Wireline (cabling)

In the oil and gas industry, the term wireline usually refers to the use of multi-conductor, single conductor, or slickline cable, also known as "wireline," which serves as a conveyance for the acquisition of subsurface petrophysical and geophysical data. Wireline is crucial for delivering well construction services such as pipe recovery, perforating, plug setting, well cleaning, and fishing. The data acquired contributes to the understanding and evaluation of subsurface geology, reservoir properties, and production characteristics.

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Wireline logging refers to the process of acquiring and analyzing geophysical and petrophysical data, along with providing related services based on along-hole depth.

Types of Wireline

There are four basic types of wireline: multi-conductor, single conductor, slickline, and braided line. Additional types include sheathed slickline and fiber-optic lines.

Multi-conductor lines feature external armor wires wrapped around a core, which generally contains either four or seven conductors. The conductors form a central core, safeguarded by the outer armor wires. These conductors deliver power to downhole instrumentation while transmitting data and commands between the surface and the downhole equipment. Primarily used in both open and cased hole applications, multi-conductor cables typically measure between 0.377 inches (9.6 mm) and 0.548 inches (13.9 mm) in diameter, with suggested working loads ranging from 6,600 to 20,000 lbf (29,000 to 89,000 N). (Note: wireline diameters and performance metrics are generally expressed in imperial units.) These cables may have smooth polymer sheathing but are often found as open wound versions.

Single-conductor cables are constructed similarly to multi-conductor cables but consist of a single conductor. Their diameters are smaller, typically between 1/10 inch (2.5 mm) and 5/16 inch (7.9 mm), and they can support suggested working loads from 800 to 7,735 lbf. Their smaller size enables use in pressurized wells, making them suitable for cased hole logging activities. Commonly, these cables facilitate well construction activities like pipe recovery, perforating, plug setting, production logging, and reservoir characterization through various logging techniques.

Slickline consists of a smooth single strand of wireline with diameters varying from 0.082 to 0.160 inches. Unlike other wirelines, slickline lacks conductors but can be found in specialized forms, such as polymer-coated slicklines. Slickline is employed for light well construction and maintenance tasks, alongside memory-reliant subsurface data gathering. Applications involve mechanical services like gauge placement and retrieval, subsurface valve manipulation, wellbore cleaning, and fishing.

Braided lines offer mechanical characteristics similar to mono-conductor wireline and are utilized in well construction and maintenance tasks, including heavy-duty fishing and wellbore cleaning operations.

Functions of Wireline

Wirelines serve to place and recover wellbore equipment, including plugs, gauges, and valves. They are single-strand, non-electric cables lowered into oil and gas wells directly from the surface. Slicklines also allow adjustments of valves and sleeves located downhole, alongside repairs to tubing within the wellbore.

Encased within a drum attached to a truck, slicklines are maneuvered in and out of the well using hydraulic reeling mechanisms.

Braided lines may feature an inner core of insulated wires that supply power to downhole equipment, often referred to as electric lines, thus allowing for electrical telemetry between the surface and the equipment at the cable's end.

Moreover, wirelines constitute electric cables transmitting well-related data. They comprise single strands or multi-strands and are pivotal in both well intervention and formation evaluation operations. In essence, wirelines play a vital role in data collection for logging activities and in workover jobs that necessitate data transmission.

First developed by Conrad and Marcel Schlumberger, wireline logs measure formation properties through electrical wiring. Unlike measurement while drilling (MWD) and mud logs, wireline logs provide constant downhole measurements communicated through the electrical wireline, assisting geologists, drillers, and engineers in making real-time decisions regarding reservoir behaviors and drilling operations. Accompanying petrophysical properties can yield a variety of geological and petrophysical analyses, including measurements of self-potential, natural gamma rays, acoustic travel time, formation density, neutron porosity, resistivity, conductivity, nuclear magnetic resonance, borehole imaging, geometry, formation dip and orientation, as well as fluid characteristics like density and viscosity.

At the end of the wireline, a logging tool, known as a sonde, measures these parameters. The sonde is lowered to a designated depth using the wireline, and measurements are recorded while retrieving it from the well. As the sonde ascends, continuous responses are documented, creating a "log" of instrument responses. Adjustments for elastic stretch of the wireline ensure accurate depth measurements. Numerous factors, including cable length, surface tension (Surf.Ten), cablehead tension (CHT), and the elastic stretch coefficient of the wireline, affect these corrections.

Remedial work required for producing wells to conserve, restore, or enhance output is called workover. This procedure often necessitates a production shut-in; however, this is not always the case.

In workover operations, a well-servicing unit is employed to efficiently winch items into and out of the wellbore. This process may involve braided steel wireline or single steel slickline to raise and lower equipment. Common operations executed during workover include well clean-up, plug placement, and perforation using explosives.

