Advanced Reservoir Monitoring - NExT · Pulsed Neutron Log (PNL) Applications (capture and...
Transcript of Advanced Reservoir Monitoring - NExT · Pulsed Neutron Log (PNL) Applications (capture and...
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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