir Management Team with Occidental de

Colombia currently working in the New Field area. He received a BS degree in Petroleum Engineering in 2009 from

Universidad Industrial de Santander. Upon graduation he was employed by Oxy Colombia and was involved inoperation and production activities, after that was in charge of the of New Field area of the Llanos RMT as a

reservoir engineer.

Helbert Mackualo is a reservoir engineer with Occidental in Bogota, Colombia. During the past 26 years he has

worked in several Los Llanos area fields and the Payoa and La Salina fields in the Middle Magdalena Valley. He has

many years experience in pressure transient analysis, PLT interpretation, reservoir simulation and petrophysics and

has also worked in waterflooding and gas injection projects. He holds a BS degree from the Universidad de

America in Bogot.

Alejandro Salgado is the Llanos Resource Management Team Lead for Occidental de Colombia. He received a BS

degree in Civil Engineering from the Universidad de Los Andes in 1986, and a MS degree in Petroleum Engineering

in 1989 from the University of Tulsa. Upon graduation he was employed by the Colombias National Oil Company-

ECOPETROL and was involved in various reservoir engineering projects such as Fluids Characterization, Reservoir

Characterization and Reservoir Simulation among others. In 2003 he joined Occidental de Colombia as a seniorreservoir engineer working in the Cano Limon field.

Juan Angel is a Senior Petrophysicist working for DCS Colombia Schlumberger. He received a MS degree in

Petroleum Engineering from Texas A & M University College Station in 2000, and BS Degree in Petroleum

Engineering in 1995 from Universidad Nacional de Colombia. Expertise from five different locations around the

world (Colombia, Ecuador, Mexico, Angola, USA) where he has been involved in open hole and cased hole logging

analysis as well as borehole reservoir engineering for a major service company.

Xavier Goddyn works for Schlumberger as a log analyst, with 16 years in the industry. He was a field engineer for

Baker Hughes Inteq (West Africa), Geosteering and software engineer for Georex (North Africa, Venezuela). He

joined Schlumberger 11 years ago as a log analyst in Libya. Currently is a Petrophysicist supporting wireline

interpretation for Peru, Ecuador and Colombia. He has a MSc of Geological Engineering from IGAL (France,

1996).

Cesar Patio is a Senior Petrophysicist working for Ecopetrol. He has 15 years working in Formation Evaluation.He has held positions as Schlumberger Field Service Manager (Latin America-Africa-Europe), Weatherford

Operations manager (Latin America) in the Wireline segment, and worked with Oxy and Ecopetrol in their

Reservoir department. He currently gives support to all Colombia basins, and international reservoir

characterization projects. Cesar holds a BS Degree in Petroleum Engineering from the Universidad Industrial de

Santander in 1997.

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