In the oil and gas industry, the term wireline usually refers to a cabling technology used by operators of oil and gas wells to lower equipment or measurement devices into the well for the purposes of well intervention and reservoir evaluation.
Braided line can contain an inner core of insulated wires which provide power to equipment located at the end of the cable, normally referred to as electric line, and provides a pathway for electrical telemetry for communication between the surface and equipment at the end of the cable.
Contents
|
A wireline tool string can be dozens of feet long with multiple separate tools installed to perform multiple operations at once.
Natural gamma-ray tools are designed to measure naturally occurring gamma radiation in the earth caused by the disintegration due to Potassium, Uranium, and Thorium. Unlike nuclear tools, these natural gamma ray tools do not emit any radiation.
Natural gamma ray tools employ a radioactive sensor, which is usually a scintillation crystal that emits a light pulse proportional to the strength of the gamma ray pulse incident on it. This light pulse is then converted to a current pulse by means of a photo multiplier tube PMT. From the photo multiplier tube, the current pulse goes to the tool's electronics for further processing and ultimately to the surface system for recording. The strength of the received gamma rays is dependent on the source emitting gamma rays, the density of the formation, and the distance between the source and the tool detector. The log recorded by this tool is used to identify lithology, estimate shale content, and depth correlation of future logs.
Nuclear tools measure formation properties through the interaction of reservoir molecules with radiation emitted from the logging tool. Most open hole nuclear tools utilize double-encapsulated chemical sources.
Density tools use gamma ray radiation to determine the lithology and porosity of the well environment. Modern density tools utilize a Cs-137 radioactive source to generate gamma rays. Density tools also have an extendable caliper arm, which is used to measure the true width of the borehole.
Gamma rays emitted from the source pass into the formation. Depending on the density of the surrounding formation, some of the gamma rays will be absorbed into the rock while others are reflected back to the tool. The ratio of returning gamma rays to absorbed gamma rays is useful in determining formation density.[1]
Neutron tools use fast neutrons to indicate porosity and lithology of the well. Neutron tools typically contain an Am241-Be chemical source or Minitron electronic source to generate the neutrons.
The hydrogen content of the formation, from oil or water, slows down the emitted neutrons until they reach a thermal or epithermal state. At the slower thermal and epithermal states, the tool is able to detect the neutrons. These counts therefore yield a count of slow neutrons, which is a clear indicator of the hydrogen content of the well.[2]
This tool is important in reservoir evaluation for determining the location of the oil-water contact. Water is far more conductive than hydrocarbons and so will give the reservoir rock it saturates a lower resistivity than rock saturated with hydrocarbons. When analysing a resistivity log, the point where the resistivity undergoes a large change is likely to be the location of the oil-water contact. It is also used as an indicator for permeability. Since most resistivity tools have different depths of investigation, a permeable formation will read different resistivities at different depths.
Sonic tools generate sound wave and measure the time it takes to reach the detectors. This is used to measure the effective porosity. Sound waves travel slower in formations in which the pores are not interconnected.
A measurement of the nuclear magnetic resonance (NMR) properties of hydrogen in the formation. There are two phases to the measurement: polarization and acquisition. First, the hydrogen atoms are aligned in the direction of a static magnetic field (B0). This polarization takes a characteristic time T1. Second, the hydrogen atoms are tipped by a short burst from an oscillating magnetic field that is designed so that they precess in resonance in a plane perpendicular to B0. The frequency of oscillation is the Larmor frequency. The precession of the hydrogen atoms induces a signal in the antenna. The decay of this signal with time is caused by transverse relaxation and is measured by the CPMG pulse sequence. The decay is the sum of different decay times, called T2. The T2 distribution is the basic output of a NMR measurement.
The NMR measurement made by both a laboratory instrument and a logging tool follow the same principles very closely. An important feature of the NMR measurement is the time needed to acquire it. In the laboratory, time presents no difficulty. In a log, there is a trade-off between the time needed for polarization and acquisition, logging speed and frequency of sampling. The longer the polarization and acquisition, the more complete the measurement. However, the longer times require either lower logging speed or less frequent samples.
