Drilling Mud Tracers for Formation Evaluation and Reservoir Engineering Applications

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Invasion Profiler TM
Drilling Mud Tracers For Formation
Evaluation Reservoir Engineering Applications
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Drilling Mud TracersPrincipal Uses
A properly executed tracer study can
provide Gold Standard answers for the following
questions

• What are the in-situ Sw hydrocarbon saturations
  of my formation?
• My formation doesnt produce water What is the
  Rw?
• Has oil-based drilling mud filtrate invaded my
  core and altered the wettability?
• Are my formation test reservoir fluids
  contaminated by oil-based mud filtrate? If so,
  what is the percent contamination?

?
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Drilling Mud Tracers for Core
A Simple Concept
Spike the coring fluid with one or more tracers
that will accompany the mud filtrate into the rock
That Requires Expert Execution
Precise controls over 1) mud design 2) mud
monitoring sampling 3) coring
procedures 4) wellsite core handling 5)
laboratory measurements
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Drilling Mud Tracers for Core

• Low Invasion Coring Practices Recommended

• Low spurt loss
• Low fluid loss
• lt 8cc for WBM
• 2-4cc for OBM
• High solids loading of properly-sized CaCO3 to
  promote filter cake formation
• Low invasion bit design
• High coring rates 80-120 feet/hour
• High Bit Weight
• High RPM

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Drilling Mud Tracers for Core

• Purpose-built Coring Fluids are Best
• Water Based Mud (WBM) Design Considerations

• Bentonite
• Starch
• Fluid Loss Agents (Low Vis PACs)
• Solids Suspension Agents (X-Gum)
• Weighting Agents (Barite)
• pH Control Agents (7.5-8.5)
• Minimize or eliminate surfactants
• Properly-sized CaCO3 Bridging Material ( 1/3
  median pore size)
• Tracer

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Drilling Mud Tracers for Core

• Purpose-built Coring Fluids are Best
• Oil-Based Mud (OBM) Design Considerations

• Base oil
• Organophilic Clay
• Spurt Loss/Fluid Loss Agents
• Low Water Content
• Emulsifiers (to minimize free water)
• High IFT (gt12 dynes/cm)
• Lime
• Electrical Stability gt 2000 volts (i.e., a tight
  emulsion)
• Tracers for both the oil and water phases

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Coring Procedures toMinimize Invasion

• Bit Design
• Conventional bits expose the core to filtrate
  invasion in three areas

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Coring Procedures toMinimize Invasion

• Bit Design

• Low invasion bits are preferred, but the best
  bits are those that allow for high ROP and low
  filtrate invasion

• Face discharge ports direct 80-90 of fluid flow
  away from the core

• No gauge cutters in the throat to disturb the
  filter cake

• Extended pilot shoe seats all the way into the
  bit throat

• Parabolic profile along cutting surface

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Coring Procedures toMinimize Invasion

• Coring Rate
• Coring rates of 80 - 120 ft/hr will minimize
  invasion

Source Rathmell et al, 1994 SPE 027045
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Drilling Mud Tracers For Cores

• Characteristics of High Quality Tracers
• No Background Use tracers that dont occur in
  the reservoir
• Soluble in the Filtrate The tracer must travel
  homogenously with the mud filtrate
• Biological Stability The tracer must not be a
  food source for bacteria (e.g., nitrates)
• Chemical Stability Thermal stability, pH
  stability, and unlikely to react or precipitate
  with anything in the reservoir
• Accurate Detection in the Laboratory Highly
  sensitive, state of the art technology able to
  detect very low concentrations

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WBM Tracers
Iodide, Lithium and Bromide Salts

• Advantages
• Thermal and chemical stability
• No cation exchange with clays for iodide and
  bromide
• Typically available in most areas

• Limitations
• Background levels in the formation can vary
• The rock samples must be destroyed to retrieve
  all of the tracer
• Lithium will react with clay minerals
• Detection limits in the range of 1-10 ppm
• High concentrations required

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WBM Tracers
Dyes and Alcohols

• Advantages
• Inexpensive
• Readily available

• Limitations
• They can react with the rock
• They can evaporate at low temperatures
• The less volatile alcohols can break down in
  crude oil

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WBM Tracers
Isotopes of Hydrogen
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WBM Tracers
Water H2O

• Deuterium Oxide
• D2O

18 Molecular Weight 20
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WBM Tracers
Deuterium Oxide

• Advantages
• Non-radioactive
• Properties similar to water

• Limitations
• Naturally occurring in water and formation brine
• Natural background can vary poor accuracy
• Requires a large dose 35 kilos/1000 barrels of
  mud
• The large volume required to overcome background
  means high cost to deploy
• Interferes with NMR logging

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WBM Tracers
Isotopes of Hydrogen
Hydrogen (H)

• Deuterium (2H)

• Tritium (3H)

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WBM Tracers
Water H2O

• Deuterium Oxide
• D2O

• Tritiated Water
• HTO

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WBM Tracers
Tritium The Optimum WBM Tracer

