Advanced Reservoir Monitoring - NExT · Pulsed Neutron Log (PNL) Applications (capture and inelastic modes): PNL is the main log used to estimate SW and Sg. Both modes needs fine

Advanced Reservoir Monitoring

Oil

Water

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Overview

Following topics will be addressed in this course:

• Saturations of Water (Sw), oil (So) and gas (Sg) are likely to

change with time.

• This is caused by fluid movements in the reservoir caused by

production and gas and water injection as part of secondary

and tertiary recovery.

• The saturation changes are not uniform, and are affected by

permeabilities, structural factors such as faults, and localised

barriers such as shale beds and tight reservoirs.

• The computations of saturations are complicated by factors

such as low salinities, unknown salinities and low porosities

which limits the resolution.

• Near wellbore conditions, such as multiple casings, damaged

zones and enlarged borehole can add to the complexity of

evaluations.

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Reservoir Monitoring Challenge

Changes in Water/oil/gas saturations during production: This is the primary

objective of saturation monitoring and to obtain that as a function of time.

Pulsed Neutron Log (PNL) Applications (capture and inelastic modes): PNL is the

main log used to estimate SW and Sg. Both modes needs fine tuning of the

interpretation parameters.

Log-Inject-Log for residual Oil Saturation (Sor) and Gravel Pack monitoring:

PNL logs can be used to evaluate for Sor very accurately and to evaluate the gravel

pack quality

Fluid sampling: A dynamic tester is used to obtain fluid samples fluid samples and

reservoir pressures behind casings.

Resistivity measurements behind Conductive and non-conductive casings:

Ideally resistivity measurements (Rt) can be of great help as a direct comparison can be

made with original open hole Rt.. Two tools are developed to measure Rt in conductive

and non-conductive casings.

Variations in the sweep water salinity:

The values of water salinity are important for estimating Sw from Pulsed Neutron Log

(PNL) capture mode. This is often unknown as the water composition is a cocktail of

formation water and the different injection waters

Deep Reading Electromagnetic Imaging (EM): Most logging tools have a limited

depth of investigations (<4 m). Well to well EM can help to image water sweep few

hundred meters between wells..

Field wide water sweep mapping: The ultimate objective is to map the water flood on

a field–wide basis for each selected zone. .

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Pulsed Neutron Logging (PNL):

• PNL is the backbone of saturation monitoring. PNL is made up as

follows:

A neutron generator emitting 20 million neutrons at an energy of

17 MeV (Fig-1).

The neutron are slowed down as they collide with the various

atoms (Fig-2) .

At high speed the atoms hit by the neutrons emit spectroscopy

(Fig-3), this is the basis of Carbon/Oxygen logging.

Eventually the neutron slow down to thermal level and get captured (Fig-4). This is the basis for capture mode (Σ)

Fig-1

Fig-2Fig-3

Fig-4

Pulsed Neutron Logging (PNL)

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3-D image of PNL Process

Pulsed Neutron Logging (PNL) Modelling:

• The image above is a 3-D model outlining the PNL process.

Y-axis: are the count rates for the neutrons and Gamma Ray

spectroscopy

Z-Axis is the energy of the emitted GR caused by inelastic

interaction.

X-axis: Is time

• Normally the emitted neutron time life is about 200 micro-sec.

• In the first 50 micro-sec, inelastic interaction takes place and the

results are shown as blue waveforms; The Y-Z plot is shown above

• In the last 100 micro-sec, capture modes takes place. This is shown

as black waveforms. The capture-time plot is also shown above.

GR

Count

GR

Count

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OH 1980PNL1983PNL1986

PNL Capture Mode to Estimate Sw

PNL Capture Mode:

• The figure on the right shows the neutron capture as a function of time

for various Sw values. This is used to obtain Sw values

• The figure on the left shows the results of open hole logs and 2 time-

lapse PNL capture mode results.

• This is used to estimate water saturation changes (shown in dark blue)

as a function of time.

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PNL Capture Mode to estimate Sg:

• The figures above shows 2 time lapse logs used to monitor the

expansion of the gas cap

• The data is accurate with an error range of 10% for porosities >=

15%..

• For lower porosities, only qualitative estimation can be made.

• In this example, the expanding gas cap is assumed to replace the oil

leaving the irreducible water saturation unchanged.

PNL Capture Mode to Estimate Sg

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Stand-alone PNL Capture Mode interpretations

in old wells with no previous data

Stand-alone PNL interpretations:

There is a large number of wells that were drilled and produced with very

limited amount of data. It is estimated that more than 50% of wells in the

Middle East fit this category. A stand-alone PNL log can be used to

provide reasonable formation evaluation to analyse old reservoirs for

missed hydrocarbon zones:• The PNL log can give GR, Porosity and capture (Σ)

• The GR is used to estimate Vsh.

• The effective porosity is computed from the neutron porosity using an

average of laminated/dispersed shale models.

• The water salinity is obtained assuming that there is a flushed zone

with a given Sro.

• The estimated Sw log is shown above on the right.

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PNL Inelastic Carbon/Oxygen

(C/O) Mode

Inelastic C/O:

• The top figure shows a classical spectroscopy obtained from elemental GR

emissions.

