Coax-slickline cable for use in well logging

A slick-line coax cable for use in downhole well-logging under conditions which would normally prevent logging with standard `stranded` line cables includes a device for preventing migration of fluid inside the cable, a coaxial conductive layer of metal to allow more efficient data transfer from the downhole logging tools to the surface recording equipment, and a seal for terminating the downhole end of the metal-encapsulated cable thereby preventing downhole pressure and fluid migration into the cable.

FIELD OF THE INVENTION 
This invention relates generally to a coax cable for use in logging an 
earth formation traversed by a borehole, and more particularly, a 
slick-line coax cable for use in downhole well-logging. 
BACKGROUND OF THE INVENTION 
Gathering petrophysical, geophysical and well production information 
through well logging techniques using instruments suspended with stranded 
cables is well known and widely practiced. Typical measurements made by 
such methods include various types of geophysical and petrophysical 
measurements as well as various types of well production information 
including, but not limited to, formation pressure, flow rate, cement 
status, water flow, corrosion and the response of the well bore 
environment to sundry electrical, acoustic, nuclear and magnetic stimuli. 
The conventional cable used in well-logging is a stranded multi-conductor 
cable which includes a layer of armor strands. One version of this cable 
has a core comprised of six outer conductors cabled around a single center 
conductor and embedded in a neoprene matrix. The outer conductors are 
formed by copper wire strands twisted around a single center strand. Each 
conductor is covered with a layer of suitable insulation material. 
Although the neoprene matrix fills substantially all the voids between 
conductors within the cable core, voids still may exist within the 
conductors themselves between and about the strands. The core is 
surrounded by a jacket of insulating material. The jacket is enclosed by a 
first and a second layer of armor strands. The core may include electrical 
conductors and/or optical fibers and electrical insulating and mechanical 
protecting sheaths immediately surrounding the electrical conductors or 
the optical fibers. In a second version of this cable, the jacket between 
the core and the armor strands is made of a thermoplastic material such as 
Polyethylene or Ethylene Propylene Copolymer (EPC). This thermoplastic 
material is such that it allows strands to embed into the jacket material. 
The armor strands lie in grooves generated on the periphery of the jacket 
and the grooves help to maintain the armor strands in a close relationship 
with the jacket/core. When tension is applied to the cable, the 
thermoplastic material fills the interstices between the armor strands, 
the armor strands embed deeper into the jacket material, and over time, 
the cable becomes permanently elongated. 
Although use of the foregoing cables is highly satisfactory for many well 
logging operations, use of either cable in a well containing substantial 
amounts of low molecular weight hydrocarbons, such as methane gas, 
involves a substantial risk of failure in the cable and/or the cable 
terminations when the cable is rewound after a logging job. Due to the 
borehole depth and a wellbore temperature in excess of 150.degree. C., 
which is quite common, the gas can permeate the matrix of the cable and 
the insulation materials of the conductors due to a phenomenon that may be 
called activated diffusion. The permeation causes pressure buildup and gas 
entrapment in the conductor voids. As the cable is removed from the well 
and wound back upon the drum at the surface, release of the entrapped gas 
is only accomplished through bleed out at the terminated ends of the 
conductors, or outright rupture of the insulation materials themselves. In 
either case, releasing the gas may result in an undesirable cable failure 
due to an electrical short. 
Other characteristics in a borehole environment, in particular downhole 
pressure, can greatly affect cable performance. Extremely high pressure 
can cause the migration of borehole fluid inside the cable. This migration 
of fluid will directly affect the transfer of data from the downhole 
logging tool to the surface. In addition, downhole pressure can enter the 
cable and damage the conductor insulation. 
For the foregoing reasons, there is a need for an apparatus which isolates 
pressure from the surface environment while simultaneously permitting the 
entrance and movement of a cable for downhole logging. 
SUMMARY OF THE INVENTION 
A single strand copper conductor is insulated with a layer of extruded high 
temperature polymer and then encapsulated inside a longitudinally welded, 
cold worked metal tube. The tube is manufactured from a material selected 
for its mechanical property and corrosion resistance. Possible materials 
include carbon steel, type 304 stainless steel, type 316 or 316L stainless 
steel, or a high nickel alloy such as Incoloy 825. The insulated conductor 
is placed inside the tube as the tube is being formed and welded. 
Because of the poor electrical characteristics of available metal tubing 
having the desired mechanical characteristics, a conductive copper layer 
is placed between the polymer insulation and the metal tube. The copper 
conductive layer may be comprised of a longitudinally `cigarette wrapped` 
tape, a helically wrapped tape either with or without a thin plastic (e.g. 
mylar) backing, a helically `served` copper shield composed of individual 
very small copper conductors, or a braided copper shield composed of 
individual very small copper conductors. 
To prevent pressure and fluid migration within the void space between the 
encapsulating metal tube and the conductive core, a method for creating 
isolated blocking dams or a continuous pressure block is provided. The 
dams may be either tape, oil, grease or a high temperature elastomer 
either with or without curatives. The dams are constructed during the tube 
fabrication without interruption of the tubing operation. Alternatively, 
the blocking tape, oil, grease or elastomer may be applied in a thin 
continuous process. 
Alternative to applying the pressure dams/layer during tubing construction, 
a method whereby the blocking substance may be injected into the void 
space is provided. The injected substance may be a viscous oil (e.g. 
silicon), a flowable grease (e.g. butyl rubber), or a fluid elastomer 
(e.g. neoprene) either with or without curatives. 
In order to prevent pressure or fluid entrance into the downhole end, a 
rubber boot is provided. The boot is manufactured from a high temperature 
rated fluoro-elastomer such as PTFE or another suitable elastomer. The 
boot has two bore diameters, one to match the outside diameter of the 
encapsulating metal tube and the other to match the diameter of the 
polymer insulation around the central copper conductor. To prevent 
extrusion of the rubber boot into the void space between the cable core 
and the encapsulating metal tube and the consequent destruction of the 
central copper conductor's insulation layer, a plastic insert (e.g. PEEK 
or another plastic not susceptible to plastic deformation at high 
temperatures and pressures) is provided. 
To prevent yielding and uncontrolled stretching of the tube as it is coiled 
and uncoiled from the surface equipment, the invention includes a sheave 
system designed to minimize the tube strain.