Wireline tools are precisely engineered instruments lowered into a wellbore via the wireline cable. Each tool is designed for specific services, such as evaluating rock properties, identifying casing collars, measuring formation pressure, assessing pore sizes, fluid identification, and sample recovery. Today's wireline tools are sophisticated and engineered to tolerate harsh conditions common in contemporary oil, gas, and geothermal wells. Certain gas wells may exhibit pressures above 30,000 psi, and temperatures in geothermal wells can exceed 500 degrees Fahrenheit. Moreover, corrosive or carcinogenic gases, such as hydrogen sulfide, may be present downhole.

To optimize the time required for operations, multiple wireline tools are often combined into a single tool string, which can extend hundreds of feet and weigh over 5,000 lbs.

Natural Gamma Ray Tools

Natural gamma ray tools are engineered to measure gamma radiation generated by the natural disintegration of elements such as potassium, uranium, and thorium. Unlike nuclear tools, these natural gamma ray instruments emit no radiation. A scintillation crystal typically serves as a radiation sensor, generating a light pulse proportional to the gamma ray intensity striking it. A photomultiplier tube converts this light pulse into a current pulse, which undergoes further processing by the tool's electronics before being relayed to the surface for recording. The intensity of received gamma rays relies on the emissions source, the surrounding formation density, and the distance between both.

The logged data from this tool aids in identifying lithology, estimating shale content, and ensuring depth correlation with subsequent logs.

Nuclear Tools

Nuclear tools gauge formation properties through interactions between reservoir molecules and radiation emitted by the logging tool. The two predominant properties measured are formation porosity and rock density. Formation porosity involves a radiation source emitting fast neutrons into the downhole environment; pore spaces occupy hydrogen-containing fluids that slow down neutrons, inducing gamma rays measured by the tool. The tool's sensors record these interactions, allowing for calibrated interpretations of porosity. Generally, open hole nuclear tools use double-encapsulated chemical sources.

Density tools apply gamma ray radiation to evaluate rock lithology and density. Contemporary density tools use a Cs-137 source to generate gamma rays that interact with rock strata. Higher density materials absorb gamma rays more effectively than lower density alternatives, enabling accurate formation density assessments by measuring the quantity and energy levels of returning gamma rays encountered with the rock matrix. Most density tools incorporate extendable caliper arms to apply the radioactive source against the bore wall, measuring the bore diameter to correct readings for variations in size.

Some advanced nuclear tools employ electronically powered sources regulated from the surface to generate neutrons. By varying neutron energies, logging engineers can deduce formation lithology expressed in fractional percentages.

In formations with porosity, pore spaces fill with oil, gas, or formation water, altering the electrical properties of the rock. A wireline resistivity tool injects current (using lateralog-type tools for conductive water-based muds or induction-type tools for resistive oil-based muds) into the rock environment and assesses resistivity based on Ohm's law. Formation resistivity primarily identifies hydrocarbon-rich pay zones while differentiating them from more conductive water zones. Most wireline tools measure resistivity across various depths into the borehole wall, enabling analysts to quantify fluid invasion levels stemming from drilling mud, thus allowing estimations of permeability.

Certain resistivity tools possess multiple electrodes mounted on articulated pads, facilitating micro-resistivity measurements with very shallow investigative depths, typically between 0.1 and 0.8 inches, ideal for borehole imaging. Both induction methods (for resistive mud systems) and direct current methods (for conductive mud systems) are employed by resistivity imagers.

Sonic and Ultrasonic Tools

Sonic tools, like the Baker Hughes XMAC-F1, consist of an array of piezoelectric transducers and receivers positioned at fixed intervals on the tool body. The transmitters send sound waves into the downhole formation at various frequencies. The signal travels from the transmitter, through the mud column and across the borehole wall to multiple receivers spaced along the tool. Travel time of the sound wave provides insights into rock properties including formation porosity, lithology, permeability, and strength. Sonic tools also evaluate the cement bond between casing and formation, primarily by calculating signal modifications post-transmission through the casing wall.

Ultrasonic tools utilize a rotating acoustic transducer to create a 360-degree image of the borehole as the logging tool ascends, which proves useful for recognizing small-scale bedding, formation dip, and drilling artifacts such as spiraling or induced fractures.

Nuclear Magnetic Resonance (NMR)

NMR tools measure hydrogen properties in formations during two phases: polarization and acquisition. Initially, hydrogen atoms align along a static magnetic field (B0), taking characteristic time T1 for polarization. Subsequently, a brief oscillating magnetic field tips these atoms, prompting precession and inducing signals detectable by antennas. The decay of this signal, characterized by transverse relaxation, is quantified using the CPMG pulse sequence to yield a T2 distribution as the fundamental NMR output.

Both laboratory instruments and logging tools closely reflect NMR measurement principles, although log acquisition time poses challenges due to the interplay of polarization, acquisition, logging speed, and sampling frequency. Extended times yield more complete measurements but necessitate trade-offs in logging speed or sampling frequency.

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A cement bond tool (CBT) assesses cement quality behind casing. CBTs determine the bond integrity between casing and cement, including cement and formation. Utilizing CBT data, companies can effectively troubleshoot cement sheath issues. Proper centralization is essential for CBT functionality.