• Since the inelastic mode takes place in the early life of neutrons, a

significant part of this spectrum is obtained from the wellbore fluids.

• A family of empirical trapizums are obtained to take into considerations

borehole conditions (casing size, borehole size, borehole fluids, porosity,

lithology, etc..).

• These trapeziums are then used to obtain the values of SW, independent

of water salinity.

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PNL Inelastic Carbon/Oxygen

(C/O) Mode

Inelastic C/O: Field Example

• A field example of C/O application

• The original open hole water saturation includes the green and black

areas.

• A C/O log was run 8 years later. The green area is the depletion

evaluated from this log.

• Below X360 there is either water zone or a residual oil zone.

• The C/O data and the OH data show a perfect fit over this bottom

interval.

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Track-1 Track-2 Track-3

Variable Flood: Field Example

• If we run both Σ and a C/O log we can solve for two unknowns: Sw and

the value of Σwater.

• The Σwater value can then be used to differentiate between injected

water and formation water.

• The example above shows 3 interpreted data:

T1: Original Open Hole saturations

T2 Sw from C/O

T3: The volumes of injected water (purple) and formation ware

(blue)

• This is used to monitor areas where the injection water has by-passed.

PNL Capture and Inelastic modes used to

obtain Sw and volume of injected water.

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Rt through Conductive Casing

Cased Hole Formation Resistance:

• Ideally, obtaining time-lapse Rt values is the easiest way to compare

changes of Rt from the original OH Rt

• Recent technology developments made that possible. This involves

sending an axial current in the casing and estimating the leakage

current over two successive intervals.

• This essentially assumes that we have two resistances in parallel;

Casing resistance

Formation resistance

• The real challenge is that the leakage current results in a change of

voltage of the order of 5*10-9 volts between two successive monitoring

intervals.

• The example above is very informative. No depletion over the top

perforations.

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Rt through Non-Conductive Casing

Cased Hole Formation Resistance- non-conductive casing:

• The induction resistivity log is designed to operate where the medium

around the tool is non-conducting. In open hole this is ideal for oil-base

mud.

• In some wells plastic casings or fibre-glass casing are used. This is

done for monitoring wells.

• In other cases the well is completed as bare-foot, with oil or gas in the

borehole.

• A slim-hole induction log is designed for this purpose. The induction log

is ideal for such data acquisition.

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X-Well Deep reading

Electromagnetic Imaging

X-Well Deep reading Electromagnetic Imaging:• This technology is developed to have deeper readings and

hence deeper imaging of water flooding.

• A series of transmitters and receivers are placed on two wells.

The position of these receivers/transmitters are changed and a

large amount of data are obtained.

• Data inversion is done to determine the fluid composition

between the 2 wells.

• The example above is for 2 wells 150 apart. Good results were

also obtained for wells 1000 ft apart.

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Cased Hole Dynamic Tester

Tool Sketch

Drilling process

Sampling process

Cased Hole Dynamic Tester:

• In few cases log data may not be conclusive.

• A tool designed to drill a hole in the casing, do a pre-test and take fluid

samples was designed and operated successfully.

• This tool has many applications:

For old wells where some of the reservoirs were not tested

Identifying un-swept hydrocarbon zones

Water sampling to determine the source of water

• The drilled hole is plugged on completion of the job by the same tool.

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Log-Inject-Log (LIL) Gravel-Pack Monitoring

Log-Inject-Log and Gravel-Pack Monitoring

Log-Inject-Log and Gravel-Pack Monitoring:

The PNL capture mode log can also be used to do other measurements. Two

important measurements are:

1- Log-Inject Log

• High salinity water is injected in the formation at a low rate, and

successive PNL Σ logs are obtained.

• The value of logged Σ will increase gradually as the water sweeps any

moved hydrocarbon. At the maximum and stable value of Σ the value of

Sro can be estimated.

2- Gravel Pack monitoring

• The gravel is made of Si and Al. When these two elements are

bombarded with neutrons, they are nuclear activated acting as a GR

source with a half life of 2.3 minutes.

• A GR tool run after the source will measure this gamma ray emission.

• Any gaps in the gravel will appear as low values of gamma ray.

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Day 1

• Reservoir fluids

• Reservoir drive mechanisms

• Inflow and outflow performance

• Justifying running reservoir monitoring logs

Day 2

• Nuclear physics of reservoir monitoring and pulsed neutron

logging (PNL)

• PNL tools and scintillation detectors

• PNL for Capture cross section measurements

Day 3

• PNL for Carbon/Oxygen (C/O) logging applications

• Combined capture and inelastic modes to monitor injection

water sweep.

• Log-Inject-Log to estimate Residual Oil Saturation

• Gravel pack monitoring

Day 4

• Stand-alone PNL data acquisition and interpretations in wells

with limited data.

• Cased Hole Formation Resistivity behind steel casing

• Formation resistivity behind non-conductive casing

• Pressure measurements and sampling behind casing

.

Day 5

• Field mapping of water flood to identify unswept zones

• X-well electromagnetic imaging

There will be daily practical workshops on each of the

topics covered using field examples

Agenda