• Tritium does not occur in formation water
• The amount of HTO found in cores is directly
  proportional to the quantity of traced filtrate
  water that entered the rock during coring
  operations!!!
• A very small quantity (10 ml) of HTO is required
  to trace a 60 foot core in a typical well because
  the detection level is gtgt 1ppb
• HTO does not interfere with NMR logging like D20

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WBM Tracers
Tritium is Safe to Use

• Tritium is widely used in watch dials, gun
  sights, luminous paints, exit signs, etc
• Once the HTO is added to the mud system, the
  drilling mud, the filtrate, and the core samples
  are completely safe to handle without any need
  for protection
• The concentration levels required for highly
  accurate results are ½ the allowable limit for
  unregulated discharge

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Tritium is Safe to Use

• At the HTO concentrations used to trace a well.
• You could drink 50-100bbls of traced mud and
  still be below the US Govts Allowable Ingestion
  Limit for HTO
• You could drink 2 liters of the traced mud every
  day for 365 days and still not have an exposure
  problem
• The weak beta radiation of the HTO will not
  penetrate a piece of paper
• You get more radiation from eating a banana than
  from drinking a cup of HTO traced drilling mud!!

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Adding HTO to theWBM System
Suction Line
Mud Mixing Hopper
Mud Pump
Swivel
Add tritiated water
Kelly
Discharge Line
Collect mud samples
Mud Pits
Drill Pipe
Shale Shaker
Return Line
Core Barrel
Reserve Pit
Collar
Bit
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Mud-Tracer Mixing

• Add the tracer while the drilling mud is
  circulating
• Add the tracer 100 to 200 feet above core point
• Circulate bottoms up several times

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Mud Sampling
Obtain mud samples during drilling and coring to
measure HTO Levels

• In order to determine the amount of filtrate
  invading the core, the amount of tracer in the
  mud must be known
• Samples of the mud should be taken before, during
  and after coring at 5 -10 foot or 30 minute
  intervals, whichever comes sooner
• Mud samples need to be time-lag depth-corrected

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Processing the Core

• Sample the core quickly to eliminate diffusion of
  tracers to the center of the core

• Experience suggests that within the first 48
  hours,
• diffusion is generally not a concern
  
• Variables such as ambient temperature and the
• permeability of the rock can influence
  diffusion rates
• When in doubt, process the core at the wellsite

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Processing Core

• All wellsite lab handling is performed so as to
  minimize adding or removing water from the rock

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Processing Core

• Trim and plug the core with cool nitrogen as a
  lubricant

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Processing Core

• Weigh the plug, wrap in saran foil, weigh it
  again, place in ProtecCore

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Analyzing Corein the Laboratory

• Meticulous attention to equipment cleaning and
  prep, and use reagent grade toluene

• Sample weights recorded at numerous stages to
  develop a weight history throughout the process

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Analyzing Corein the Laboratory
Desiccant
Core
Condenser
Calibrated receiver
Core sample in extraction thimble
Boiling flask
Solvent
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Analyzing Corein the Laboratory
Desiccant
Condenser
Calibrated receiver
Core sample in extraction thimble
Boiling flask
Solvent
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Analyzing Corein the Laboratory

• Immediately transferred to sealed vials

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Determining HTO Concentrationsin the Laboratory

• The HTO concentration is detected in a
  state-of-the-art liquid scintillation counter

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Example Results
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Core Analysis for Rw

• Determining Rw in traced cores starts with
  obtaining companion plug sets

B
A
B
A
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Core Analysis for Rw

• PLUG SET A
• Clean leach
• Dry
• Measure pore volume (Vp)
• Saturate with brine
• Quantify CBW from NMR T2 distribution

A
Martin Dacy, 2004 SPWLA
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Core Analysis for Rw

• PLUG SET B
• Dean Stark to obtain total Vw
• Dry, weigh crush
• Leach salt
• Measure total Cl-
• Correct for filtrate Cl-
• Calculate corrected Cl- content

B
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Core Analysis for Rw
Remove Vcbw determined from NMR
Corrected Cl-
Formation Water
Yields Formation Brine Salinity Rw
Vfmw
Remove Vfil determined from tracer analysis
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Core Log Sw Well 1

• Clean, high quality cemented sand
• Limited diamond core
• Validated log f
• Provided HgPc model
• 10-35 Sw discrepancy Pc Core to Log
• Pc model thought correct given clean sand with
  unambiguous FWL but uncertainties remained
• Development decisions pending
• Decision to cut high quality low invasion core in
  Well 2 to resolve Sw and other production
  uncertainties

Hg Pc model
Source Rathmell, et al SCA 9902, SPE 57318,
1999
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Tracer-Corrected Core SwWell 2 - Low Invasion
WBM
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Core Log SwWell 2 - Low Invasion WBM
Source Rathmell, et al SCA 9902, SPE 57318,
1999
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Core Log SwWell 2 - Low Invasion WBM

• Well 2
• Pc Model Corrected Core Sw agreed
• Salinity (Rw) gradient and variable saturation
  exponent (n) were employed for log Sw calculation
• When core analysis findings were included in log
  Sw model