Two primary problems identified by CBTs include channeling and micro-annulus. Micro-annulus refers to microscopic cracks in the cement sheath, while channeling involves substantial contiguous voids, often due to poor casing centralization. Both issues can be rectified through remedial electric line work.

A CBT measures by generating rapid compressional waves through the wellbore into the pipe, cement, and formation. The transmitter emits sounds at the top of the tool, producing rapid clicking sounds. Typically, two receivers record the compressional wave arrival time. This information gets logged as travel times and micro-seismograms.

Recent logging advancements allow receivers to assess cement integrity across a 360-degree rate, creating radial cement maps and individual sector arrival times.

Casing collar locator tools (CCL) are essential instruments in cased hole electric lines, crucial for depth correlation and identifying line overspeed during heavy fluid logging.

A CCL operates based on Faraday’s Law of Induction, comprising two magnets separated by copper wire coils. As the device passes a casing joint, metallic variance induces a current spike, recorded in the log as a "collar kick."

A cased hole gamma perforator performs mechanical functions such as shooting perforations, setting downhole tubing, and executing tracer surveys. Like open hole natural gamma ray tools, gamma perforators detect gamma emissions from naturally radioactive elements, processing gamma counts which are logged for depth verification.

Setting tools facilitate the placement of completion elements like production packers or bridge plugs utilizing gas energy from explosive charges for hydraulic piston operation. This assembly connects to the plug or packer via a setting mandrel and sliding sleeve, compressing elastomer components for secure installation.

Expansion tools operate similarly, featuring an internal bi-directional piston assembly, designed to avoid impacting production by leaving hardware in the well.

The cable head serves as the uppermost aspect of the wireline toolstring, connecting the conductor wire to the remaining toolstring. Cable heads are custom-built, tailored to depth, pressure, and wellbore fluid types. Weak points in electric lines occur here, allowing for easier separation in case the tool gets stuck.

Tractors are electrical tools designed to push toolstrings into holes, mitigating wireline gravity constraints. These are especially useful in highly deviated and horizontal wells, utilizing either wheels or a wormlike motion against the borehole sides.

The measuring head contacts the wireline first during operations, comprising several wheels that support the wire while collecting key wireline data.

For oilfield activities, the wireline is stored on the surface, coiled around a large spool. Operators may employ portable spools or integrate them into drilling rigs. A motor-driven system turns the spool, facilitating the movement of equipment into and out of the well.

During wireline operations, pressure control systems ensure the containment of wellbore pressure. Open hole electric line operations may require countering pressure surges, while cased hole applications often deal with high-pressure production. Pressure equipment must be rated well above expected pressures, with standard equipment ratings of 5,000, 10,000, and 15,000 psi. Advanced developments include equipment capable of managing 20,000 psi and beyond.

A flange connects to the top of the Christmas tree, possibly with an adapter for pressure control components. A metal gasket is installed between the Christmas tree top and flange to secure wellbore pressures.

A wireline control valve (or blowout preventer) is an enclosed device featuring one or more rams capable of sealing over the wireline during emergencies. Dual wireline valves contain two ram sets, some with grease injection capabilities between the rams to mitigate well pressure.

The term "lubricators" denotes pressure-tested pipe sections that seal wireline tools during pressurization, connecting tools for runs into and out of the well. Valves enable pressure bleed-off for tool maintenance and other tasks.

Pump-in subs (or flow Ts) facilitate fluid injection into the pressure control string, commonly for wellsite pressure testing between each run. These subs may also manage pressure relief after a run or administer kill fluids for wild well control.

The grease injector head controls well pressure during runs into the hole. Using small pipes called flow tubes, it reduces well pressure while injecting high-pressure grease into the lower grease head section.

Pack-off subs manage hydraulic pressure on brass fittings which compress a rubber seal to create a barrier around the wireline. Whether hand-pumped or motorized, these subs ensure secure connections.

Line wipers function similarly to pack-off subs; however, they utilize softer rubber elements, exerting light pressure on the wireline to cleanse grease and well fluids.

A quick test sub (QTS) streamlines pressure testing of pressure control equipment (PCE) for repetitive operations. The PCE undergoes pressure tests before disconnection at the QTS, which has two O-rings to ensure ongoing pressure retention.

If the wireline detaches from the tool, a ball check valve can seal off the well from the surface. Throughout operations, a steel ball positioned in a confined area within the grease head blocks the hole once the wireline exits under pressure, effectively halting surface pressure.

A head catcher (or tool catcher) resides at the lubricator's top, securing tools in place to prevent accidental drops into the well. When activated, it clamps onto the cable head's fishing neck, necessitating hydraulic pressure for release.

A tool trap serves similarly to a head catcher, preventing dropped tools. Located above well control valves, it needs to open for tool descent while allowing recovery even when closed.

A subassembly device bolted atop the BOP stack is engineered to replace traditional bolt-flanges with tapered-wedge and lock ring structures. This ensures both reliable pressure control connection security and significant time savings.

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