A cement bond tool, or CBT, is an acoustic tool used to measure the quality of the cement behind the casing. Using a CBT, the bond between the casing and cement as well as the bond between cement and formation can be determined. Using CBT data, a company can troubleshoot problems with the cement sheath if necessary.[3] This tool must be centralized in the well to function properly.
Two of the largest problems found in cement by CBT's are channelling and micro-annulus. A micro annulus is the formation of microscopic cracks in the cement sheath. Channelling is where large, contiguous voids in the cement sheath form, typically caused by poor centralization of the casing. Both of these situations can, if necessary, be fixed by remedial electric line work.
A CBT gains its measurements by rapidly pulsing out compressional waves across the well bore and into the pipe, cement, and formation. The compressional pulse originates in a transmitter at the top of the tool, which, when powered up on surface sounds like a rapid clicking sound. The tool typically has two receivers, one three feet away from the receiver, and another at five feet from the transmitter. These receivers record the arrival time of the compressional waves.[4] The information from these receivers are logged as traveltimes for the three and five foot receivers and as a micro-seismogram.
Recent advances in logging technologies have allowed the receivers to measure 360 degrees of cement integrity and can be represented on a log as a radial cement map and as 6-8 individual sector arrival times.[5]
Casing collar locator tools, or CCL's, are among the simplest and most essential in cased hole electric line. CCL's are typically used for depth correlation and can be an indicator of line overspeed when logging in heavy fluids.[6]
A CCL operates on Faraday's Law of Induction. Two magnets are separated by a coil of copper wire. As the CCL passes by a casing joint, or collar, the difference in metal thickness across the two magnets induces a current spike in the coil. This current spike is sent uphole and logged as what's called a collar kick on the cased hole log.
A cased hole gamma perforator is used to perform mechanical services, such as shooting perforations, setting downhole tubing/casing elements, dumping remedial cement, tracer surveys, etc. Typically, a gamma perforator will have some sort of explosively initiated device attached to it, such as a perforating gun, a setting tool, or a dump bailor. In certain instances, the gamma perforator is used to merely spot objects in the well, as in tubing conveyed perforating operations and tracer surveys.
Gamma perforators operate in much the same way as an open hole natural gamma ray tool. Gamma rays given off from naturally occurring radioactive elements bombard the tool. The tool processes the gamma ray counts and sends the data uphole where it is put onto a log. The information is then used to ensure that the depth shown on the log is correct. After that, power can be applied through the tool to set off explosive charges for things like perforating, setting plugs or packers, dumping cement, etc.
Setting tools are used to set downhole completion elements. Setting tools are typically large steel tools onto which a downhole completion can be screwed. One of the most common setting tools is manufactured by Baker Hughes.
Setting tools are explosively driven devices. A shooting CCL or a gamma perforator is used to apply power to detonate a low explosive in the setting tool. The gas pressure created by the deflagrating low explosive exerts a large force on a piston holding back oil. The pneumatic pressure of the piston pushes the oil, which hydraulically separates the setting tool from the plug or packer. The downhole completion is now set in place.
Not only for completions, Setting tools can also run bridge plugs. Which are most commonly used to abandon a well. A certain amount of oil well cement must then be placed on top of the plug. A bond log is also common protocol, the cement must be bonded with the casing to abandon a well, if not, there must be squeeze guns shot. So they can pump cement down the casing and through the squeeze perforations and to the outside of the casing.
The cable head is the upper most portion of the toolstring on any given type of wireline. The cable head is where the conductor wire is made into an electrical connection that can be connected to the rest of the toolstring. Cable heads are typically custom built by the wireline operator for every job and depend greatly on depth, pressure and the type of wellbore fluid.
Electric line weakpoints are also located in the cable head. If the tool is to become stuck in the well, the weak point is where the tool would first separate from the wireline. If the wireline were severed anywhere else along the line, the tool becomes much more difficult to fish.
