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Electrical
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Depth Measurement
Temperature Rating
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Derivation of Fault
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Open Hole
DEPTH MEASUREMENTS
In open hole logging, the cable is the only method of determining the depth of the instruments.
The characteristics of the cable and methods of measuring the cable length all contribute to
accurate depth measurements.
-
Inelastic Stretch--New Cable. The inelastic or permanent stretch of
the cable is the
stretch that always occurs when
the cable is first put into service. This elongation of the
cable is permanent and in normal
use this will occur completely in the first 30 runs in a
well designed cable. This
stretch is of the order of 1 foot per 1000 feet of cable. Once a
well designed cable has been
"seasoned" it will act like an elastic member without
further elongation. Factors
effecting inelastic stretch in new cables are:
-
Voids in cable core
-
Embedment of
armor into core
-
Inner armor coverage
-
Hot pretensioning
-
Post tensioning
-
Storage of Cable
-
Inelastic Stretch--Old Cables. Under certain conditions
even old or seasoned cables
may experience additional
permanent elongation or inelastic stretch. This can be
caused by:
-
Excessive tension
Excessive
temperature
Spudding
Low inner armor coverage
Storage of cable for long
periods at low tension
-
Elastic Stretch .
The cable is a very elastic member. However, as long as the tension and
the
elastic stretch coefficient of the cable are known, the true length of the cable
can be
determined. The stretch
coefficient K of the cable is obtained by using an
extensionmeter or "stretch
meter" and measuring the elongation of the cable when the
tension is varied a precise
amount.
K
= 5
L
/ ( L 5T
)
5L
= Change in length ft.
L = Sample length ft.
5T = Change in
tension lbs.
K = Stretch coefficient ft/ft/lb.
Values for K for the 7J46
cable are typically 0.77 ft/1000ft/1000lb.
Factors that can cause the
stretch coefficient to change are:
-
Age of Cable
Rotation
Temperature
-
Cable Tension.
Before the stretch of
the cable in a bore hole can be determined, it is
necessary
to know the tension distribution in the cable. Several types of tension gauges
are used including hydraulic
and strain gauges. These are usually mounted on the truck
spooling the cable or mounted
on a turn around sheave in the derrick.
The strain gauge mounted in
the derrick is the most accurate method. Using this device
the cable tension for a cable in the hole would be:
| Ts = W1
L + f L + W1T
Ts = (W1
+ f)L + W1T
where
Ts - Tension
in cable at surface
W1 - effective
weight per unit length of cable
f - frictional drag of cable at logging speed
up hole
W1T
- effective weight of tool |
 |
Effective weights refer to
the weight of the tool and the cable in the bore hole fluids. The
specific gravity is given in the
Camesa catalog for all Camesa cables and can be used to
determine the effective cable
weight in the bore hole.
The tension developed by (w'
+ f) goes from zero at the bottom to (w' + f ) L at the
surface. Therefore the average
tension over the length of the cable is 1/2 (w' + f )L. The effective
weight of the tool WT'
acts over the entire length of the cable. The total stretch
of the cable,
5LT
, can be
computed as:
5LT
= K L 5T= K L [ (W' + F) L / 2 + WT'
5LT
= K L [ (W' + F) L + 2 WT'
] / 2
5LT
= K L (TS + WT')
/ 2
-
Depth
Measuring Systems .
There are two basic types
of depth measuring systems:
Calibrated Wheel. There are many types of wheel systems .
-
Wrap
around wheel in measuring head
-
Tangent wheel in
measuring head
-
Calibrated sheave
wheel
-
Twin measuring wheels
To obtain accurate depth with
a wheel it is first necessary to have
a perfect
wheel
a memory to accumulate the tension history of the cable as it goes
in and out of the
hole
a computational device to add tension corrections to the length continually.
There
are many problems in obtaining an accurate wheel measurement.
-
Cable diameter
Cable condition
Slippage
Indentation
Worm gear effect
Wear of wheel surface
In addition to the
problems with the wheel, it is still necessary to know the tension history of
the cable as it passes 1000 lbs between going in the hole and coming out then
even a perfect wheel will measure 7.7 more feet coming out of the hole than
going into the hole for a length of 10,000 ft., with a stretch coefficient of
0.77 ft/1000ft/1000lbs.
-
Calibrated
Cable--This system requires that the cable be measured and marked
magnetically (or with shims)
while the cable is under standard reference tension TO
- usually 1000 lbs. Using
this system, the true depth at any point in time can be
obtained by locating a magnetic mark (or shim) and making a differential
stretch correction to the
depth indicated by the mark. This differential stretch correction,
L,
is the difference in stretch (L0)
the cable had when it is marked at
reference tension T0
and the stretch of the cable
at some field tension T, 5LT
5L
=
5LT
- 5L0
5L
= (K L / 2) ( TS
-
WT')
K L T0
5L
= (K L / 2)
(TS
- WT'
- 2 T0)
This is the equation used to
prepare some stretch correction charts for field use.
To use this chart consider the following example.
| L |
Depth
indicated from magnetic mark = 10,000 ft. |
| TS |
Cable
tension at the surface = 3,000 lbs. |
| WT |
Effective
weight of tool = 500 lbs. |
| T0 |
Tension
at which cable was marked = 1000 lbs. |
| K |
0.77
ft./1000 ft/1000 lbs. |
| 5L |
differential
stretch correction - 100 ft. |
5L
- (0.77) (10) (3 +0.5 - 2)/2 = 5.8 ft.
Therefore the true depths
L
- L + 5L
= 10,000 + 5.8 = 10,005.8 ft.
Cables
can be calibrated and marked by
- Measuring with 100
surveyors tape
- Using an Automatic Cable
Marking Unit
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