DETAILED DESCRIPTION OF THE INVENTION 
The implementation of the slick line cable according to the present 
invention is illustrated in FIG. 1. A cable 11 is shown supporting a well 
logging sonde 12, for example, in a borehole 13 drilled into the earth. 
The cable 11 passes over a pulley 14 attached to a structure 15 erected on 
the earth surface. The upper end of the cable is secured to a conventional 
winch 16 by a means which will enable the sonde 12 be lowered into and 
withdrawn from the well 13. The winch 16 may be mounted on a truck 17 
incorporating the usual electronic devices for the transmission, 
processing, display or other like processing steps of the data issued from 
the sonde 12, as well as for the control of the operation of the sonde 12. 
The cable of the present invention is shown in FIG. 2. This cable comprises 
a slick line conductor 20 for transmitting data. Conductor 20 is comprised 
of a single, solid wire having an approximate diameter between 0.067" and 
1.1875". In a preferred embodiment, conductor 20 is comprised of a solid 
copper wire. A layer of extruded high temperature polymer insulation 
material 21, such as PFA, PFE, FEP, ETFE, TEFZEL.TM., TEFLON.TM., or a 
similar material, coaxially surrounds the conductor 20. This material 21 
serves to insulate the conductor 20 from the conductive copper layer 22 
and metal tube 23. A layer of stranded copper wire 22 surrounds the 
insulation layer 21. This layer 22 serves to enhance telemetry 
characteristics. The copper conductor 20, insulating polymer 21 and the 
stranded copper wire 22 are all encapsulated inside a longitudinally 
welded, cold worked metal tube 23. The tube is manufactured from a 
material chosen for its mechanical property and corrosion resistance. 
Possible materials include carbon steel, type 304 stainless steel, type 
316 or 316L stainless steel or a high nickel alloy such as Incoloy 825. 
The insulated conductor 20, 21 is placed inside the steel tube 23 as the 
tube is being formed and welded. 
FIG. 3 shows a cross-sectional view of the cable as encapsulated in the 
tube 23. However, not shown between the copper strands layer 22 and the 
steel tube 23 are void spaces due to component geometries. The existence 
of these voids increases the possibility of pressure and fluid migration 
into and within these spaces during cable operations. As shown in FIG. 4, 
to prevent these pressure and fluid migrations, isolated blocking dams 30 
or continuous pressure blocks are provided inside the cable. The dams may 
be either tape, oil, grease or a high temperature elastomer either with or 
without curatives. The blocking dams 30 are constructed during the steel 
tube fabrication without interruption of the tubing operation. 
Alternatively, the blocking tape, oil, grease glue or elastomer may be 
applied in a thin continuous process. 
As an alternative to applying the pressure dams 30 during tubing 
construction, a blocking substance is injected into the void space. The 
injected blocking substance can be a viscous oil such as silicon, a 
flowable grease such as butyl rubber or a fluid elastomer such as neoprene 
either with or without curatives. 
In order to prevent pressure or fluid entrance into the downhole end of the 
cable, a rubber boot 32 is provided to serve as a seal for that end of the 
cable. The boot 32 is manufactured from high temperature rated 
fluoro-elastomer, either PTFE or other suitable elastomer. The boot has 
two diameters 32a and 32b. One diameter matches the diameter of the 
encapsulating metal tube 23 and the other diameter matches the diameter of 
the polymer insulation 21 around the central copper conductor 20. To 
prevent the extrusion of the rubber boot 32 into the void space between 
the cable core and encapsulating metal tube, which would lead to the 
destruction of the insulation layer 21, a plastic insert 33 is provided. 
This insert 33 can be of PEEK or other plastic not susceptible to 
deformation at high temperatures and pressures. 
The present invention is constructed by forming a continuous flat strip of 
metal into a tubular member 23 with edges of the metal strip being 
juxtaposed. The edges of the strip are then welded together to provide a 
fluid-tight tubular member. Furthermore, the electrical conductor 20 is 
fed into the tube 23 simultaneously with the forming and welding of the 
tubular member. 
The foregoing description of the preferred and alternate embodiments of the 
present invention have been presented for purposes of illustration and 
description. It is not intended to be exhaustive or limit the invention to 
the precise form disclosed. Obviously, many modifications and variations 
will be apparent to those skilled in the art. The embodiments were chosen 
and described in order to best explain the principles of the invention and 
its practical application thereby enabling others skilled in the art to 
understand the invention for various embodiments and with various 
modifications as are suited to the particular use contemplated. It is 
intended that the scope of the invention be defined by the accompanying 
claims and their equivalents.