THERMAL DECAY TOOL LOGGING TRAININGTABLE OF
CONTENTS1.0HISTORY2.0APPLICATION3.0THEORY OF MEASUREMENT3.1
Operation3.2 Depth of Investigation and Vertical
Resolution4.0CALIBRATION5.0LIMITATIONS AND PRESENTATION5.1
Limitations5.2 Presentation6.0TOOL COMBINATIONS6.1 Associated
Mnemonics6.2 Typical Log Readings7.0LQC, CORRECTIONS AND
INTERPRETATION7.1 Log Quality Control7.2 Corrections7.3
Interpretation8.0ADDITIONAL READING1.0 HISTORY TDT was first
introduced in the 1960's2.0 APPLICATION Evaluating reservoirs
through casing or tubing. Measure the production of hydrocarbons
and formation water over the production life of a well. Help
determine work over options to increase well production.3.0 THEORY
OF MEASUREMENT3.1 Operation
Figure 1: Energy decay of neutrons in open holeStandard open
hole neutron tools emit high-energy neutrons into the formation
that are rapidly slowed by hydrogen atoms in the formation. Once
the neutrons reach the thermal energy level they are then measured
by the neutron tool.
Figure 2: Energy decay of neutrons in cased holeThe TDT tool
emits short bursts of high-energy neutrons (14MeV) into the
formation. The energy decay of neutrons in a cased hole has an
initial rapid decay caused by the borehole and casing. Past this
the rate of decay of the neutrons is a function of the formation
fluid. Oil, water and gas each have different decay rates.
Figure 3: TDT sampling of neutron energy decayThermal neutron
energy decays according to:N = No e^(-t/T)Where No = the initial
number of neutronst = elapsed timeT = thermal decay time in usec.
(time required for 63% of neutron decay)The TDT measures the slope
of the neutron energy decay by assigning gates to the decay
plot.3.2 Depth of Investigation and Vertical ResolutionVertical
resolution is approximately 1 foot.3.3 Specifications20,000psi, 325
degFMaximum logging speed is 1800ft/hr4.0 CALIBRATIONA shop
calibration needs to be performed for the TDT. The minitron device
emits the neutrons from the TDT tool and needs to be warmed up for
at least 20 minutes while inside the shop calibration tank. The
minitron is electrically activated by a surface power source in the
logging unit.5.0 LIMITATIONS AND PRESENTATION5.1 LimitationsFresh
water and oil have long decay times and can affect the measurements
of fast decay fluids in the formation like shales and salty
formation water. Charts are available to correct for this.5.2
Presentation
Figure 3: TDT PresentationColumn 2 and 3 display the standard
Log Analysis results before production. Columns 4, 5 and 6 display
the TDT results (Sigma 1, 2 and 3) after well production and the
relative water and hydrocarbon saturation.6.0 TOOL COMBINATIONS6.1
Associated MnemonicsSWT1 - Water saturation from pass # 1
PSW1 - Volume of water (PHI*SW)
6.2 Typical Log ReadingsCapture Cross Section
Shale35-55
Matrix8-12
Fresh Water22
Formation Water (Salt)22-120
Gas0-12
Oil18-22
7.0 LQC, CORRECTIONS AND INTERPRETATION7.1 Log Quality
Control7.2 Corrections7.3 Interpretation7.3.1 Estimation of Water
Saturation:The TDT tool requires the volume of clay and porosity
determined from the standard open hole-logging suite. The relative
concentration of the fluids in the porosity is what the TDT will
help determine. Each TDT survey will indicate some hydrocarbon
production by virtue of a drop in hydrocarbon saturation from the
previous survey and also from the initial well conditions before
production.8.0 ADDITIONAL READINGGeneral Literature:Formation
evaluation chart books.Open hole Log Analysis texts.Formation
Evaluation texts.