Sw Log Sw Core Sw Pc
Source Rathmell, et al SCA 9902, SPE 57318,
1999
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OBM Tracers
CMT-1000 The Optimum OBM Tracer

• CMT-1000 is a deuterated hydrocarbon where one or
  more of the hydrogen atoms has been replaced with
  deuterium
• Excellent detection level (lt5 ppb)
• Completely safe to use
• Very small quantities used, so no impact on the
  base oil properties
• Irrefutable fingerprint

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OBM Tracers
CMT-1000 The Optimum OBM Tracer
D
H
H
D
H
H
H
H
D
Mass 78
79
80
81
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OBM Tracers Finding uncontaminated rock and FT
samples

• Oil-based muds (OBMs) are the mud of choice for
  drilling poorly consolidated formations
• OBMs tend to improve ROP and core recovery, as
  well as produce in-gauge boreholes
• OBMs can minimize adverse rock-fluid interactions
  related to clay stability around the wellbore
• BUT
• The oil phase and associated surfactants can
  alter the wettability and relative perm
  characteristics of the core material
• By tracing the OBM, we can identify fresh-state
  rock and/or quantify filtrate contamination
  determine if our FT oil samples are contaminated

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OBM Tracers Determining In-Situ Saturations

• OBMs can reduce (or eliminate) filtrate water
  invasion
• BUT
• OBMs are rarely devoid of water. In a typical
  OBM, 10-45 of the mud volume is water
• The common assumption is that the OBMs never give
  up water to the formation. This may or may not
  be the case

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OBM Tracers for Cores

• The best solution is to trace both the water and
  oil phases of the mud!
• This allows independent verification of any
  alteration of the in-situ oil and/or water
  saturations
• If any filtrate has entered the core, well
  quantify it

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OBM Tracers
Multiphase Tracing
OIL
WATER
Add CMT-1000 to the oil phase
Add HTO to the water phase
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OBM Tracers
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OBM Tracers
CMT-1000 Marker Peak
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Tracers for Formation Test (FT) Sample
Contamination Studies

• FT oil samples are preserved at reservoir
  conditions, minimizing or eliminating
  irreversible changes in fluid properties that can
  occur with changes in pressure and temperature
• However, OMB filtrate can contaminate FT samples
  and alter the very fluid properties that are
  being sought (viscosity, asphaltene content, Bo,
  IFT, Psat, etc)
• Without precise fluid properties, there is a risk
  of inaccuracy in reservoir and production
  engineering calculations

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Tracers for Formation Test Sample Contamination
Studies
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Tracers for Formation Test-Sample Contamination
Studies
- Case History -

• 18 RFT samples taken and analyzed by 2 methods
• CMT-1000 tracer analysis - contamination ranges
  from 3.1 to 16.0
• Passive (GC spectrum deconvolution) fingerprint
  analysis contamination ranges from 0.0 to
  24.0
• Client company created a blind test program of 33
  samples to gauge confidence levels for the 2
  methods

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Tracers for Formation Test-Sample Contamination
Studies
15 of 33 samples analyzed were splits of the same
sample!

• CMT-1000
• Contamination ranged from
• 8.5-9.5
• Rsd 3.9 (0.003)
• Passive Fingerprint
• Contamination ranged from
• 7.5 -13.5
• Rsd 23.2 (0.020)

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Tracers for Formation Test-Sample Contamination
Studies
Impact of OMB Filtrate Contamination on Fluid
Properties
Contaminated Oil Sample (7.7Mole NovaPlus)
Uncontaminated Oil Sample
Psat, psia _at_ 142ºF
4062
5772
1250
2012
GOR, scf/bbl
Bo
1.679
2.019
Viscosity, cp
0.344
0.283
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Summary
Drilling Mud Tracers forFormation Evaluation
andReservoir EngineeringApplications
Properly executed tracer jobs require tight
control over

• 1) Mud Design
• 2) Mud Monitoring Sampling
• 3) Coring Procedures
• 4) Wellsite Core Handling
• 5) Laboratory Measurements

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Summary
Drilling Mud Tracers forFormation Evaluation
andReservoir EngineeringApplications
The optimum drilling mud tracers have these
characteristics

• No Background
• Soluble in the Filtrate
• Biological Stability
• Chemical Stability
• Accurate Detection in the Laboratory

Tritium (for WBM) and deuterated hydrocarbons
(for OBM) meet all of these criteria and offer
best in class performance.
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Summary
Drilling Mud Tracers forFormation Evaluation
andReservoir EngineeringApplications

• Formation Evaluation Applications
• Determine Sw and hydrocarbon saturation
• Determine Rw in reservoirs that do not produce
  water
• Identify fresh-state OBM core samples for
  relative permeability and wettability
  measurements (i.e., accelerate analytical
  programs)
• Reservoir Engineering Applications
• Decrease uncertainty of formation test oil
  analyses and improved facility design
• Determine suitability of oil samples for core
  analysis testing (sample aging, etc)

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Invasion Profiler TM
Drilling Mud Tracers For Formation
Evaluation Reservoir Engineering Applications