These are electrical tools used to push the toolstring into hole, overcoming wireline's disadvantage of being gravity dependent. These are used for in highly deviated and horizontal wells where gravity is insufficient, even with roller stem. They push against the side of the wellbore either through the use of wheels or through a wormlike motion. The technology has been in place for more than 10 years, Welltec has operation factors of over 98% with their wireline tractors. The leading operator on the Norwegian Continental Shelf, has successfully applied this technology since 1996 and has concluded that it is a reliable as well as a cost-efficient technology. According to the group’s calculations, they save approximately NOK 500 million ($80 million USD) annually on tractor operations and from 1996 to 2005, tractors have covered an accumulated distance of more than 3,000 kilometers through horizontal wells for the company.[1]
A measuring head is the first piece of equipment the wireline comes into contact with off the drum. The measuring head is composed of several wheels which support the wireline on its way to the winch and they also measure crucial wireline data.
A measuring head records tension, depth, and speed. Current models use optical encoders to derive the revolutions of a wheel with a known circumference, which in turn is used to figure speed and depth. A wheel with a pressure sensor is used to figure tension.
For oilfield work, the wireline resides on the surface, wound around a large (3 to 10 feet in diameter) spool. Operators may use a portable spool (on the back of a special truck) or a permanent part of the drilling rig. A motor and drive train turn the spool and raise and lower the equipment into and out of the well – the winch.
The pressure control employed during wireline operations is intended to contain pressure originating from the well bore. During open hole electric line operations, the pressure might be the result from a well kicking. During cased hole electric line, this is most likely the result of a well producing at high pressures. Pressure equipment must be rated to well over the expected well pressures. Normal ratings for wireline pressure equipment is 5,000, 10,000, and 15,000 pounds per square inch. Some wells are contained with 20,000 psi and 30,000 psi equipment is in development also
A flange attaches to the top of the Christmas tree, usually with some sort of adapter for the rest of the pressure control. A metal gasket is placed between the top of the Christmas tree and the flange to keep in well pressures.
A wireline valve, also called a wireline blow out preventer(BOP), is an enclosed device with one or more rams capable of closing over the wireline in an emergency. A dual wireline valve has two sets of rams and some have the capability of pumping grease in the space between the rams to counterbalance the well pressure.
Lubricator is the term used for sections of pressure tested pipe that act to seal in wireline tools during pressurization.
Pump-in subs (also known as a flow T) allow for the injection of fluid into the pressure control string. Normally these are used for wellsite pressure testing, which is typically performed between every run into the well. They can also be used to bleed off pressure from the string after a run in the well, or to pump in kill fluids to control a wild well.
The grease injector head is the main apparatus for controlling well pressure while running into the hole. The grease head uses a series of very small pipes, called flow tubes, to decrease the pressure head of the well. Grease is injected at high pressure into the bottom portion of the grease head to counteract the remaining well pressure.
Pack-off subs utilize grease pressure on a rubber sealing element to create an impermeable seal around the wireline. pack-off subs can be hand pumped or compressed through a motorized pumping unit.
A line wiper operates in much the same way as a pack-off sub, except that the rubber element is much softer. Grease pumps exert force on the rubber element until a light pressure is exerted on the wireline, cleaning grease and well fluid off the line in the process.
A Quick Test Sub (QTS) is used when pressure testing the pressure control equipment (PCE) that will be used during explosives operations. The PCE is pressure tested and then broke at the QTS. The explosives are then attached to the tool string and pulled back in to the lubricator. The PCE is then reconnected at the QTS. The QTS has two O-rings where it was disconnected that can be tested with hydraulic pressure to confirm the PCE can still hold the pressure it was tested to.
If the wireline were to become severed from the tool, a ball check valve can seal the well off from the surface. During wireline operations, a steel ball sits to the side of a confined area within the grease head while the cable runs in and out of the hole. If the wireline exits that confined area under pressure, the pressure will force the steel ball up towards the hole where the wireline had been. The ball's diameter is larger than that of the hole, so the ball effectively seals off pressure to the surface.
A head catcher is a device placed at the top of the lubricator section. Should the wireline tools be forced into the top of the lubricator section, the head catcher, which looks like a small 'claw,' will clamp down on the fishing neck of the tool. This action prevents the tools from falling downhole should the line pull out of the rope socket. Pressure is bled off of the head catcher to release the tools.
A tool trap has the same purpose as a head catcher in that it prevents the tools from inadvertently dropping down the hole.